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Wastewater Treatment

ContentsArticlesIntroduction 1

Wastewater 1Sewage treatment 5Biochemical oxygen demand 20Effluent 23Biofilter 24Trickling filter 27Chemical oxygen demand 31Chlorination 34Ozone 37Ultraviolet germicidal irradiation 54Water treatment 58Settling 61Flocculation 64Activated sludge 67Slow sand filter 72Aerated lagoon 75Advanced oxidation process 77Aerobic treatment system 78Anaerobic digestion 81Bioreactor 97Carbon filtering 100Constructed wetland 101Dissolved air flotation 113Desalination 115Electrocoagulation 129Expanded granular sludge bed digestion 133Fine bubble diffusers 134Sedimentation 135Membrane bioreactor 137Retention basin 145Reverse osmosis 146Rotating biological contactor 154

API oil-water separator 158Septic tank 160Stabilization pond 164Ultrafiltration (industrial) 166Treatment pond 167Wet oxidation 170

ReferencesArticle Sources and Contributors 171Image Sources, Licenses and Contributors 175

Article LicensesLicense 178

1

Introduction

WastewaterWastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquidwaste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass awide range of potential contaminants and concentrations. In the most common usage, it refers to the municipalwastewater that contains a broad spectrum of contaminants resulting from the mixing of wastewaters from differentsources.Sewage is correctly the subset of wastewater that is contaminated with feces or urine, but is often used to mean anywaste water. "Sewage" includes domestic, municipal, or industrial liquid waste products disposed of, usually via apipe or sewer or similar structure, sometimes in a cesspool emptier.The physical infrastructure, including pipes, pumps, screens, channels etc. used to convey sewage from its origin tothe point of eventual treatment or disposal is termed sewerage.

OriginWastewater or sewage can come from (text in brackets indicates likely inclusions or contaminants):• Human waste (fæces, used toilet paper or wipes, urine, or other bodily fluids), also known as blackwater, usually

from lavatories;• Cesspit leakage;• Septic tank discharge;• Sewage treatment plant discharge;• Washing water (personal, clothes, floors, dishes, etc.), also known as greywater or sullage;• Rainfall collected on roofs, yards, hard-standings, etc. (generally clean with traces of oils and fuel);• Groundwater infiltrated into sewage;• Surplus manufactured liquids from domestic sources (drinks, cooking oil, pesticides, lubricating oil, paint,

cleaning liquids, etc.);• Urban rainfall runoff from roads, carparks, roofs, sidewalks, or pavements (contains oils, animal fæces, litter, fuel

or rubber residues, metals from vehicle exhausts, etc.);• Seawater ingress (high volumes of salt and micro-biota);• Direct ingress of river water (high volumes of micro-biota);• Direct ingress of manmade liquids (illegal disposal of pesticides, used oils, etc.);• Highway drainage (oil, de-icing agents, rubber residues);• Storm drains (almost anything, including cars, shopping trolleys, trees, cattle, etc.);• Blackwater (surface water contaminated by sewage);• Industrial waste• industrial site drainage (silt, sand, alkali, oil, chemical residues);

• Industrial cooling waters (biocides, heat, slimes, silt);• Industrial process waters;• Organic or bio-degradable waste, including waste from abattoirs, creameries, and ice cream manufacture;• Organic or non bio-degradable/difficult-to-treat waste (pharmaceutical or pesticide manufacturing);• extreme pH waste (from acid/alkali manufacturing, metal plating);• Toxic waste (metal plating, cyanide production, pesticide manufacturing, etc.);

Wastewater 2

• Solids and Emulsions (paper manufacturing, foodstuffs, lubricating and hydraulic oil manufacturing, etc.);• agricultural drainage, direct and diffuse.

Wastewater constituentsThe composition of wastewater varies widely. This is a partial list of what it may contain:• Water ( > 95%) which is often added during flushing to carry waste down a drain;• Pathogens such as bacteria, viruses, prions and parasitic worms;• Non-pathogenic bacteria;• Organic particles such as feces, hairs, food, vomit, paper fibers, plant material, humus, etc.;• Soluble organic material such as urea, fruit sugars, soluble proteins, drugs, pharmaceuticals, etc.;• Inorganic particles such as sand, grit, metal particles, ceramics, etc.;• Soluble inorganic material such as ammonia, road-salt, sea-salt, cyanide, hydrogen sulfide, thiocyanates,

thiosulfates, etc.;• Animals such as protozoa, insects, arthropods, small fish, etc.;• Macro-solids such as sanitary napkins, nappies/diapers, condoms, needles, children's toys, dead animals or plants,

etc.;• Gases such as hydrogen sulfide, carbon dioxide, methane, etc.;• Emulsions such as paints, adhesives, mayonnaise, hair colorants, emulsified oils, etc.;• Toxins such as pesticides, poisons, herbicides, etc.• Pharmaceuticals and other hormones.

Wastewater quality indicatorsAny oxidizable material present in a natural waterway or in an industrial wastewater will be oxidized both bybiochemical (bacterial) or chemical processes. The result is that the oxygen content of the water will be decreased.Basically, the reaction for biochemical oxidation may be written as:

Oxidizable material + bacteria + nutrient + O2 → CO

2 + H

2O + oxidized inorganics such as NO

3 or

SO4

Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:S-- + 2 O

2 → SO

4--

NO2- + ½ O

2 → NO

3-

Since all natural waterways contain bacteria and nutrients, almost any waste compounds introduced into suchwaterways will initiate biochemical reactions (such as shown above). Those biochemical reactions create what ismeasured in the laboratory as the Biochemical oxygen demand (BOD). Such chemicals are also liable to be brokendown using strong oxidizing agents and these chemical reactions create what is measured in the laboratory as theChemical oxygen demand (COD). Both the BOD and COD tests are a measure of the relative oxygen-depletioneffect of a waste contaminant. Both have been widely adopted as a measure of pollution effect. The BOD testmeasures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand ofoxidizable pollutants.The so-called 5-day BOD measures the amount of oxygen consumed by biochemical oxidation of wastecontaminants in a 5-day period. The total amount of oxygen consumed when the biochemical reaction is allowed toproceed to completion is called the Ultimate BOD. Because the Ultimate BOD is so time consuming, the 5-day BODhas been almost universally adopted as a measure of relative pollution effect.There are also many different COD tests of which the 4-hour COD is probably the most common.

Wastewater 3

There is no generalized correlation between the 5-day BOD and the ultimate BOD. Similarly there is no generalizedcorrelation between BOD and COD. It is possible to develop such correlations for a specific waste contaminants in aspecific waste water stream but such correlations cannot be generalized for use with any other waste contaminants orwaste water streams. This is because the composition of any waste water stream is different. As an example andeffluent consisting of a solution of simple sugars that might discharge from a confectionery factory is likely to haveorganic components that degrade very quickly. In such a case the 5 day BOD and the ultimate BOD would be verysimilar . I.e there would be very little organic material left after 5 days. . However a final effluent of a sewagetreatment works serving a large industrialised area might have a discharge where the ultimate BOD was muchgreater than the 5 day BOD because much of the easily degraded material would have been removed in the sewagetreatment process and many industrial processes discharge difficult to degrade organic molecules.The laboratory test procedures for the determining the above oxygen demands are detailed in many standard texts.American versions include the "Standard Methods For the Examination Of Water and Wastewater" [1]

Sewage disposal

Industrial wastewater effluent with neutralized pH from tailing runoff. Taken inPeru.

In some urban areas, sewage is carriedseparately in sanitary sewers and runofffrom streets is carried in storm drains.Access to either of these is typically througha manhole. During high precipitationperiods a sanitary sewer overflow can occur,forcing untreated sewage to flow back intothe environment. This can pose a seriousthreat to public health and the surroundingenvironment.

Sewage may drain directly into majorwatersheds with minimal or no treatment.When untreated, sewage can have seriousimpacts on the quality of an environmentand on the health of people. Pathogens cancause a variety of illnesses. Some chemicalspose risks even at very low concentrationsand can remain a threat for long periods of time because of bioaccumulation in animal or human tissue.

TreatmentThere are numerous processes that can be used to clean up waste waters depending on the type and extent of contamination. Most wastewater is treated in industrial-scale wastewater treatment plants (WWTPs) which may include physical, chemical and biological treatment processes. However, the use of septic tanks and other On-Site Sewage Facilities (OSSF) is widespread in rural areas, serving up to one quarter of the homes in the U.S.[2] The most important aerobic treatment system is the activated sludge process, based on the maintenance and recirculation of a complex biomass composed by micro-organisms able to absorb and adsorb the organic matter carried in the wastewater. Anaerobic processes are widely applied in the treatment of industrial wastewaters and biological sludge. Some wastewater may be highly treated and reused as reclaimed water. For some waste waters ecological approaches using reed bed systems such as constructed wetlands may be appropriate. Modern systems include tertiary treatment by micro filtration or synthetic membranes. After membrane filtration, the treated wastewater is indistinguishable from waters of natural origin of drinking quality. Nitrates can be removed from wastewater by

Wastewater 4

microbial denitrification, for which a small amount of methanol is typically added to provide the bacteria with asource of carbon. Ozone Waste Water Treatment is also growing in popularity, and requires the use of an ozonegenerator, which decontaminates the water as Ozone bubbles percolate through the tank.Disposal of wastewaters from an industrial plant is a difficult and costly problem. Most petroleum refineries,chemical and petrochemical plants[3] [4] have onsite facilities to treat their wastewaters so that the pollutantconcentrations in the treated wastewater comply with the local and/or national regulations regarding disposal ofwastewaters into community treatment plants or into rivers, lakes or oceans. Other Industrial processes that producea lot of waste-waters such as paper and pulp production has created environmental concern leading to developmentof processes to recycle water use within plants before they have to be cleaned and disposed of.[5]

ReuseTreated wastewater can be reused as drinking water, in industry (cooling towers), in artificial recharge of aquifers, inagriculture (70% of Israel's irrigated agriculture is based on highly purified wastewater) and in the rehabilitation ofnatural ecosystems (Florida's Everglades).

Algal fuelWoods Hole Oceanographic Institution and Harbor Branch Oceanographic Institution, following the conclusions ofthe USDOE´s Aquatic Species Program, use wastewater for breeding algae. The wastewater from domestic andindustrial sources contain rich organic compounds, which accelerate the growth of algae. This algae can be used toproduce algal fuels[6]

Algaewheel, based in Indianapolis, Indiana, presented a proposal to build a new wastewater treatment facility inCedar Lake, Indiana that uses algae to treat municipal wastewater and uses the sludge byproduct to producebiofuel.[7] [8]

EtymologyThe words "sewage" and "sewer" came from Old French essouier = "to drain", which came from Latin exaquāre.Their formal Latin antecedents are exaquāticum and exaquārium.

Legislation

European UnionCouncil Directive 91/271/EEC on Urban Waste Water Treatment was adopted on 21 May 1991,[9] amended by theCommission Directive 98/15/EC.[10] Commission Decision 93/481/EEC defines the information that Member Statesshould provide the Commission on the state of implementation of the Directive.[11]

Wastewater 5

References[1] Standard Methods of the Examination of Water and Wastewater (http:/ / www. standardmethods. org)[2] "Septic Systems" US EPA. 2011 (http:/ / cfpub. epa. gov/ owm/ septic/ septic. cfm?page_id=261)[3] Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834.[4] Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc. (4th

ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0.[5] J. F. Byrd, M. D. Ehrke, J. I. Whitfield. (1984) "New Bleached Kraft Pulp Plant in Georgia: State of the Art Environmental Control" (http:/ /

www. jstor. org/ stable/ 25042250) Water pollution control federation 56(4): 378–385.[6] Biofuels from industrial/domestic wastewater (http:/ / www. merinews. com/ catFull. jsp?articleID=135399)[7] "Algaewheel — Wastewater Treatment Specialists" (http:/ / www. algaewheel. com). . Retrieved 2008-06-18.[8] "Indiana Company to Submit Proposal to Utilize Algae to Treat Wastewater and Create Renewable Energy" (http:/ / www. ewire. com/

display. cfm/ Wire_ID/ 4808). E-Wire. 2008-06-12. . Retrieved 2008-06-18.[9] http:/ / eur-lex. europa. eu/ LexUriServ/ LexUriServ. do?uri=CELEX:31991L0271:EN:NOT[10] http:/ / eur-lex. europa. eu/ LexUriServ/ LexUriServ. do?uri=CELEX:31998L0015:EN:NOT[11] http:/ / eur-lex. europa. eu/ LexUriServ/ LexUriServ. do?uri=CELEX:31993D0481:EN:NOT

Sewage treatment

The objective of sewage treatment is to produce a disposable effluentwithout causing harm to the surrounding environment, and also

prevent pollution.[1]

Sewage treatment, or domestic wastewatertreatment, is the process of removing contaminantsfrom wastewater and household sewage, both runoff(effluents) and domestic. It includes physical, chemical,and biological processes to remove physical, chemicaland biological contaminants. Its objective is to producean environmentally-safe fluid waste stream (or treatedeffluent) and a solid waste (or treated sludge) suitablefor disposal or reuse (usually as farm fertilizer). Usingadvanced technology it is now possible to re-usesewage effluent for drinking water, although Singaporeis the only country to implement such technology on aproduction scale in its production of NEWater.[2]

Origins of sewage

Sewage is created by residential, institutional, and commercial and industrial establishments and includes householdwaste liquid from toilets, baths, showers, kitchens, sinks and so forth that is disposed of via sewers. In many areas,sewage also includes liquid waste from industry and commerce. The separation and draining of household waste intogreywater and blackwater is becoming more common in the developed world, with greywater being permitted to beused for watering plants or recycled for flushing toilets.

Sewage may include stormwater runoff. Sewerage systems capable of handling stormwater are known as combinedsystems. Combined sewer systems are usually avoided now because precipitation causes widely varying flowsreducing sewage treatment plant efficiency. Combined sewers require much larger, more expensive, treatmentfacilities than sanitary sewers. Heavy storm runoff may overwhelm the sewage treatment system, causing a spill oroverflow. Sanitary sewers are typically much smaller than combined sewers, and they are not designed to transportstormwater. Backups of raw sewage can occur if excessive Infiltration/Inflow is allowed into a sanitary sewersystem.Modern sewered developments tend to be provided with separate storm drain systems for rainwater.[3] As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment,

Sewage treatment 6

heavy metals, organic compounds, animal waste, and oil and grease. (See urban runoff.)[4] Some jurisdictions requirestormwater to receive some level of treatment before being discharged directly into waterways. Examples oftreatment processes used for stormwater include retention basins, wetlands, buried vaults with various kinds ofmedia filters, and vortex separators (to remove coarse solids).

Process overviewSewage can be treated close to where it is created, a decentralised system, (in septic tanks, biofilters or aerobictreatment systems), or be collected and transported via a network of pipes and pump stations to a municipaltreatment plant, a centralised system, (see sewerage and pipes and infrastructure). Sewage collection and treatment istypically subject to local, state and federal regulations and standards. Industrial sources of wastewater often requirespecialized treatment processes (see Industrial wastewater treatment).Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment.• Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle

to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials areremoved and the remaining liquid may be discharged or subjected to secondary treatment.

• Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typicallyperformed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require aseparation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment.

• Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allowrejection into a highly sensitive or fragile ecosystem (estuaries, low-flow rivers, coral reefs,...). Treated water issometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior to dischargeinto a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park.If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

Process Flow Diagram for a typical large-scale treatment plant

Process Flow Diagram for a typical treatment plant via Subsurface Flow ConstructedWetlands (SFCW)

Sewage treatment 7

Pre-treatmentPre-treatment removes materials that can be easily collected from the raw waste water before they damage or clogthe pumps and skimmers of primary treatment clarifiers (trash, tree limbs, leaves, etc.).

Screening

The influent sewage water is screened to remove all large objects like cans, rags, sticks, plastic packets etc. carried inthe sewage stream.[5] This is most commonly done with an automated mechanically raked bar screen in modernplants serving large populations, whilst in smaller or less modern plants a manually cleaned screen may be used. Theraking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/orflow rate. The solids are collected and later disposed in a landfill or incinerated. Bar screens or mesh screens ofvarying sizes may be used to optimize solids removal. If gross solids are not removed they become entrained in pipesand moving parts of the treatment plant and can cause substantial damage and inefficiency in the process.[6] :9

Grit removal

Pre-treatment may include a sand or grit channel or chamber where the velocity of the incoming wastewater isadjusted to allow the settlement of sand, grit, stones, and broken glass. These particles are removed because theymay damage pumps and other equipment. For small sanitary sewer systems, the grit chambers may not be necessary,but grit removal is desirable at larger plants.[6] :10

An empty sedimentation tank at the treatment plant in Merchtem, Belgium.

Fat and grease removal

In some larger plants, fat and grease isremoved by passing the sewage through asmall tank where skimmers collect the fatfloating on the surface. Air blowers in thebase of the tank may also be used to helprecover the fat as a froth. Many plants,however, use primary clarifiers withmechanical surface skimmers for fat andgrease removal.

Primary treatment

In the primary sedimentation stage,sewage flows through large tanks,commonly called "primary clarifiers" or

Sewage treatment 8

"primary sedimentation tanks." The tanks are used to settle sludge while grease and oils rise to the surface and areskimmed off. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drivethe collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities.[6] :9-11

Grease and oil from the floating material can sometimes be recovered for saponification.The dimensions of the tank should be designed to effect removal of a high percentage of the floatables and sludge. Atypical sedimentation tank may remove from 50 to 70 percent of suspended solids, and from 30 to 35 percent ofbiochemical oxygen demand (BOD) from the sewage.

Secondary treatmentSecondary treatment is designed to substantially degrade the biological content of the sewage which are derivedfrom human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquorusing aerobic biological processes. To be effective, the biota require both oxygen and food to live. The bacteria andprotozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbonmolecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified asfixed-film or suspended-growth systems.• Fixed-film or attached growth systems include trickling filters, Moving Bed Biofilm Reactors (MBBR [7]), and

rotating biological contactors, where the biomass grows on media and the sewage passes over its surface.• Suspended-growth systems include activated sludge, where the biomass is mixed with the sewage and can be

operated in a smaller space than fixed-film systems that treat the same amount of water. However, fixed-filmsystems are more able to cope with drastic changes in the amount of biological material and can provide higherremoval rates for organic material and suspended solids than suspended growth systems.[6] :11-13

Roughing filters are intended to treat particularly strong or variable organic loads, typically industrial, to allow themto then be treated by conventional secondary treatment processes. Characteristics include filters filled with media towhich wastewater is applied. They are designed to allow high hydraulic loading and a high level of aeration. Onlarger installations, air is forced through the media using blowers. The resultant wastewater is usually within thenormal range for conventional treatment processes.

A generalized, schematic diagram of an activated sludge process.

A filter removes a small percentage of thesuspended organic matter, while themajority of the organic matter undergoes achange of character, only due to thebiological oxidation and nitrification takingplace in the filter. With this aerobicoxidation and nitrification, the organicsolids are converted into coagulatedsuspended mass, which is heavier andbulkier, and can settle to the bottom of atank. The effluent of the filter is thereforepassed through a sedimentation tank, calleda secondary clarifier, secondary settling tankor humus tank.

Activated sludge

In general, activated sludge plants encompass a variety of mechanisms and processes that use dissolved oxygen topromote the growth of biological floc that substantially removes organic material.[6] :12-13

The process traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrateultimately to nitrogen gas. (See also denitrification).

Sewage treatment 9

A Typical Surface-Aerated Basin (using motor-driven floating aerators)

Surface-aerated basins (Lagoons)

Many small municipal sewage systemsin the United States (1 million gal./dayor less) use aerated lagoons.[8]

Most biological oxidation processes fortreating industrial wastewaters have incommon the use of oxygen (or air) andmicrobial action. Surface-aerated basinsachieve 80 to 90 percent removal ofBOD with retention times of 1 to 10days.[9] The basins may range in depthfrom 1.5 to 5.0 metres and usemotor-driven aerators floating on thesurface of the wastewater.[9]

In an aerated basin system, the aeratorsprovide two functions: they transfer air into the basins required by the biological oxidation reactions, and theyprovide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater andmicrobes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kgO2/kW·h. However, they do not provide as good mixing as is normally achieved in activated sludge systems andtherefore aerated basins do not achieve the same performance level as activated sludge units.[9]

Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of biologicalreactions increase with temperature. Most surface aerated vessels operate at between 4 °C and 32 °C.[9]

Constructed wetlands

Constructed wetlands (can either be surface flow or subsurface flow, horizontal or vertical flow), include engineeredreedbeds and belong to the family of phytorestoration and ecotechnologies; they provide a high degree of biologicalimprovement and depending on design, act as a primary, secondary and sometimes tertiary treatment, also seephytoremediation. One example is a small reedbed used to clean the drainage from the elephants' enclosure atChester Zoo in England; numerous CWs are used to recycle the water of the city of Honfleur in France andnumerous other towns in Europe, the US, Asia and Australia. They are known to be highly productive systems asthey copy natural wetlands, called the "Kidneys of the earth" for their fundamental recycling capacity of thehydrological cycle in the biosphere. Robust and reliable, their treatment capacities improve as time go by, at theopposite of conventional treatment plants whose machinery age with time. They are being increasingly used,although adequate and experienced design are more fundamental than for other systems and space limitation mayimpede their use.

Filter beds (oxidizing beds)

In older plants and those receiving variable loadings, trickling filter beds are used where the settled sewage liquor is spread onto the surface of a bed made up of coke (carbonized coal), limestone chips or specially fabricated plastic media. Such media must have large surface areas to support the biofilms that form. The liquor is typically distributed through perforated spray arms. The distributed liquor trickles through the bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological films of bacteria, protozoa and fungi form on the media’s surfaces and eat or otherwise reduce the organic content.[6] :12 This biofilm is often grazed by insect larvae, snails, and worms which help maintain an optimal thickness. Overloading of beds increases the thickness of the film leading to clogging of the filter media and ponding on the surface. Recent

Sewage treatment 10

advances in media and process micro-biology design overcome many issues with Trickling filter designs.

Soil Bio-Technology

A new process called Soil Bio-Technology (SBT) developed at IIT Bombay has shown tremendous improvements inprocess efficiency enabling total water reuse, due to extremely low operating power requirements of less than 50joules per kg of treated water.[10] Typically SBT systems can achieve chemical oxygen demand (COD) levels lessthan 10 mg/L from sewage input of COD 400 mg/L.[11] SBT plants exhibit high reductions in COD values andbacterial counts as a result of the very high microbial densities available in the media. Unlike conventional treatmentplants, SBT plants produce insignificant amounts of sludge, precluding the need for sludge disposal areas that arerequired by other technologies.[12]

In the Indian context, conventional sewage treatment plants fall into systemic disrepair due to 1) high operatingcosts, 2) equipment corrosion due to methanogenesis and hydrogen sulphide, 3) non-reusability of treated water dueto high COD (>30 mg/L) and high fecal coliform (>3000 NFU) counts, 4) lack of skilled operating personnel and 5)equipment replacement issues. Examples of such systemic failures has been documented by Sankat MochanFoundation at the Ganges basin after a massive cleanup effort by the Indian government in 1986 by setting upsewage treatment plants under the Ganga Action Plan failed to improve river water quality.

Biological aerated filters

Biological Aerated (or Anoxic) Filter (BAF) or Biofilters combine filtration with biological carbon reduction,nitrification or denitrification. BAF usually includes a reactor filled with a filter media. The media is either insuspension or supported by a gravel layer at the foot of the filter. The dual purpose of this media is to support highlyactive biomass that is attached to it and to filter suspended solids. Carbon reduction and ammonia conversion occursin aerobic mode and sometime achieved in a single reactor while nitrate conversion occurs in anoxic mode. BAF isoperated either in upflow or downflow configuration depending on design specified by manufacturer.

Schematic diagram of a typical rotating biological contactor (RBC). The treated effluentclarifier/settler is not included in the diagram.

Rotating biological contactors

Rotating biological contactors (RBCs)are mechanical secondary treatmentsystems, which are robust and capableof withstanding surges in organic load.RBCs were first installed in Germanyin 1960 and have since been developedand refined into a reliable operatingunit. The rotating disks support thegrowth of bacteria andmicro-organisms present in thesewage, which break down andstabilise organic pollutants. To besuccessful, micro-organisms need bothoxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As themicro-organisms grow, they build up on the media until they are sloughed off due to shear forces provided by therotating discs in the sewage. Effluent from the RBC is then passed through final clarifiers where themicro-organisms in suspension settle as a sludge. The sludge is withdrawn from the clarifier for further treatment.

A functionally similar biological filtering system has become popular as part of home aquarium filtration and purification. The aquarium water is drawn up out of the tank and then cascaded over a freely spinning corrugated fiber-mesh wheel before passing through a media filter and back into the aquarium. The spinning mesh wheel

Sewage treatment 11

develops a biofilm coating of microorganisms that feed on the suspended wastes in the aquarium water and are alsoexposed to the atmosphere as the wheel rotates. This is especially good at removing waste urea and ammoniaurinated into the aquarium water by the fish and other animals.

Membrane bioreactors

Membrane bioreactors (MBR) combine activated sludge treatment with a membrane liquid-solid separation process.The membrane component uses low pressure microfiltration or ultra filtration membranes and eliminates the need forclarification and tertiary filtration. The membranes are typically immersed in the aeration tank; however, someapplications utilize a separate membrane tank. One of the key benefits of an MBR system is that it effectivelyovercomes the limitations associated with poor settling of sludge in conventional activated sludge (CAS) processes.The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS)concentration than CAS systems, which are limited by sludge settling. The process is typically operated at MLSS inthe range of 8,000–12,000 mg/L, while CAS are operated in the range of 2,000–3,000 mg/L. The elevated biomassconcentration in the MBR process allows for very effective removal of both soluble and particulate biodegradablematerials at higher loading rates. Thus increased sludge retention times, usually exceeding 15 days, ensure completenitrification even in extremely cold weather.The cost of building and operating an MBR is usually higher than conventional wastewater treatment. Membranefilters can be blinded with grease or abraded by suspended grit and lack a clarifier's flexibility to pass peak flows.The technology has become increasingly popular for reliably pretreated waste streams and has gained wideracceptance where infiltration and inflow have been controlled, however, and the life-cycle costs have been steadilydecreasing. The small footprint of MBR systems, and the high quality effluent produced, make them particularlyuseful for water reuse applications.[13]

Secondary sedimentation

Secondary Sedimentation tank at a ruraltreatment plant.

The final step in the secondary treatment stage is to settle out the biologicalfloc or filter material through a secondary clarifier and to produce sewagewater containing low levels of organic material and suspended matter.

Tertiary treatment

The purpose of tertiary treatment is to provide a final treatment stage to raisethe efluennt quality before it is discharged to the receiving environment (sea,river, lake, ground, etc.). More than one tertiary treatment process may beused at any treatment plant. If disinfection is practiced, it is always the finalprocess. It is also called "effluent polishing."

Sewage treatment 12

Filtration

Sand filtration removes much of the residual suspended matter.[6] :22-23 Filtration over activated carbon, also calledcarbon adsorption, removes residual toxins.[6] :19

Lagooning

A sewage treatment plant and lagoon in Everett,Washington, United States.

Lagooning provides settlement and further biological improvementthrough storage in large man-made ponds or lagoons. These lagoonsare highly aerobic and colonization by native macrophytes, especiallyreeds, is often encouraged. Small filter feeding invertebrates such asDaphnia and species of Rotifera greatly assist in treatment byremoving fine particulates.

Nutrient removal

Wastewater may contain high levels of the nutrients nitrogen andphosphorus. Excessive release to the environment can lead to a buildup of nutrients, called eutrophication, which can in turn encourage theovergrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause an algal bloom, a rapid growth inthe population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition ofthe algae by bacteria uses up so much of oxygen in the water that most or all of the animals die, which creates moreorganic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species producetoxins that contaminate drinking water supplies. Different treatment processes are required to remove nitrogen andphosphorus.

Nitrogen removal

The removal of nitrogen is effected through the biological oxidation of nitrogen from ammonia to nitrate(nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to theatmosphere and thus removed from the water.Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation ofammonia (NH3) to nitrite (NO2

−) is most often facilitated by Nitrosomonas spp. (nitroso referring to the formation ofa nitroso functional group). Nitrite oxidation to nitrate (NO3

−), though traditionally believed to be facilitated byNitrobacter spp. (nitro referring the formation of a nitro functional group), is now known to be facilitated in theenvironment almost exclusively by Nitrospira spp.Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. It isfacilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen,but the activated sludge process (if designed well) can do the job the most easily.[6] :17-18 Since denitrification is thereduction of nitrate to dinitrogen gas, an electron donor is needed. This can be, depending on the wastewater, organicmatter (from faeces), sulfide, or an added donor like methanol. The sludge in the anoxic tanks (denitrification tanks)must be mixed well (mixture of recirculated mixed liquor, return activated sludge [RAS], and raw influent) e.g. byusing submersible mixers in order to achieve the desired denitrification.Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment.Many sewage treatment plants use axial flow pumps to transfer the nitrified mixed liquor from the aeration zone tothe anoxic zone for denitrification. These pumps are often referred to as Internal Mixed Liquor Recycle (IMLR)pumps.

Sewage treatment 13

Phosphorus removal

Phosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water systems. (For adescription of the negative effects of algae, see Nutrient removal). It is also particularly important for water reusesystems where high phosphorus concentrations may lead to fouling of downstream equipment such as reverseosmosis.Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In thisprocess, specific bacteria, called polyphosphate accumulating organisms (PAOs), are selectively enriched andaccumulate large quantities of phosphorus within their cells (up to 20 percent of their mass). When the biomassenriched in these bacteria is separated from the treated water, these biosolids have a high fertilizer value.Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g. ferric chloride),aluminum (e.g. alum), or lime.[6] :18 This may lead to excessive sludge production as hydroxides precipitates and theadded chemicals can be expensive. Chemical phosphorus removal requires significantly smaller equipment footprintthan biological removal, is easier to operate and is often more reliable than biological phosphorus removal. Anothermethod for phosphorus removal is to use granular laterite.Once removed, phosphorus, in the form of a phosphate-rich sludge, may be stored in a land fill or resold for use infertilizer.

DisinfectionThe purpose of disinfection in the treatment of waste water is to substantially reduce the number of microorganismsin the water to be discharged back into the environment for the later use of drinking, bathing, irrigation, etc. Theeffectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type ofdisinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Cloudywater will be treated less successfully, since solid matter can shield organisms, especially from ultraviolet light or ifcontact times are low. Generally, short contact times, low doses and high flows all militate against effectivedisinfection. Common methods of disinfection include ozone, chlorine, ultraviolet light, or sodium hypochlorite.[6]

:16 Chloramine, which is used for drinking water, is not used in waste water treatment because of its persistence.After multiple steps of disinfection, the treated water is ready to be released back into the water cycle by means ofthe nearest body of water or agriculture. Afterwards, the water can be transferred to reserves for everyday humanuses.Chlorination remains the most common form of waste water disinfection in North America due to its low cost andlong-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generatechlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine orchloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further,because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated,adding to the complexity and cost of treatment.Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, thetreated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UVradiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable ofreproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacementand the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UVradiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In theUnited Kingdom, UV light is becoming the most common means of disinfection because of the concerns about theimpacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receivingwater. Some sewage treatment systems in Canada and the US also use UV light for their effluent waterdisinfection.[14] [15]

Sewage treatment 14

Ozone (O3) is generated by passing oxygen (O2) through a high voltage potential resulting in a third oxygen atombecoming attached and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comesin contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorinebecause, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozoneis generated onsite as needed. Ozonation also produces fewer disinfection by-products than chlorination. Adisadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements forspecial operators.

Odour ControlOdours emitted by sewage treatment are typically an indication of an anaerobic or "septic" condition.[16] Early stagesof processing will tend to produce smelly gases, with hydrogen sulfide being most common in generatingcomplaints. Large process plants in urban areas will often treat the odours with carbon reactors, a contact media withbio-slimes, small doses of chlorine, or circulating fluids to biologically capture and metabolize the obnoxiousgases.[17] Other methods of odour control exist, including addition of iron salts, hydrogen peroxide, calcium nitrate,etc. to manage hydrogen sulfide levels.

Package plants and batch reactorsTo use less space, treat difficult waste and intermittent flows, a number of designs of hybrid treatment plants havebeen produced. Such plants often combine at least two stages of the three main treatment stages into one combinedstage. In the UK, where a large number of wastewater treatment plants serve small populations, package plants are aviable alternative to building a large structure for each process stage. In the US, package plants are typically used inrural areas, highway rest stops and trailer parks.[18]

One type of system that combines secondary treatment and settlement is the sequencing batch reactor (SBR).Typically, activated sludge is mixed with raw incoming sewage, and then mixed and aerated. The settled sludge isrun off and re-aerated before a proportion is returned to the headworks.[19] SBR plants are now being deployed inmany parts of the world.The disadvantage of the SBR process is that it requires a precise control of timing, mixing and aeration. Thisprecision is typically achieved with computer controls linked to sensors. Such a complex, fragile system is unsuitedto places where controls may be unreliable, poorly maintained, or where the power supply may be intermittent.Extended aeration package plants use separate basins for aeration and settling, and are somewhat larger than SBRplants with reduced timing sensitivity.[20]

Package plants may be referred to as high charged or low charged. This refers to the way the biological load isprocessed. In high charged systems, the biological stage is presented with a high organic load and the combined flocand organic material is then oxygenated for a few hours before being charged again with a new load. In the lowcharged system the biological stage contains a low organic load and is combined with flocculate for longer times.

Sewage treatment 15

Sludge treatment and disposalThe sludges accumulated in a wastewater treatment process must be treated and disposed of in a safe and effectivemanner. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causingmicroorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobicdigestion, and composting. Incineration is also used albeit to a much lesser degree.[6] :19-21

Sludge treatment depends on the amount of solids generated and other site-specific conditions. Composting is mostoften applied to small-scale plants with aerobic digestion for mid sized operations, and anaerobic digestion for thelarger-scale operations.

Anaerobic digestionAnaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either bethermophilic digestion, in which sludge is fermented in tanks at a temperature of 55°C, or mesophilic, at atemperature of around 36°C. Though allowing shorter retention time (and thus smaller tanks), thermophilic digestionis more expensive in terms of energy consumption for heating the sludge.Anaerobic digestion is the most common (mesophilic) treatment of domestic sewage in septic tanks, which normallyretain the sewage from one day to two days, reducing the BOD by about 35 to 40 percent. This reduction can beincreased with a combination of anaerobic and aerobic treatment by installing Aerobic Treatment Units (ATUs) inthe septic tank.One major feature of anaerobic digestion is the production of biogas (with the most useful component beingmethane), which can be used in generators for electricity production and/or in boilers for heating purposes.

Aerobic digestionAerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteriarapidly consume organic matter and convert it into carbon dioxide. The operating costs used to be characteristicallymuch greater for aerobic digestion because of the energy used by the blowers, pumps and motors needed to addoxygen to the process.Aerobic digestion can also be achieved by using diffuser systems or jet aerators to oxidize the sludge. Fine bubblediffusers are typically the more cost-efficient diffusion method, however, plugging is typically a problem due tosediment settling into the smaller air holes. Coarse bubble diffusers are more commonly used in activated sludgetanks (generally a side process in waste water management) or in the flocculation stages. A key component forselecting diffuser type is to ensure it will produce the required oxygen transfer rate.

CompostingComposting is also an aerobic process that involves mixing the sludge with sources of carbon such as sawdust, strawor wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon sourceand, in doing so, produce a large amount of heat.[6] :20

IncinerationIncineration of sludge is less common because of air emissions concerns and the supplemental fuel (typically naturalgases or fuel oil) required to burn the low calorific value sludge and vaporize residual water. Stepped multiple hearthincinerators with high residence time and fluidized bed incinerators are the most common systems used to combustwastewater sludge. Co-firing in municipal waste-to-energy plants is occasionally done, this option being lessexpensive assuming the facilities already exist for solid waste and there is no need for auxiliary fuel.[6] :20-21

Sewage treatment 16

Sludge disposalWhen a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Typically,sludges are thickened (dewatered) to reduce the volumes transported off-site for disposal. There is no process whichcompletely eliminates the need to dispose of biosolids. There is, however, an additional step some cities are taking tosuperheat sludge and convert it into small pelletized granules that are high in nitrogen and other organic materials. InNew York City, for example, several sewage treatment plants have dewatering facilities that use large centrifugesalong with the addition of chemicals such as polymer to further remove liquid from the sludge. The removed fluid,called centrate, is typically reintroduced into the wastewater process. The product which is left is called "cake" andthat is picked up by companies which turn it into fertilizer pellets. This product is then sold to local farmers and turffarms as a soil amendment or fertilizer, reducing the amount of space required to dispose of sludge in landfills. Muchsludge originating from commercial or industrial areas is contaminated with toxic materials that are released into thesewers from the industrial processes.[21] Elevated concentrations of such materials may make the sludge unsuitablefor agricultural use and it may then have to be incinerated or disposed of to landfill.

Treatment in the receiving environment

The outlet of the Karlsruhe sewage treatmentplant flows into the Alb.

Many processes in a wastewater treatment plant are designed to mimicthe natural treatment processes that occur in the environment, whetherthat environment is a natural water body or the ground. If notoverloaded, bacteria in the environment will consume organiccontaminants, although this will reduce the levels of oxygen in thewater and may significantly change the overall ecology of thereceiving water. Native bacterial populations feed on the organiccontaminants, and the numbers of disease-causing microorganisms arereduced by natural environmental conditions such as predation orexposure to ultraviolet radiation. Consequently, in cases where thereceiving environment provides a high level of dilution, a high degreeof wastewater treatment may not be required. However, recentevidence has demonstrated that very low levels of specific contaminants in wastewater, including hormones (fromanimal husbandry and residue from human hormonal contraception methods) and synthetic materials such asphthalates that mimic hormones in their action, can have an unpredictable adverse impact on the natural biota andpotentially on humans if the water is re-used for drinking water.[22] In the US and EU, uncontrolled discharges ofwastewater to the environment are not permitted under law, and strict water quality requirements are to be met, asclean drinking water is essential. (For requirements in the US, see Clean Water Act.) A significant threat in thecoming decades will be the increasing uncontrolled discharges of wastewater within rapidly developing countries.

Effects on BiologySewage treatment plants can have multiple effects on nutrient levels in the water that the treated sewage flows into.These effects on nutrients can have large effects on the biological life in the water in contact with the effluent.Stabilization ponds (or treatment ponds) can include any of the following:• Oxidation ponds, which are aerobic bodies of water usually 1–2 meters in depth that receive effluent from

sedimentation tanks or other forms of primary treatment.• Dominated by algae

• Polishing ponds are similar to oxidation ponds but receive effluent from an oxidation pond or from a plant with anextended mechanical treatment.• Dominated by zooplankton

Sewage treatment 17

• Facultative lagoons, raw sewage lagoons, or sewage lagoons are ponds where sewage is added with no primarytreatment other than coarse screening. These ponds provide effective treatment when the surface remains aerobic;although anaerobic conditions may develop near the layer of settled sludge on the bottom of the pond.[23]

• Anaerobic lagoons are heavily loaded ponds.• Dominated by bacteria

• Sludge lagoons are aerobic ponds, usually 2–5 meters in depth, that receive anaerobically digested primarysludge, or activated secondary sludge under water.• Upper layers are dominated by algae [24]

Phosphorus limitation is a possible result from sewage treatment and results in flagellate-dominated plankton,particularly in summer and fall.[25]

At the same time a different study found high nutrient concentrations linked to sewage effluents. High nutrientconcentration leads to high chlorophyll a concentrations, which is a proxy for primary production in marineenvironments. High primary production means high phytoplankton populations and most likely high zooplanktonpopulations because zooplankton feed on phytoplankton. However, effluent released into marine systems also leadsto greater population instability.[26]

A study done in Britain found that the quality of effluent affected the planktonic life in the water in direct contactwith the wastewater effluent. Turbid, low-quality effluents either did not contain ciliated protozoa or contained onlya few species in small numbers. On the other hand, high-quality effluents contained a wide variety of ciliatedprotozoa in large numbers. Due to these findings, it seems unlikely that any particular component of the industrialeffluent has, by itself, any harmful effects on the protozoan populations of activated sludge plants.[27]

The planktonic trends of high populations close to input of treated sewage is contrasted by the bacterial trend. In astudy of Aeromonas spp. in increasing distance from a wastewater source, greater change in seasonal cycles wasfound the furthest from the effluent. This trend is so strong that the furthest location studied actually had an inversionof the Aeromonas spp. cycle in comparison to that of fecal coliforms. Since there is a main pattern in the cycles thatoccurred simultaneously at all stations it indicates seasonal factors (temperature, solar radiation, phytoplankton)control of the bacterial population. The effluent dominant species changes from Aeromonas caviae in winter toAeromonas sobria in the spring and fall while the inflow dominant species is Aeromonas caviae, which is constantthroughout the seasons.[28]

Sewage treatment in developing countriesFew reliable figures on the share of the wastewater collected in sewers that is being treated in the world exist. Inmany developing countries the bulk of domestic and industrial wastewater is discharged without any treatment orafter primary treatment only. In Latin America about 15% of collected wastewater passes through treatment plants(with varying levels of actual treatment). In Venezuela, a below average country in South America with respect towastewater treatment, 97 percent of the country’s sewage is discharged raw into the environment.[29] In a relativelydeveloped Middle Eastern country such as Iran, the majority of Tehran's population has totally untreated sewageinjected to the city’s groundwater.[30] However now the construction of major parts of the sewage system, collectionand treatment, in Tehran is almost complete, and under development, due to be fully completed by the end of 2012.In Israel, about 50 percent of agricultural water usage (total use was 1 billion cubic metres in 2008) is providedthrough reclaimed sewer water. Future plans call for increased use of treated sewer water as well as moredesalination plants.[31]

Most of sub-Saharan Africa is without wastewater treatment.

Sewage treatment 18

References[1] Khopkar, S. M. (2004). Environmental Pollution Monitoring And Control (http:/ / books. google. com/ ?id=TAk21grzDZgC). New Delhi:

New Age International. p. 299. ISBN 8122415075. . Retrieved 2009-06-28.[2] History of the NEWater (http:/ / www. pub. gov. sg/ about/ historyfuture/ Pages/ NEWater. aspx)[3] Burrian, Steven J., et al. (1999). "The Historical Development of Wet-Weather Flow Management." (http:/ / www. epa. gov/ nrmrl/ pubs/

600ja99275/ 600ja99275. pdf) US Environmental Protection Agency (EPA). National Risk Management Research Laboratory, Cincinnati,OH. Document No. EPA/600/JA-99/275.

[4] Stormwater Effects Handbook: A Toolbox for Watershed Managers, Scientists, and Engineers (http:/ / unix. eng. ua. edu/ ~rpitt/ Publications/BooksandReports/ Stormwater Effects Handbook by Burton and Pitt book/ MainEDFS_Book. html). New York: CRC/Lewis Publishers. 2001.ISBN 0-87371-924-7. . Chapter 2.

[5] Water and Environmental Health at London and Loughborough (1999). "Waste water Treatment Options." (http:/ / www. lut. ac. uk/ well/resources/ technical-briefs/ 64-wastewater-treatment-options. pdf) Technical brief no. 64. London School of Hygiene & Tropical Medicineand Loughborough University.

[6] EPA. Washington, DC (2004). "Primer for Municipal Waste water Treatment Systems." (http:/ / www. epa. gov/ owm/ primer. pdf)Document no. EPA 832-R-04-001.

[7] http:/ / www. waterworld. com/ index/ webcasts/ webcast-display/ 5792747027/ webcasts/ waterworld/ live-events/ evaluation_-application.html

[8] Maine Department of Environmental Protection. Augusta, ME. "Aerated Lagoons - Wastewater Treatment." (http:/ / www. lagoonsonline.com) Maine Lagoon Systems Task Force. Accessed 2010-07-11.

[9] Beychok, M.R. (1971). "Performance of surface-aerated basins". Chemical Engineering Progress Symposium Series 67 (107): 322–339.Available at CSA Illumina website (http:/ / md1. csa. com/ partners/ viewrecord. php?requester=gs& collection=ENV& recid=7112203& q=&uid=788301038& setcookie=yes)

[10] Kadam, A.; Ozaa, G.; Nemadea, P.; Duttaa, S.; Shankar, H. (2008). "Municipal wastewater treatment using novel constructed soil filtersystem". Chemosphere (Elsevier) 71 (5): 975–981. doi:10.1016/j.chemosphere.2007.11.048. PMID 18207216.

[11] Nemade, P.D.; Kadam, A.M.; Shankar, H.S. (2009). "Wastewater renovation using constructed soil filter (CSF): A novel approach" (http:/ /www. che. iitb. ac. in/ online/ bibliography/ wastewater-renovation-using-constructed-soil-filter-csf-a-novel-approach). Journal of HazardousMaterials (Elsevier) 170 (2-3): 657–665. doi:10.1016/j.jhazmat.2009.05.015. PMID 19501460. .

[12] A documentary video detailing a 3 MLD SBT plant deployed at the Brihanmumbai Municipal Corporation for Mumbai city can be seen at"SBT at BMC Mumbai." (http:/ / www. youtube. com/ watch?v=dKWVtZ81mY0)

[13] EPA. Washington, DC (2007). "Membrane Bioreactors." (http:/ / www. epa. gov/ owm/ mtb/ etfs_membrane-bioreactors. pdf) WastewaterManagement Fact Sheet.

[14] Das, Tapas K. (08 2001). "Ultraviolet disinfection application to a wastewater treatment plant". Clean Technologies and EnvironmentalPolicy (Springer Berlin/Heidelberg) 3 (2): 69–80. doi:10.1007/S100980100108.

[15] Florida Department of Environmental Protection. Talahassee, FL. "Ultraviolet Disinfection for Domestic Waste water." (http:/ / www. dep.state. fl. us/ water/ wastewater/ dom/ domuv. htm) 2010-03-17.

[16] Harshman, Vaughan; Barnette, Tony (05 2000). "Wastewater Odor Control: An Evaluation of Technologies" (http:/ / www. wwdmag. com/Wastewater-Odor-Control-An-Evaluation-of-Technologies-article1698). Water Engineering & Management. ISSN 0273-2238. .

[17] Walker, James D. and Welles Products Corporation (1976). "Tower for removing odors from gases." (http:/ / www. freepatentsonline. com/4421534. html) U.S. Patent No. 4421534.

[18] EPA. Washington, DC (2000). "Package Plants." (http:/ / www. epa. gov/ owm/ mtb/ package_plant. pdf) Wastewater Technology FactSheet. Document no. EPA 832-F-00-016.

[19] EPA. Washington, DC (1999). "Sequencing Batch Reactors." (http:/ / www. epa. gov/ owm/ mtb/ sbr_new. pdf) Wastewater TechnologyFact Sheet. Document no. EPA 832-F-99-073.

[20] Hammer, Mark J. (1975). Water and Waste-Water Technology. John Wiley & Sons. pp. 390–391. ISBN 0-471-34726-4.[21] ORGANIC CONTAMINANTS IN SEWAGE SLUDGE FOR AGRICULTURAL USE, European Commission Joint Research Centre

Institute for Environment and Sustainability Soil and Waste Unit H. Langenkamp & P. Part (http:/ / ec. europa. eu/ environment/ waste/sludge/ pdf/ organics_in_sludge. pdf)

[22] Environment-agency.gov.uk (http:/ / www. environment-agency. gov. uk/ business/ 444304/ 1290036/ 1290100/ 1290353/ 1294402/1314667/ )

[23] Metcalf & Eddy, Inc. (1972). Wastewater Engineering. McGraw-Hill Book Company. pp. 552–554. ISBN 0-07-041675-3.[24] Haughey, A. (1968) The Planktonic Algae of Auckland Sewage Treatment Ponds, New Zealand Journal of Marine and Freshwater Research[25] Nutrients and Phytoplankton in Lake Washington Edmondson, WT; Nutrients and Eutrophication: The Limiting Nutrient Controversy,

American Society of Limnology and Oceanography Special Symposia Vol.1[26] Caperon, Cattell, and Krasnick (1971) Phytoplankton Kinetics in a Subtropical Estuary: Eutrophication, Limnology and Oceanography[27] Curds and Cockburn (1969) Protozoa in Biological Sewage-Treatment Processes -- I. A Survey of the Protozoan Fauna of British

Percolating filters and Activated-Sludge Plants, Water Research[28] Monfort and Baleux (1990) Dynamics of Aeromonas hydrophila, Aeromonas sobria, and Aeromonas caviae in a Sewage Treatment Pond,

Applied and Environmental Microbiology

Sewage treatment 19

[29] Caribbean Environment Programme (1998). Appropriate Technology for Sewage Pollution Control in the Wider Caribbean Region (http:/ /www. cep. unep. org/ publications-and-resources/ technical-reports/ tr40en. pdf). Kingston, Jamaica: United Nations EnvironmentProgramme. . Retrieved 2009-10-12. Technical Report No. 40.

[30] Massoud Tajrishy and Ahmad Abrishamchi, Integrated Approach to Water and Wastewater Management for Tehran, Iran, WaterConservation, Reuse, and Recycling: Proceedings of the Iranian-American Workshop, National Academies Press (2005)

[31] Martin, Andrew (2008-08-10). "Farming in Israel, without a drop to spare" (http:/ / www. iht. com/ articles/ 2008/ 08/ 10/ business/ 10feed.php). New York Times. .

External links• "Anaerobic Industrial Wastewater Treatment: Perspectives for Closing Water and Resource Cycles." (http:/ /

edepot. wur. nl/ 39480) Jules B. van Lier, Wageningen University, The Netherlands• Arcata, California Constructed Wetland: A Cost-Effective Alternative for Wastewater Treatment (http:/ /

ecotippingpoints. org/ our-stories/ indepth/ usa-california-arcata-constructed-wetland-wastewater. html)• Boston Sewage Tour (http:/ / seagrant. mit. edu/ education/ resources/ bostonsewage/ introduction. html) - MIT

Sea Grant• Interactive Diagram of Wastewater Treatment - "Go with the Flow" (http:/ / wef. org/ apps/ gowithflow/ theflow.

htm) - Water Environment Federation• Phosphorus Recovery (http:/ / www. phosphorus-recovery. tu-darmstadt. de) - Technische Universität Darmstadt

& CEEP• Heavy metals recovery (http:/ / enviropark. ru/ course/ category. php?id=10) - Mendeleev University Science Park• Sewer History (http:/ / www. sewerhistory. org)• The Straight Dope - What happens to all the stuff that goes down the toilet? (http:/ / www. straightdope. com/

mailbag/ msolidwaste. html) - Syndicated column by Cecil Adams• Tour of a Washington state sewage plant written by an employee (http:/ / www. poopreport. com/ Consumer/

poop_plant. html)• National Water Engineering of Pakistan - Wastewater Treatment Plants in Pakistan (http:/ / www. nwepk. com/ )

Biochemical oxygen demand 20

Biochemical oxygen demandBiochemical oxygen demand or BOD is a chemical procedure for determining the amount of dissolved oxygenneeded by aerobic biological organisms in a body of water to break down organic material present in a given watersample at certain temperature over a specific time period. It is not a precise quantitative test, although it is widelyused as an indication of the organic quality of water.[1] It is most commonly expressed in milligrams of oxygenconsumed per litre of sample during 5 days of incubation at 20 °C and is often used as a robust surrogate of thedegree of organic pollution of water.BOD can be used as a gauge of the effectiveness of wastewater treatment plants. It is listed as a conventionalpollutant in the U.S. Clean Water Act.

The BOD5 testThere are two commonly recognized methods for the measurement of BOD.

Dilution methodTo ensure that all other conditions are equal, a very small amount of micro-organism seed is added to each samplebeing tested. This seed is typically generated by diluting activated sludge with de-ionized water. The BOD test iscarried out by diluting the sample with oxygen saturated de-ionized water, inoculating it with a fixed aliquot of seed,measuring the dissolved oxygen (DO) and then sealing the sample to prevent further oxygen dissolving in. Thesample is kept at 20 °C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, andthe dissolved oxygen is measured again. The difference between the final DO and initial DO is the BOD.The loss of dissolved oxygen in the sample, once corrections have been made for the degree of dilution, is called theBOD5. For measurement of carbonaceous BOD (cBOD), a nitrification inhibitor is added after the dilution waterhas been added to the sample. The inhibitor hinders the oxidation of nitrogen.BOD can be calculated by:• Undiluted: Initial DO - Final DO = BOD• Diluted: ((Initial DO - Final DO)- BOD of Seed) x Dilution FactorBOD is similar in function to chemical oxygen demand (COD), in that both measure the amount of organiccompounds in water. However, COD is less specific, since it measures everything that can be chemically oxidized,rather than just levels of biologically active organic matter.

Manometric methodThis method is limited to the measurement of the oxygen consumption due only to carbonaceous oxidation.Ammonia oxidation is inhibited.The sample is kept in a sealed container fitted with a pressure sensor. A substance that absorbs carbon dioxide(typically lithium hydroxide) is added in the container above the sample level. The sample is stored in conditionsidentical to the dilution method. Oxygen is consumed and, as ammonia oxidation is inhibited, carbon dioxide isreleased. The total amount of gas, and thus the pressure, decreases because carbon dioxide is absorbed. From thedrop of pressure, the sensor electronics computes and displays the consumed quantity of oxygen.The main advantages of this method compared to the dilution method are:• simplicity: no dilution of sample required, no seeding, no blank sample.• direct reading of BOD value.• continuous display of BOD value at the current incubation time.

Biochemical oxygen demand 21

Test LimitationsThe test method involves variables limiting reproducibility. Tests normally show observations varying plus or minusten to twenty percent around the mean.[2] :82

ToxicitySome wastes contain chemicals capable of suppressing microbiological growth or activity. Potential sources includeindustrial wastes, antibiotics in pharmaceutical or medical wastes, sanitizers in food processing or commercialcleaning facilities, chlorination disinfection used following conventional sewage treatment, and odor-controlformulations used in sanitary waste holding tanks in passenger vehicles or portable toilets. Suppression of themicrobial community oxidizing the waste will lower the test result.[2] :85

Appropriate Microbial PopulationThe test relies upon a microbial ecosystem with enzymes capable of oxidizing the available organic material. Somewaste waters, such as those from biological secondary sewage treatment, will already contain a large population ofmicroorganisms acclimated to the water being tested. An appreciable portion of the waste may be utilized during theholding period prior to commencement of the test procedure. On the other hand, organic wastes from industrialsources may require specialized enzymes. Microbial populations from standard seed sources may take some time toproduce those enzymes. A specialized seed culture may be appropriate to reflect conditions of an evolved ecosystemin the receiving waters.[2] :85-87

History of the use of BODThe Royal Commission on River Pollution, which was established in 1865 and the formation of the RoyalCommission on Sewage Disposal in 1898 led to the selection in 1908 of BOD5 as the definitive test for organicpollution of rivers. Five days was chosen as an appropriate test period because this is supposedly the longest timethat river water takes to travel from source to estuary in the U.K. In 1912, the commission also set a standard of 20ppm BOD5 as the maximum concentration permitted in sewage works discharging to rivers, provided that there wasat least an 8:1 dilution available at dry weather flow. This was contained in the famous 20:30 (BOD:SuspendedSolids) + full nitrification standard which was used as a yardstick in the U.K. up to the 1970s for sewage workseffluent quality.The United States includes BOD effluent limitations in its secondary treatment regulations. Secondary sewagetreatment is generally expected to remove 85 percent of the BOD measured in sewage and produce effluent BODconcentrations with a 30-day average of less than 30 mg/L and a 7-day average of less than 45 mg/L. The regulationsalso describe "treatment equivalent to secondary treatment" as removing 65 percent of the BOD and producingeffluent BOD concentrations with a 30-day average less than 45 mg/L and a 7-day average less than 65 mg/L.[3]

Typical BOD valuesMost pristine rivers will have a 5-day carbonaceous BOD below 1 mg/L. Moderately polluted rivers may have aBOD value in the range of 2 to 8 mg/L. Municipal sewage that is efficiently treated by a three-stage process wouldhave a value of about 20 mg/L or less. Untreated sewage varies, but averages around 600 mg/L in Europe and as lowas 200 mg/L in the U.S., or where there is severe groundwater or surface water Infiltration/Inflow. (The generallylower values in the U.S. derive from the much greater water use per capita than in other parts of the world.)[1]

Biochemical oxygen demand 22

BOD BiosensorAn alternative to measure BOD is the development of biosensors, which are devices for the detection of an analytethat combines a biological component with a physicochemical detector component. Biosensors can be used toindirectly measure BOD via a fast (usually <30 min) to be determined BOD substitute and a correspondingcalibration curve method (pioneered by Karube et al., 1977). Consequently, biosensors are now commerciallyavailable, but they do have several limitations such as their high maintenance costs, limited run lengths due to theneed for reactivation, and the inability to respond to changing quality characteristics as would normally occur inwastewater treatment streams; e.g. diffusion processes of the biodegradable organic matter into the membrane anddifferent responses by different microbial species which lead to problems with the reproducibility of results (Praet etal., 1995). Another important limitation is the uncertainty associated with the calibration function for translating theBOD substitute into the real BOD (Rustum et al, 2008).

BOD Software sensorRustum et al. (2008) proposed the use the KSOM to develop intelligent models for making rapid inferences aboutBOD using other easy to measure water quality parameters, which, unlike BOD, can be obtained directly andreliably using on-line hardware sensors. This will make the use of BOD for on-line process monitoring and control amore plausible proposition. In comparison to other data-driven modeling paradigms such as multi-layer perceptronsartificial neural networks (MLP ANN) and classical multi-variate regression analysis, the KSOM is not negativelyaffected by missing data. Moreover, time sequencing of data is not a problem when compared to classical time seriesanalysis.

References• Lenore S. Clescerl, Arnold E. Greenberg, Andrew D. Eaton (1999). Standard Methods for Examination of Water

& Wastewater (20th ed.). Washington, DC: American Public Health Association. ISBN 0-87553-235-7. Alsoavailable by online subscription at www.standardmethods.org [4]

• Rustum R., A. J. Adeloye, and M. Scholz (2008) Applying Kohonen Self-organizing Map as a Software Sensor toPredict the Biochemical Oxygen Demand, Water Environment Research, 80 (1), 32 – 40.

Notes[1] Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin (2003). Chemistry for Environmental Engineering and Science (5th ed.). New York:

McGraw-Hill. ISBN 0-07-248066-1.[2] Hammer, Mark J. (1975). Water and Waste-Water Technology. John Wiley & Sons. ISBN 0-471-34726-4.[3] U.S. Environmental Protection Agency (EPA). Washington, DC. "Secondary Treatment Regulation." (http:/ / www. access. gpo. gov/ nara/

cfr/ waisidx_07/ 40cfr133_07. html) Code of Federal Regulations, 40 CFR Part 133.[4] http:/ / www. standardmethods. org

External links• BOD Doctor (http:/ / www. boddoctor. com/ wiki/ index. php?title=Main_Page) - a troubleshooting wiki for this

problematic test

Effluent 23

Effluent

Wastewater discharge

Effluent is an outflowing of water or gas from a natural body of water,or from a human-made structure.

Effluent is defined by the United States Environmental ProtectionAgency as “wastewater - treated or untreated - that flows out of atreatment plant, sewer, or industrial outfall. Generally refers to wastesdischarged into surface waters”.[1] The Compact Oxford EnglishDictionary defines effluent as “liquid waste or sewage discharged intoa river or the sea”.[2]

Effluent in the artificial sense is generally considered to be waterpollution, such as the outflow from a sewage treatment facility or the wastewater discharge from industrial facilities.An effluent sump pump, for instance, pumps waste from toilets installed below a main sewage line.

In the context of waste water treatment plants, effluent that has been treated is sometimes called secondary effluent,or treated effluent. This cleaner effluent is then used to feed the bacteria in biofilters.In the context of a thermal power station, the output of the cooling system may be referred to as the effluent coolingwater, which is noticeably warmer than the environment. Effluent emissions may also refer to the air emissions ofthe plant, which come from the flue gas stack in the case of combustion plants and contain carbon dioxide, watervapor, and contaminants. In the case of nuclear power stations, some effluent emissions containing trace radioactiveisotopes are gaseous.In sugar beet processing, effluent is often settled in water tanks which allow the mud-contaminated water to settle.The mud sinks to the bottom, leaving the top section of water clear, free to be pumped back into the river or bereused in the process again.The Mississippi River's effluent of fresh water is so massive (7,000 to 20,000 m³/sec, or 200,000 to 700,000 ft³/sec)that a plume of fresh water is detectable by the naked eye from space, even as it rounds Florida and up to the coast ofGeorgia.

References[1] "Terms of Environment Beginning With "E"" (http:/ / www. epa. gov/ OCEPAterms/ eterms. html). United States Environmental Protection

Agency. 2006-10-03. . Retrieved 2010-06-09.[2] "AskOxford: effluent" (http:/ / www. askoxford. com/ concise_oed/ effluent?view=uk). Ask Oxford.com. Oxford University Press,. 2010. .

Retrieved 2010-06-09.

Biofilter 24

BiofilterBiofiltration is a pollution control technique using living material to capture and biologically degrade processpollutants. Common uses include processing waste water, capturing harmful chemicals or silt from surface runoff,and microbiotic oxidation of contaminants in air.

Biofilter installation at a commercialcomposting facility.

Examples of biofiltration include;• Bioswales, Biostrips, Biobags, Bioscrubbers, and Trickling filters• Constructed wetlands and Natural wetlands• Slow sand filters• Treatment ponds• Green belts• Living walls• Riparian zones, Riparian forests, Bosques

Control of air pollutionWhen applied to air filtration and purification, biofilters use microorganisms to remove air pollution.[1] The air flowsthrough a packed bed and the pollutant transfers into a thin biofilm on the surface of the packing material.Microorganisms, including bacteria and fungi are immobilized in the biofilm and degrade the pollutant. Tricklingfilters and bioscrubbers rely on a biofilm and the bacterial action in their recirculating waters.The technology finds greatest application in treating malodorous compounds and water-soluble volatile organiccompounds (VOCs). Industries employing the technology include food and animal products, off-gas fromwastewater treatment facilities, pharmaceuticals, wood products manufacturing, paint and coatings application andmanufacturing and resin manufacturing and application, etc. Compounds treated are typically mixed VOCs andvarious sulfur compounds, including hydrogen sulfide. Very large airflows may be treated and although a large area(footprint) has typically been required -- a large biofilter (>200,000 acfm) may occupy as much or more land than afootball field -- this has been one of the principal drawbacks of the technology. Engineered biofilters, designed andbuilt since the early 1990s, have provided significant footprint reductions over the conventional flat-bed, organicmedia type.

Air cycle system at biosolids composting plant.Large duct in foreground is exhaust air into

biofilter shown in next photo

One of the main challenges to optimum biofilter operation ismaintaining proper moisture throughout the system. The air isnormally humidified before it enters the bed with a watering (spray)system, humidification chamber, bioscrubber, or biotrickling filter.Properly maintained, a natural, organic packing media like peat,vegetable mulch, bark or wood chips may last for several years butengineered, combined natural organic and synthetic componentpacking materials will generally last much longer, up to 10 years. Anumber of companies offer these types or proprietary packing materialsand multi-year guarantees, not usually provided with a conventionalcompost or wood chip bed biofilter.

Biofilter 25

Biosolids composting plant biofilter mound - notesprinkler visible front right to maintain proper

moisture level for optimum functioning

Although widely employed, the scientific community is still unsure ofthe physical phenomena underpinning biofilter operation, andinformation about the microorganisms involved continues to bedeveloped. A biofilter/bio-oxidation system is a fairly simple device toconstruct and operate and offers a cost-effective solution provided thepollutant is biodegradable within a moderate time frame (increasingresidence time = increased size and capital costs), at reasonableconcentrations (and lb/hr loading rates) and that the airstream is at anorganism-viable temperature. For large volumes of air, a biofilter maybe the only cost-effective solution. There is no secondary pollution(unlike the case of incineration where additional CO2 and NOx areproduced from burning fuels) and degradation products form additional biomass, carbon dioxide and water. Mediairrigation water, although many systems recycle part of it to reduce operating costs, has a moderately highbiochemical oxygen demand (BOD) and may require treatment before disposal. However, this "blowdown water",necessary for proper maintenance of any bio-oxidation system, is generally accepted by municipal POTWs withoutany pretreatment.

Biofilters are being utilized in Columbia Falls, Montana at Plum Creek Timber Company's fiberboard plant.[2] Thebiofilters decrease the pollution emitted by the manufacturing process and the exhaust emitted is 98% clean. Thenewest, and largest, biofilter addition to Plum Creek cost $9.5 million, yet even though this new technology isexpensive, in the long run it will cost less overtime than the alternative exhaust-cleaning incinerators fueled bynatural gas (which are not as environmentally friendly). The biofilters use trillions of microscopic bacteria thatcleanse the air being released from the plant.

Water treatment

A typical complete trickling filter system fortreating wastewaters.[3]

Trickling filters have been used to filter water for various end uses foralmost two centuries. Biological treatment has been used in Europe tofilter surface water for drinking purposes since the early 1900s and isnow receiving more interest worldwide. Biological treatment methodsare also common in wastewater treatment, aquaculture and greywaterrecycling as a way to minimize water replacement while increasingwater quality.

For drinking water, biological water treatment involves the use ofnaturally occurring micro-organisms in the surface water to improvewater quality. Under optimum conditions, including relatively lowturbidity and high oxygen content, the organisms break down materialin the water and thus improve water quality. Slow sand filters orcarbon filters are used to provide a place on which thesemicro-organisms grow. These biological treatment systems effectivelyreduce water-borne diseases, dissolved organic carbon, turbidity andcolour in surface water, improving overall water quality.

Biofilter 26

Image 1: A schematic cross-section of the contactface of the bed media in a trickling filter.

Use in aquaculture

The use of biofilters are commonly used on closed aquaculturesystems, such as recirculating aquaculture systems (RAS). Manydesigns are used, with different benefits and drawbacks, however thefunction is the same: reducing water exchanges by convertingammonia to nitrate. Ammonia (NH4

+ and NH3) originates from thebrachial excretion from the gills of aquatic animals and from thedecomposition of organic matter. As ammonia-N is highly toxic, this isconverted to a less toxic form of nitrite (by Nitrosomonas sp.) and thento an even less toxic form of nitrate (by Nitrobacter sp.). This"nitrification" process requires oxygen (aerobic conditions), withoutwhich the biofilter can crash. Furthermore, as this nitrification cycle produces H+, the pH can decrease whichnecessitates the use of buffers such as lime.

References[1] Joseph S. Devinny, Marc A. Deshusses and Todd S. Webster (1999). Biofiltration for Air Pollution Control. Lewis Publishers.

ISBN 1-56670-289-5.[2] Lynch, Keriann (2008-10-26). "'Bug farm' a breath of fresh air" (http:/ / www. spokesman. com/ stories/ 2008/ oct/ 26/

bug-farm-a-breath-of-fresh-air). Spokesman Review. .[3] Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st Edition ed.). John Wiley & Sons Ltd. LCCN

67019834.

External links• Bioswales and strips for storm runoff (http:/ / www. dot. ca. gov/ hq/ env/ stormwater/ ongoing/ pilot_studies/

bmps/ details/ bs_strips/ ) - California Dept. of Transportation (CalTrans)

Trickling filter 27

Trickling filter

Image 1: A schematic cross-section of the contact face of the bed ofmedia in a trickling filter

A trickling filter consists of a fixed bed of rocks, lava,coke, gravel, slag, polyurethane foam, sphagnum peatmoss, ceramic, or plastic media over which sewage orother wastewater flows downward and causes a layer ofmicrobial slime (biofilm) to grow, covering the bed ofmedia. Aerobic conditions are maintained by splashing,diffusion, and either by forced air flowing through thebed or natural convection of air if the filter medium isporous.

The terms trickle filter, trickling biofilter, biofilter,biological filter and biological trickling filter areoften used to refer to a trickling filter. These systemshave also been described as roughing filters,intermittent filters, packed media bed filters, alternativeseptic systems, percolating filters, attached growthprocesses, and fixed film processes.

OperationThe removal of pollutants from the wastewater stream involves both absorption and adsorption of organiccompounds by the layer of microbial biofilm. The filter media is typically chosen to provide a very high surface areato volume. Typical materials are often porous and have considerable internal surface area in addition to the externalsurface of the medium. Passage of the wastewater over the media furnishes dissolved air, the oxygen which the slimelayer requires for the biochemical oxidation of the organic compounds and releases carbon dioxide gas, water andother oxidized end products. As the biofilm layer thickens, it eventually sloughs off into the treated effluent andsubsequently forms part of the secondary sludge. Typically, a trickling filter is followed by a clarifier orsedimentation tank for the separation and removal of the sloughing. Other filters utilizing higher-density media suchas sand, foam and peat moss do not produce a sludge that must be removed, but require forced air blowers andbackwashing or an enclosed anaerobic environment.The treatment of sewage or other wastewater with trickling filters is among the oldest and most well characterizedtreatment technologies.

TypesThe three basic types of trickle filters are used for:• the treatment of small individual residential or rural sewage• large centralized systems for treatment of municipal sewage• systems applied to the treatment of industrial wastewater.

Septic system leach fieldThis is the simplest form of waste liquid disposal system, typically using pipes buried in loose sand or gravel todissipate the liquid outflow from a septic tank. Liquid purification is performed by a biofilm which naturally formsas a coating on the sand and gravel in the absorption field and feeds on the dissolved nutrients in the waste stream.

Trickling filter 28

Due to the system being completely buried and generally isolated from the surface environment, the process of wastebreakdown is slow and requires a relatively large surface area to absorb and process liquid wastes. If too much liquidwastes enter the field too quickly, the wastes may pass out of the biofilm before waste consumption can occur,leading to pollution of groundwater.In order to prolong the life of a leaching field, one method of construction is to build two fields of pipingside-by-side, and use a rotating flow valve to direct waste into one field at a time, switching between fields everyyear or two. This allows a period of rest to let the microorganisms have time to break down the wastes built up in thegravel bed.In areas where the ground is insufficiently absorptive (fails the percolation test) a homeowner may be required toconstruct a mound system which is a special engineered waste disposal bed of sand and gravel mounded on thesurface of the ground with poor liquids absorption.

Leach field dosing

Generally it is better if the biofilm is permitted a period of time to rest between liquid influxes and for the liquids tobe evenly distributed through the leaching bed to promote biofilm growth throughout the pipe network. Typicallyflows from septic systems are either small surges (handwashing) or very large surges (clothes washer emptying),resulting in highly erratic liquid outflow into the field and uneven biofilm growth concentrating primarily around thefield inlet and dropping off in the outer reaches of the piping system.For this reason it is common for engineered mound systems to include an electrically powered dosing system whichconsists of a large capacity underground storage tank and lift pump after the septic tank. When the tank fills to apredetermined level, it is emptied into the leaching field.The storage tank collects small outflows such as from handwashing and saves them for dosing when the tank fillsfrom other sources. During this fill period the field is able to rest continuously. When full, the discharge dose fillsout the entire field completely to the same degree of flow, every time, promoting an even biofilm growth throughoutthe system.Dosing systems have maintenance requirements over traditional non-powered surge systems. The pump and floatsystem can break down and require replacement, and the dosing system also needs electricity. However, the systemcan be designed so that in the event of power failure the storage tank overflows to the field operating in thetraditional surge-flow manner until power is restored or repairs can be done.

Soil Compaction issues

The biofilm is most productive if the absorption field is loosely packed, to permit easy air infiltration down into thebiofilm bed. Consequently the land over the leaching field is often a restricted area where large vehicles cannot beallowed to drive, because the heavy weight will compact the bed, and potentially cause system failure due tohindering of biofilm growth.One method to help prevent compaction of the field is to place a U-shaped cover over gravel trenches in the bed,with a dosing pipe suspended above the bed by the cover. Any weight from above is passed to the sides of the trenchkeeping the bed directly under the cover free from compaction.

Trickling filter 29

Sewage treatment trickle filtersOnsite sewage facilities (OSSF) are recognized as viable, low-cost, long-term, decentralized approaches to sewagetreatment if they are planned, designed, installed, operated and maintained properly (USEPA, 1997).Sewage trickling filters are used in areas not serviced by municipal wastewater treatment plants (WWTP). They aretypically installed in areas where the traditional septic tank system are failing, cannot be installed due to sitelimitations, or where improved levels of treatment are required for environmental benefits such as preventingcontamination of ground water or surface water.Sites with a high water table, high bedrock, heavy clay, small land area, or which require minimal site destruction(for example, tree removal) are ideally suited for trickling filters.All varieties of sewage trickling filters have a low and sometimes intermittent power consumption. They can besomewhat more expensive than traditional septic tank-leach field systems, however their use allows for bettertreatment, a reduction in size of disposal area, less excavation, and higher density land development.

Configurations and components

All sewage trickling filter systems share the same fundamental components:• a septic tank for fermentation and primary settling of solids• a filter medium upon which beneficial microbes (biomass, biofilm) are promoted and developed• a container which houses the filter medium• a distribution system for applying wastewater to be treated to the filter medium• a distribution system for disposal of the treated effluent or percolation ponds.By treating septic tank effluent before it is distributed into the ground, higher treatment levels are obtained andsmaller disposal means such as leach field, shallow pressure trench or area beds are required.Systems can be configured for single-pass use where the treated water is applied to the trickling filter once beforebeing disposed of, or for multi-pass use where a portion of the treated water is cycled back to the septic tank andre-treated via a closed-loop. Multi-pass systems result in higher treatment quality and assist in removing TotalNitrogen (TN) levels by promoting nitrification in the aerobic media bed and denitrification in the anaerobic septictank.Trickling filters differ primarily in the type of filter media used to house the microbial colonies. Types of media mostcommonly used include plastic matrix material, open-cell polyurethane foam, sphagnum peat moss, recycled tires,clinker, gravel, sand and geotextiles. Ideal filter medium optimizes surface area for microbial attachment, wastewaterretention time, allows air flow, resists plugging and does not degrade. Some residential systems require forcedaeration units which will increase maintenance and operational costs.

Regulatory approvals

Third-party verification of trickling filters has proven them to be a reliable alternative to septic systems withincreased levels of treatment performance and nitrogen removal. Typical effluent quality parameters are BiochemicalOxygen Demand (BOD), Total suspended solids (TSS), Total Kjeldahl Nitrogen (TKN), and fecal coliforms.The leading testing facility in the United States is the Massachusetts Alternative Septic System Test Center, aprogram of the Buzzards Bay National Estuary Program. Testing conducted here includes the stringentEnvironmental Technology Initiative (ETI) where systems are tested in triplicate over two years, and theEnvironmental Technology Verification (ETV) program which is funded by the U.S. Environmental ProtectionAgency (EPA) and includes stress testing as well as evaluation of nitrogen removal over 14 months. Systems areapproved for installation by local, state and federal regulations and controls.

Trickling filter 30

A typical complete trickling filter system

Industrial wastewater treatment tricklefilters

Wastewaters from a variety of industrial processeshave been treated in trickling filters. Such industrialwastewater trickling filters consist of two types:• Large tanks or concrete enclosures filled with

plastic packing or other media.[1]

• Vertical towers filled with plastic packing orother media.[2] [3]

The availability of inexpensive plastic towerpackings has led to their use as trickling filter bedsin tall towers, some as high as 20 meters.[4] As earlyas the 1960s, such towers were in use at: the GreatNorthern Oil's Pine Bend Refinery in Minnesota; theCities Service Oil Company Trafalgar Refinery inOakville, Ontario and at a kraft paper mill.[5]

The treated water effluent from industrialwastewater trickling filters is very oftensubsequently processed in a clarifier-settler toremove the sludge that sloughs off the microbial slime layer attached to the trickling filter media (see Image 1above).Currently, some of the latest trickle filter technology involves aerated biofilters which are essentially trickle filtersconsisting of plastic media in vessels using blowers to inject air at the bottom of the vessels, with either downflow orupflow of the wastewater.[6]

References[1] King Fahd University of Petroleum and Minerals, Course ChE 101.11 (http:/ / faculty. kfupm. edu. sa/ CE/ msaleh/ ChE 107. 11March06.

ppt) Saudi Aramco Engineering Development Program, pages 62-65 including Figure 11[2] Biological filter and process (http:/ / www. google. com/ patents?hl=en& lr=& vid=USPAT4351729& id=U3c8AAAAEBAJ& oi=fnd&

dq=trickle+ filter+ "packed+ tower"+ wastewater) U.S. patent 4,351,729, September 28, 1982, Assigned to Celanese Corporation[3] Lecture (http:/ / www. ag. auburn. edu/ ~davisda/ classes/ facilities/ presentations/ Recirc_overview. pdf) by Dr. Allen Davis, Auburn

University, page 6 of 8 pdf pages including schematic of packed tower trickling filter)[4] Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st Edition ed.). John Wiley & Sons Ltd. LCCN

67019834.[5] E.H. Bryan and D.H. Moeller, Aerobic Biological Oxidation Using Dowpac, Paper 42, Conference on Biological Waste Treatment,

Manhattan College, April 20, 1960. (http:/ / cobweb. ecn. purdue. edu/ ~alleman)[6] Marcus Van Sperling (2007). Activated Sludge and Aerobic Biofilm Reactors. IWA Publications. ISBN 1-84339-165-1.

External links• Overview of Biological Wastewater Treatment (http:/ / web. deu. edu. tr/ atiksu/ ana52/ atgrow. ppt)• The History of Fixed-Film Wastewater Treatment Systems (http:/ / web. deu. edu. tr/ atiksu/ ana52/ ani4041.

html)

Chemical oxygen demand 31

Chemical oxygen demandIn environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure theamount of organic compounds in water. Most applications of COD determine the amount of organic pollutants foundin surface water (e.g. lakes and rivers), making COD a useful measure of water quality. It is expressed in milligramsper liter (mg/L), which indicates the mass of oxygen consumed per liter of solution. Older references may expressthe units as parts per million (ppm).

OverviewThe basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strongoxidizing agent under acidic conditions. The amount of oxygen required to oxidize an organic compound to carbondioxide, ammonia, and water is given by:

This expression does not include the oxygen demand caused by the oxidation of ammonia into nitrate. The process ofammonia being converted into nitrate is referred to as nitrification. The following is the correct equation for theoxidation of ammonia into nitrate.

It is applied after the oxidation due to nitrification if the oxygen demand from nitrification must be known.Dichromate does not oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard chemicaloxygen demand test.The International Organization for Standardization describes a standard method for measuring chemical oxygendemand in ISO 6060 [1].

HistoryFor many years, the strong oxidizing agent potassium permanganate (KMnO4) was used for measuring chemicaloxygen demand. Measurements were called oxygen consumed from permanganate, rather than the oxygen demand oforganic substances. Potassium permanganate's effectiveness at oxidizing organic compounds varied widely, and inmany cases biochemical oxygen demand (BOD) measurements were often much greater than results from CODmeasurements. This indicated that potassium permanganate was not able to effectively oxidize all organiccompounds in water, rendering it a relatively poor oxidizing agent for determining COD.Since then, other oxidizing agents such as ceric sulphate, potassium iodate, and potassium dichromate have beenused to determine COD. Of these, potassium dichromate (K2Cr2O7) has been shown to be the most effective: it isrelatively cheap, easy to purify, and is able to nearly completely oxidize almost all organic compounds.In these methods, a fixed volume with a known excess amount of the oxidant is added to a sample of the solutionbeing analyzed. After a refluxing digestion step, the initial concentration of organic substances in the sample iscalculated from a titrimetric or spectrophotometric determination of the oxidant still remaining in the sample.

Chemical oxygen demand 32

Using potassium dichromatePotassium dichromate is a strong oxidizing agent under acidic conditions. (Acidity is usually achieved by theaddition of sulfuric acid.) The reaction of potassium dichromate with organic compounds is given by:

where d = 2n/3 + a/6 - b/3 - c/2. Most commonly, a 0.25 N solution of potassium dichromate is used for CODdetermination, although for samples with COD below 50 mg/L, a lower concentration of potassium dichromate ispreferred.In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (sincein all redox reactions, one reagent is oxidized and the other is reduced), forming Cr3+. The amount of Cr3+ isdetermined after oxidization is complete, and is used as an indirect measure of the organic contents of the watersample.

BlanksBecause COD measures the oxygen demand of organic compounds in a sample of water, it is important that nooutside organic material be accidentally added to the sample to be measured. To control for this, a so-called blanksample is required in the determination of COD (and BOD -biochemical oxygen demand - for that matter). A blanksample is created by adding all reagents (e.g. acid and oxidizing agent) to a volume of distilled water. COD ismeasured for both the water and blank samples, and the two are compared. The oxygen demand in the blank sampleis subtracted from the COD for the original sample to ensure a true measurement of organic matter.

Measurement of excessFor all organic matter to be completely oxidized, an excess amount of potassium dichromate (or any oxidizing agent)must be present. Once oxidation is complete, the amount of excess potassium dichromate must be measured toensure that the amount of Cr3+ can be determined with accuracy. To do so, the excess potassium dichromate istitrated with ferrous ammonium sulfate (FAS) until all of the excess oxidizing agent has been reduced to Cr3+.Typically, the oxidation-reduction indicator Ferroin is added during this titration step as well. Once all the excessdichromate has been reduced, the Ferroin indicator changes from blue-green to reddish-brown. The amount offerrous ammonium sulfate added is equivalent to the amount of excess potassium dichromate added to the originalsample. and also we can determine COD by boiling the water sample and we can determine CO2 ratio by theinfra-red analyzer

Preparation Ferroin Indicator reagentA solution of 1.485 g 1,10-phenanthroline monohydrate is added to a solution of 695 mg FeSO4·7H2O in water, andthe resulting red solution is diluted to 100 mL.

CalculationsThe following formula is used to calculate COD:

where b is the volume of FAS used in the blank sample, s is the volume of FAS in the original sample, and n is thenormality of FAS. If milliliters are used consistently for volume measurements, the result of the COD calculation isgiven in mg/L.The COD can also be estimated from the concentration of oxidizable compound in the sample, based on its stoichiometric reaction with oxygen to yield CO2 (assume all C goes to CO2), H2O (assume all H goes to H2O), and

Chemical oxygen demand 33

NH3 (assume all N goes to NH3), using the following formula:COD = (C/FW)(RMO)(32)

Where C = Concentration of oxidizable compound in the sample,FW = Formula weight of the oxidizable compound in the sample,RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO2,water, and ammonia

For example, if a sample has 500 wppm of phenol:C6H5OH + 7O2 → 6CO2 + 3H2OCOD = (500/94)(7)(32) = 1191 wppm

Inorganic interferenceSome samples of water contain high levels of oxidizable inorganic materials which may interfere with thedetermination of COD. Because of its high concentration in most wastewater, chloride is often the most serioussource of interference. Its reaction with potassium dichromate follows the equation:

Prior to the addition of other reagents, mercuric sulfate can be added to the sample to eliminate chloride interference.The following table lists a number of other inorganic substances that may cause interference. The table also listschemicals that may be used to eliminate such interference, and the compounds formed when the inorganic moleculeis eliminated.

Inorganic molecule Eliminated by Elimination forms

Chloride Mercuric sulfate Mercuric chloride complex

Nitrite Sulfamic acid N2 gas

Ferrous iron - -

Sulfides - -

Government regulationMany governments impose strict regulations regarding the maximum chemical oxygen demand allowed inwastewater before they can be returned to the environment. For example, in Switzerland, a maximum oxygendemand between 200 and 1000 mg/L must be reached before wastewater or industrial water can be returned to theenvironment [2].

References• Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin (2003). Chemistry for Environmental Engineering and

Science (5th ed.). New York: McGraw-Hill. ISBN 0-07-248066-1.• Lenore S. Clescerl, Arnold E. Greenberg, Andrew D. Eaton. Standard Methods for Examination of Water &

Wastewater (20th ed.). Washington, DC: American Public Health Association. ISBN 0-87553-235-7.

Chemical oxygen demand 34

External links• ISO 6060: Water quality - Determination of the chemical oxygen demand [1]

• Water chemical oxygen demand [3] (Food and Agriculture Organization of the United Nations)

References[1] http:/ / www. iso. org/ iso/ en/ CatalogueDetailPage. CatalogueDetail?CSNUMBER=12260& ICS1=13& ICS2=60& ICS3=50[2] http:/ / www. csem. ch/ corporate/ Report2002/ pdf/ p56. pdf[3] http:/ / www. fao. org/ gtos/ tems/ variable_show. jsp?VARIABLE_ID=123

ChlorinationChlorination is the process of adding the element chlorine to water as a method of water purification to make it fitfor human consumption as drinking water. Water which has been treated with chlorine is effective in preventing thespread of waterborne disease.The chlorination of public drinking supplies was originally met with resistance, as people were concerned about thehealth effects of the practice. The use of chlorine has greatly reduced the prevalence of waterborne disease as it iseffective against almost all bacteria and viruses, as well as amoeba.Chlorination is also used to sanitize the water in swimming pools and as a disinfection stage in sewage treatment.Shock chlorination is a process used in many swimming pools, water wells, springs, and other water sources toreduce the bacterial and algal residue in the water. Shock chlorination is performed by mixing a large amount ofsodium hypochlorite, which can be in the form of a powder or a liquid such as chlorine bleach, into the water. Waterthat is being shock chlorinated should not be swum in or drunk until the sodium hypochlorite count in the water goesdown to three ppm or less.

HistoryThe first scientists to suggest disinfecting water with chlorine were Louis-Bernard Guyton de Morveau (in France)and William Cumberland Cruikshank (in England), both around the year 1800.[1]

The technique of purification of drinking water by use of compressed liquefied chlorine gas was developed in 1910by U.S. Army Major (later Brig. Gen.) Carl Rogers Darnall (1867–1941), Professor of Chemistry at the ArmyMedical School.[2] Shortly thereafter, Major (later Col.) William J. L. Lyster (1869–1947) of the Army MedicalDepartment used a solution of calcium hypochlorite in a linen bag to treat water.For many decades, Lyster's method remained the standard for U.S. ground forces in the field and in camps,implemented in the form of the familiar Lyster Bag (also spelled Lister Bag). Darnall's work became the basis forpresent day systems of chlorination of municipal water supplies, which were perfected in the 1930s and widelyestablished in the United States by World War II.[3]

Chlorination 35

Chemistry in waterWhen chlorine is added to water, it reacts to form a pH dependent equilibrium mixture of chlorine, hypochlorousacid and hydrochloric acid[4] :

Cl2 + H2O → HOCl + HClDepending on the pH, hypochlorous acid partly dissociates to hydrogen and hypochlorite ions:

HClO → H+ + ClO-

In acidic solution, the major species are Cl2 and HOCl while in alkaline solution effectively only ClO- is present.Very small concentrations of ClO2

-, ClO3-, ClO4

- are also found.[5]

DrawbacksDisinfection by chlorination can be problematic, in some circumstances. Chlorine can react with naturally occurringorganic compounds found in the water supply to produce compounds known as disinfection byproducts (DBPs). Themost common DBPs are trihalomethanes (THMs) and haloacetic acids (HAAs). Due to the potential carcinogenicityof these compounds, drinking water regulations across the developed world require regular monitoring of theconcentration of these compounds in the distribution systems of municipal water systems. The World HealthOrganization has stated that the "Risks to health from DBPs are extremely small in comparison with inadequatedisinfection."There are also other concerns regarding chlorine, including its volatile nature which causes it to disappear tooquickly from the water system, and aesthetic concerns such as taste and odour.

AlternativesChlorine in water is more than three times more effective as a disinfectant against Escherichia coli than anequivalent concentration of bromine, and is more than six times more effective than an equivalent concentration ofiodine.[6]

Several alternatives to traditional chlorination exist, and have been put into practice to varying extents. Ozonation isused by many European countries and also in a few municipalities in the United States. Due to current regulations,systems employing ozonation in the United States still must maintain chlorine residuals comparable to systemswithout ozonation.Disinfection with chloramine is also becoming increasingly common. Unlike chlorine, chloramine has a longer halflife in the distribution system and still maintains effective protection against pathogens. The reason chloraminespersist in the distribution is due to the relatively lower redox potential in comparison to free chlorine. Chloramine isformed by the addition of ammonia into drinking water to form mono-, di-, and trichloramines. WhereasHelicobacter pylori can be many times more resistant to chlorine than Escherichia coli, both organisms are aboutequally susceptible to the disinfecting effect of chloramine.[7]

Water treated by filtration may not need further disinfection; a very high proportion of pathogens are removed bymicroorganisms in the filter bed. Filtered water must be used soon after it is filtered, as the low amount of remainingmicrobes may proliferate over time.The advantage of chlorine in comparison to ozone is that the residual persists in the water for an extended period oftime. This feature allows the chlorine to travel through the water supply system, effectively controlling pathogenicbackflow contamination. In a large system this may not be adequate, and so chlorine levels may be boosted at pointsin the distribution system, or chloramine may be used, which remains in the water for longer before reacting ordissipating.Another method which is gaining popularity is UV disinfection. UV treatment leaves no residue in the water due to use of light instead of chemical disinfectants. However, this method alone (as well as chlorination alone) will not

Chlorination 36

remove bacterially produced toxins, pesticides, heavy metals, etc. from water. Often, multiple steps are taken incommercially sold water.Yet another method is using silver for its disinfecting properties.

References[1] Rideal, Samuel (1895). Disinfection and Disinfectants (http:/ / books. google. com/ books?id=HDpDAAAAIAAJ& pg=PA57&

dq="Disinfection+ and+ Disinfectants"+ AND+ 1895+ AND+ Cruikshank& hl=en& ei=w7GcTNKPHcX6lweRzLn2CQ& sa=X&oi=book_result& ct=result& resnum=1& ved=0CCkQ6AEwAA#v=onepage& q& f=false), p. 57. J.B. Lippincott Co.

[2] Darnall C.R. (1911), "The Purification of Water by Anhydrous Chlorine", American Journal of Public Health; 1: 783–97.[3] Hodges, L. (1977). Environmental Pollution (2nd ed.). New York: Rinehart and Winston. p. 189.[4] Fair, G. M., J. Corris, S. L. Chang, I. Weil, and R. P. Burden. 1948. The behavior of chlorine as a water disinfectant. J. Am. Water Works

Assoc. 40:1051-1061.[5] .Shunji Nakagawara, Takeshi Goto, Masayuki Nara, Youichi Ozaqa, Kunimoto Hotta and Yoji Arata, "Spectroscopic Characterization and the

pH Dependence of Bactericidal Activity of the Aqueous Chlorine Solution", Analytical Sciences, 14, 69, 1998.[6] Koski TA, Stuart LS, Ortenzio LF (1 March 1966). "Comparison of chlorine, bromine, iodine as disinfectants for swimming pool water.".

Applied Microbiology 14 (2): 276–279. PMC 546668. PMID 4959984.[7] Baker KH, Hegarty JP, Redmond B, Reed NA, Herson DS (2002). "Effect of oxidizing disinfectants (chlorine, monochloramine, and ozone)

on Helicobacter pylori." (http:/ / aem. asm. org/ cgi/ reprint/ 68/ 2/ 981) (PDF). Applied and Environmental Microbiology 68 (2): 981–984.doi:10.1128/AEM.68.2.981-984.2002. PMC 126689. PMID 11823249. .

External links• City of Milwaukee, Wisconsin Water Works (http:/ / www. mpw. net/ Pages/ WaterWorks. html)• Emergency Disinfection of Drinking Water (http:/ / www. epa. gov/ safewater/ faq/ emerg. html) (US EPA)• National Pollutant Inventory - Chlorine (http:/ / www. npi. gov. au/ database/ substance-info/ profiles/ 20. html)• Chlorinated Drinking Water (http:/ / monographs. iarc. fr/ ENG/ Monographs/ vol52/ volume52. pdf) (IARC

Monograph)• NTP Study Report TR-392: Chlorinated & Chloraminated Water (http:/ / ntp. niehs. nih. gov/ ntp/ htdocs/

LT_rpts/ tr392. pdf) (US NIH)• American Chemistry Council's Chlorine Chemistry Division (http:/ / www. americanchemistry. com/ chlorine/ )• Disinfection Practices (http:/ / www. enceechlor. com/ download/ Drinking Water Chlorination - A Review of

Disinfection Practices. pdf)

Ozone 37

Ozone

Ozone

[[File:Ozone-1,3-dipole.pngSkeletal formula of ozone with partialcharges shown and assorted dimensions]] [[File:Ozone-CRC-MW-3D-vdW.png

Spacefill modelof ozone]]

[[Image:Ozone-CRC-MW-3D-balls.png Ball and stick model of ozone]]

[[Image:Ozone-elpot-3D-vdW.png Electrostatic potential map of ozone]]

Identifiers

CAS number 10028-15-6 [1] 

PubChem 24823 [2]

ChemSpider 23208 [3] 

UNII 66H7ZZK23N [4] 

EC number 233-069-2 [5]

MeSH Ozone [6]

ChEBI CHEBI:25812 [7]

RTECS number RS8225000

Gmelin Reference 1101

Jmol-3D images Image 1 [8]

Image 2 [9]

Properties

Molecular formula O3

Molar mass 48 g mol−1

Exact mass 47.984743866 g mol−1

Appearance Pale, blue gas

Density 0.002144 g cm-3 (at 0 °C)

Melting point -192 °C, 81 K, -314 °F

Boiling point -112 °C, 161 K, -170 °F

Solubility in water 1.05 g dm-3 (at 0 °C)

Refractive index (nD) 1.2226 (liquid)

Structure

Space group C2v

Coordinationgeometry

Digonal

Molecular shape Dihedral

Hybridisation sp2 for O1

Ozone 38

Dipole moment 0.53 D

Thermochemistry

Std enthalpy offormation ΔfH

o298

142.67 kJ mol−1

Standard molarentropy So

298

238.92 J K−1 mol−1

Hazards

EU classification

ONFPA 704

Related compounds

Related compounds Sulfur dioxideThiozone

(what is this?)   (verify) [10]

Except where noted otherwise, data are given for materials in their standardstate (at 25 °C, 100 kPa)

Infobox references

Ozone (O3, pronounced /ˈoʊzoʊn/), or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an

allotrope of oxygen that is much less stable than the diatomic allotrope (O2). Ozone in the lower atmosphere is an airpollutant with harmful effects on the respiratory systems of animals and will burn sensitive plants; however, theozone layer in the upper atmosphere is beneficial, preventing potentially damaging electromagnetic radiation fromreaching the Earth's surface.[11] [12] Ozone is present in low concentrations throughout the Earth's atmosphere. It hasmany industrial and consumer applications.

HistoryOzone, the first allotrope of a chemical element to be recognized, was proposed as a distinct chemical substance byChristian Friedrich Schönbein in 1840, who named it after the Greek verb ozein (ὄζειν, "to smell"), from the peculiarodor in lightning storms.[13] [14] The formula for ozone, O3, was not determined until 1865 by Jacques-LouisSoret[15] and confirmed by Schönbein in 1867.[13] [16]

Physical propertiesOzone is a pale blue gas, slightly soluble in water and much more soluble in inert non-polar solvents such as carbontetrachloride or fluorocarbons, where it forms a blue solution. At –112 °C, it condenses to form a dark blue liquid. Itis dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquidozone can detonate. At temperatures below –193 °C, it forms a violet-black solid.[17]

Most people can detect about 0.01 μmol/mol of ozone in air where it has a very specific sharp odor somewhatresembling chlorine bleach. Exposure of 0.1 to 1 μmol/mol produces headaches, burning eyes, and irritation to therespiratory passages.[18] Even low concentrations of ozone in air are very destructive to organic materials such aslatex, plastics, and animal lung tissue.Ozone is diamagnetic, which means that its electrons are all paired. In contrast, O2 is paramagnetic, containing twounpaired electrons.

Ozone 39

StructureAccording to experimental evidence from microwave spectroscopy, ozone is a bent molecule, with C2v symmetry(similar to the water molecule). The O – O distances are 127.2 pm. The O – O – O angle is 116.78°.[19] The centralatom is sp² hybridized with one lone pair. Ozone is a polar molecule with a dipole moment of 0.53 D.[20] Thebonding can be expressed as a resonance hybrid with a single bond on one side and double bond on the otherproducing an overall bond order of 1.5 for each side.

ReactionsOzone is a powerful oxidizing agent, far stronger than O2. It is also unstable at high concentrations, decaying toordinary diatomic oxygen (with a half-life of about half an hour in atmospheric conditions):[21]

2 O3 → 3 O2This reaction proceeds more rapidly with increasing temperature and increased pressure. Deflagration of ozone canbe triggered by a spark, and can occur in ozone concentrations of 10 wt% or higher.[22]

With metalsOzone will oxidize most metals (except gold, platinum, and iridium) to oxides of the metals in their highestoxidation state. For example:

2 Cu+ + 2 H3O+ + O3 → 2 Cu2+ + 3 H2O + O2

With nitrogen and carbon compoundsOzone also oxidizes nitric oxide to nitrogen dioxide:

NO + O3 → NO2 + O2This reaction is accompanied by chemiluminescence. The NO2 can be further oxidized:

NO2 + O3 → NO3 + O2The NO3 formed can react with NO2 to form N2O5:Solid nitryl perchlorate can be made from NO2, ClO2, and O3 gases:

2 NO2 + 2 ClO2 + 2 O3 → 2 NO2ClO4 + O2Ozone does not react with ammonium salts but it oxidizes with ammonia to ammonium nitrate:

2 NH3 + 4 O3 → NH4NO3 + 4 O2 + H2OOzone reacts with carbon to form carbon dioxide, even at room temperature:

C + 2 O3 → CO2 + 2 O2

Ozone 40

With sulfur compoundsOzone oxidizes sulfides to sulfates. For example, lead(II) sulfide is oxidised to lead(II) sulfate:

PbS + 4 O3 → PbSO4 + 4 O2Sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide:

S + H2O + O3 → H2SO43 SO2 + 3 H2O + O3 → 3 H2SO4

In the gas phase, ozone reacts with hydrogen sulfide to form sulfur dioxide:H2S + O3 → SO2 + H2O

In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, andone to produce sulfuric acid:

H2S + O3 → S + O2 + H2O3 H2S + 4 O3 → 3 H2SO4

Other substratesAll three atoms of ozone may also react, as in the reaction of tin(II) chloride with hydrochloric acid and ozone:

3 SnCl2 + 6 HCl + O3 → 3 SnCl4 + 3 H2OIodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:

I2 + 6 HClO4 + O3 → 2 I(ClO4)3 + 3 H2O

CombustionOzone can be used for combustion reactions and combusting gases; ozone provides higher temperatures thancombusting in dioxygen (O2). The following is a reaction for the combustion of carbon subnitride which can alsocause higher temperatures:

3 C4N2 + 4 O3 → 12 CO + 3 N2Ozone can react at cryogenic temperatures. At 77 K (−196 °C), atomic hydrogen reacts with liquid ozone to form ahydrogen superoxide radical, which dimerizes:[23]

H + O3 → HO2 + O2 HO2 → H2O4

Reduction to ozonidesReduction of ozone gives the ozonide anion, O3

– . Derivatives of this anion are explosive and must be stored atcryogenic temperatures. Ozonides for all the alkali metals are known. KO3, RbO3, and CsO3 can be prepared fromtheir respective superoxides:

KO2 + O3 → KO3 + O2Although KO3 can be formed as above, it can also be formed from potassium hydroxide and ozone:[24]

2 KOH + 5 O3 → 2 KO3 + 5 O2 + H2ONaO3 and LiO3 must be prepared by action of CsO3 in liquid NH3 on an ion exchange resin containing Na+ or Li+

ions:[25]

CsO3 + Na+ → Cs+ + NaO3A solution of calcium in ammonia reacts with ozone to give to ammonium ozonide and not calcium ozonide:[23]

3 Ca + 10 NH3 + 6 O3 → Ca·6NH3 + Ca(OH)2 + Ca(NO3)2 + 2 NH4O3 + 2 O2 + H2

Ozone 41

ApplicationsOzone can be used to remove manganese from water, forming a precipitate which can be filtered:

2 Mn2+ + 2 O3 + 4 H2O → 2 MnO(OH)2 (s) + 2 O2 + 4 H+

Ozone will also detoxify cyanides by converting it to cyanate, which is a thousand times less toxic.CN- + O3 → CNO− + O2

Ozone will also completely decompose urea:[26]

(NH2)2CO + O3 → N2 + CO2 + 2 H2OOzone will cleave alkenes to form carbonyl compounds in the ozonolysis process.

Ozone in Earth's atmosphere

The distribution of atmospheric ozone in partialpressure as a function of altitude

Concentration of ozone as measured by theNimbus-7 satellite

The standard way to express total ozone levels (the amount of ozone ina vertical column) in the atmosphere is by using Dobson units. Pointmeasurements are reported as mole fractions in nmol/mol (parts perbillion, ppb) or as concentrations in μg/m3.

Ozone layer

The highest levels of ozone in the atmosphere are in the stratosphere,in a region also known as the ozone layer between about 10 km and50 km above the surface (or between about 6 and 31 miles). Here itfilters out photons with shorter wavelengths (less than 320 nm) ofultraviolet light, also called UV rays, (270 to 400 nm) from the Sunthat would be harmful to most forms of life in large doses. These samewavelengths are also among those responsible for the production ofvitamin D in humans. Ozone in the stratosphere is mostly producedfrom ultraviolet rays reacting with oxygen:

O2 + photon (radiation < 240 nm) → 2 OO + O2 + M → O3 + M

It is destroyed by the reaction with atomic oxygen:O3 + O → 2 O2

The latter reaction is catalysed by the presence of certain free radicals,of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). Inrecent decades the amount of ozone in the stratosphere has been declining mostly because of emissions of CFCs andsimilar chlorinated and brominated organic

Ozone 42

Total ozone concentration in June 2000 asmeasured by EP-TOMS satellite instrument

molecules, which have increased the concentration of ozone-depletingcatalysts above the natural background. Ozone only makes up0.00006% of the atmosphere.

Low level ozone

Low level ozone (or tropospheric ozone) is an atmosphericpollutant.[27] It is not emitted directly by car engines or by industrialoperations, but formed by the reaction of sunlight on air containinghydrocarbons and nitrogen oxides that react to form ozone directly atthe source of the pollution or many kilometers down wind.

Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but theproducts are themselves key components of smog. Ozone photolysis by UV light leads to production of the hydroxylradical OH and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creationof components of smog such as peroxyacyl nitrates which can be powerful eye irritants. The atmospheric lifetime oftropospheric ozone is about 22 days; its main removal mechanisms are being deposited to the ground, the abovementioned reaction giving OH, and by reactions with OH and the peroxy radical HO2· (Stevenson et al., 2006).[28]

There is evidence of significant reduction in agricultural yields because of increased ground-level ozone andpollution which interferes with photosynthesis and stunts overall growth of some plant species.[29] [30] The UnitedStates Environmental Protection Agency is proposing a secondary regulation to reduce crop damage, in addition tothe primary regulation designed for the protection of human health.Certain examples of cities with elevated ozone readings are Houston, Texas, and Mexico City, Mexico. Houston hasa reading of around 41 nmol/mol, while Mexico City is far more hazardous, with a reading of about 125nmol/mol.[30]

Ozone cracking

Ozone cracking in natural rubber tubing

Ozone gas attacks any polymer possessing olefinic or double bondswithin its chain structure, such as natural rubber, nitrile rubber, andstyrene-butadiene rubber. Products made using these polymers areespecially susceptible to attack, which causes cracks to grow longerand deeper with time, the rate of crack growth depending on the loadcarried by the product and the concentration of ozone in theatmosphere. Such materials can be protected by adding antiozonants,such as waxes, which bond to the surface to create a protective film orblend with the material and provide long term protection. Ozone cracking used to be a serious problem in car tiresfor example, but the problem is now seen only in very old tires. On the other hand, many critical products likegaskets and O-rings may be attacked by ozone produced within compressed air systems. Fuel lines are often madefrom reinforced rubber tubing and may also be susceptible to attack, especially within engine compartments wherelow levels of ozone are produced from electrical equipment. Storing rubber products in close proximity to DCelectric motors can accelerate the rate at which ozone cracking occurs. The commutator of the motor creates sparkswhich in turn produce ozone.

Ozone 43

Ozone as a greenhouse gas

Although ozone was present at ground level before the Industrial Revolution, peak concentrations are now far higherthan the pre-industrial levels, and even background concentrations well away from sources of pollution aresubstantially higher.[31] [32] This increase in ozone is of further concern because ozone present in the uppertroposphere acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth. Quantifying thegreenhouse gas potency of ozone is difficult because it is not present in uniform concentrations across the globe.However, the most widely accepted scientific assessments relating to climate change (e.g. the IntergovernmentalPanel on Climate Change Third Assessment Report[33] ) suggest that the radiative forcing of tropospheric ozone isabout 25% that of carbon dioxide.The annual global warming potential of tropospheric ozone is between 918-1022 tons carbon dioxide equivalent/tonstropospheric ozone. This means on a per-molecule basis, ozone in the troposphere has a radiative forcing effectroughly 1,000 times as strong as carbon dioxide. However, tropospheric ozone is a short-lived greenhouse gas,which decays in the atmosphere much more quickly than carbon dioxide. This means that over a 20 year horizon, theglobal warming potential of tropospheric ozone is much less, roughly 62 to 69 tons carbon dioxide equivalent / tonstropospheric ozone.[34]

Because of its short-lived nature, tropospheric ozone does not have strong global effects, but has very strongradiative forcing effects on regional scales. In fact, there are regions of the world where tropospheric ozone has aradiative forcing up to 150% of carbon dioxide.[35]

Health effects

Air pollution

Red Alder leaf, showing the typicaldiscolouration caused by ozone pollution[36]

Signboard in Gulfton, Houston indicating anozone watch

Ground-level ozone is created near the Earth's surface by the action ofdaylight UV rays on a group of pollutants called ozone precursors.There is a great deal of evidence to show that ground level ozone canharm lung function and irritate the respiratory system.[] [37] Exposureto ozone and the pollutants that produce it is linked to premature death,asthma, bronchitis, heart attack, and other cardiopulmonary problems.

Long-term exposure to ozone has been shown to increase risk of deathfrom respiratory illness. A study of 450,000 people living in UnitedStates cities showed a significant correlation between ozone levels andrespiratory illness over the 18-year follow-up period. The studyrevealed that people living in cities with high ozone levels such asHouston or Los Angeles had an over 30% increased risk of dying fromlung disease.[38] [39]

Air quality guidelines such as those from the World HealthOrganization, the United States Environmental Protection Agency(EPA) and the European Union are based on detailed studies designedto identify the levels that can cause measurable ill health effects.

According to scientists with the EPA, susceptible people can beadversely affected by ozone levels as low as 40 nmol/mol.[40]

In the EU, the current target value for ozone concentrations is120 µg/m³ which is about 60 nmol/mol. This target applies to all

Ozone 44

member states in accordance with Directive 2008/50/EC [41]. Ozone concentration is measured as a maximum dailymean of 8 hour averages and the target should not be exceeded on more than 25 calendar days per year, starting fromJanuary 2010. Whilst the directive requires in the future a strict compliance with 120 µg/m³ limit (i.e. mean ozoneconcentration not to be exceeded on any day of the year), there is no date set for this requirement and this is treatedas a long-term objective. [42]

The Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, includingground-level ozone, and counties out of compliance with these standards are required to take steps to reduce theirlevels. In May 2008, the EPA lowered its ozone standard from 80 nmol/mol to 75 nmol/mol. This provedcontroversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60nmol/mol, and the World Health Organization recommends 51 nmol/mol. Many public health and environmentalgroups also supported the 60 nmol/mol standard.[43] On January 7, 2010, the U.S. Environmental Protection Agency(EPA) announced proposed revisions to the National Ambient Air Quality Standard (NAAQS) for the pollutantozone, the principal component of smog:

... EPA proposes that the level of the 8-hour primary standard, which was set at 0.075 μmol/mol in the 2008final rule, should instead be set at a lower level within the range of 0.060 to 0.070 μmol/mol, to provideincreased protection for children and other ‘‘at risk’’ populations against an array of O3- related adverse healtheffects that range from decreased lung function and increased respiratory symptoms to serious indicators ofrespiratory morbidity including emergency department visits and hospital admissions for respiratory causes,and possibly cardiovascular-related morbidity as well as total non- accidental and cardiopulmonarymortality....[44]

The EPA has developed an Air Quality Index (AQI) to help explain air pollution levels to the general public. Underthe current standards, eight-hour average ozone mole fractions of 85 to 104 nmol/mol are described as "unhealthy forsensitive groups," 105 nmol/mol to 124 nmol/mol as "unhealthy," and 125 nmol/mol to 404 nmol/mol as "veryunhealthy."[45]

Ozone can also be present in indoor air pollution, partly as a result of electronic equipment such as photocopiers. Aconnection has also been known to exist between the increased pollen, fungal spores, and ozone caused bythunderstorms and hospital admissions of asthma sufferers.[46]

In the Victorian era, one British folk myth held that the smell of the sea was caused by ozone. In fact, thecharacteristic "smell of the sea" is caused by dimethyl sulfide a chemical generated by phytoplankton. VictorianBritish folk considered the resulting smell "bracing," but in high concentrations, dimethyl sulfide is actually toxic.[47]

PhysiologyOzone, along with reactive forms of oxygen such as superoxide, singlet oxygen, hydrogen peroxide, andhypochlorite ions, is naturally produced by white blood cells and other biological systems (such as the roots ofmarigolds) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, whenozone breaks down to dioxygen it gives rise to oxygen free radicals, which are highly reactive and capable ofdamaging many organic molecules. Moreover, it is believed that the powerful oxidizing properties of ozone may be acontributing factor of inflammation. The cause-and-effect relationship of how the ozone is created in the body andwhat it does is still under consideration and still subject to various interpretations, since other body chemicalprocesses can trigger some of the same reactions. A team headed by Dr. Paul Wentworth Jr. [48] of the Department ofChemistry at the Scripps Research Institute has shown evidence linking the antibody-catalyzed water-oxidationpathway of the human immune response to the production of ozone. In this system, ozone is produced byantibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen.[49]

When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a

Ozone 45

class of secosterols termed atheronals, generated by ozonolysis of cholesterol's double bond to form a 5,6secosterol[50] as well as a secondary condensation product via aldolization.[51]

Ozone has been implicated to have an adverse effect on plant growth: "... ozone reduced total chlorophylls,carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) contentand ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and soluteleakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protectivemechanisms against oxidative stress in citrus."[52]

Safety regulationsDue to the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes andrespiratory systems and can be hazardous at even low concentrations. The Canadian Center for Occupation Safetyand Health reports that:

"Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. Theseverity of injury depends on both by the concentration of ozone and the duration of exposure. Severeand permanent lung injury or death could result from even a very short-term exposure to relatively lowconcentrations." [53]

To protect workers potentially exposed to ozone, U.S. Occupational Safety and Health Administration hasestablished a permissible exposure limit (PEL) of 0.1 μmol/mol (29 CFR 1910.1000 table Z-1), calculated as an 8hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established anImmediately Dangerous to Life and Health Limit (IDLH) of 5 μmol/mol.[54] Work environments where ozone isused or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor forozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available fromseveral suppliers.Elevated ozone exposure can occur on passenger aircraft, with levels depending on altitude and atmosphericturbulence.[55] United States Federal Aviation Authority regulations set a limit of 250 nmol/mol with a maximumfour-hour average of 100 nmol/mol.[56] Some planes are equipped with ozone converters in the ventilation system toreduce passenger exposure.[55]

ProductionOzone often forms in nature under conditions where O2 will not react.[18] Ozone used in industry is measured inμmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m3, mg/hr (milligrams per hour) or weightpercent. The regime of applied concentrations ranges from 1 to 5% in air and from 6 to 14% in oxygen for oldergeneration methods. New electrolytic methods can achieve up 20 to 30% dissolved ozone concentrations in outputwater.Temperature and humidity plays a large role in how much ozone is being produced using traditional generationmethods such as corona discharge and ultraviolet light. Old generation methods will produce less than 50% itsnominal capacity if operated with humid ambient air than when it operates in very dry air. New generators usingelectrolytic methods can achieve higher purity and dissolution through using water molecules as the source of ozoneproduction.

Ozone 46

Corona discharge methodThis is the most common type of ozone generator for most industrial and personal uses. While variations of the "hotspark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozonegenerators, these units usually work by means of a corona discharge tube.[57] They are typically cost-effective and donot require an oxygen source other than the ambient air to produce ozone concentrations of 3-6%. Fluctuations inambient air, due to weather or other environmental conditions, cause variability in ozone production. However, theyalso produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation byremoving water vapor and increase ozone production. Use of an oxygen concentrator can further increase the ozoneproduction and further reduce the risk of nitric acid formation by removing not only the water vapor, but also thebulk of the nitrogen.

Ultraviolet lightUV ozone generators, or vacuum-ultraviolet (VUV) ozone generators, employ a light source that generates anarrow-band ultraviolet light, a subset of that produced by the Sun. The Sun's UV sustains the ozone layer in thestratosphere of Earth.[58]

While standard UV ozone generators tend to be less expensive, they usually produce ozone with a concentration ofabout 0.5% or lower. Another disadvantage of this method is that it requires the air (oxygen) to be exposed to the UVsource for a longer amount of time, and any gas that is not exposed to the UV source will not be treated. This makesUV generators impractical for use in situations that deal with rapidly moving air or water streams (in-duct airsterilization, for example). Production of ozone is one of the potential dangers of ultraviolet germicidal irradiation.VUV ozone generators are used in swimming pool and spa applications ranging to millions of gallons of water. VUVozone generators, unlike corona discharge generators, do not produce harmful nitrogen by-products and also unlikecorona discharge systems, VUV ozone generators work extremely well in humid air environments. There is also notnormally a need for expensive off-gas mechanisms, and no need for air driers or oxygen concentrators which requireextra costs and maintenance.

Cold plasmaIn the cold plasma method, pure oxygen gas is exposed to a plasma created by dielectric barrier discharge. Thediatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.Cold plasma machines utilize pure oxygen as the input source and produce a maximum concentration of about 5%ozone. They produce far greater quantities of ozone in a given space of time compared to ultraviolet production.However, because cold plasma ozone generators are very expensive, they are found less frequently than the previoustwo types.The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. Inorder to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodesand to prevent arcing.Some cold plasma units also have the capability of producing short-lived allotropes of oxygen which include O4, O5,O6, O7, etc. These species are even more reactive than ordinary O3.

ElectrolyticElectrolytic ozone generation (EOG) splits water molecules into H2, O2, and O3. In most EOG methods, thehydrogen gas will be removed to leave oxygen and ozone as the only reaction products. Therefore, EOG can achievehigher dissolution in water without other competing gases found in corona discharge method, such as nitrogen gasespresent in ambient air. This method of generation can achieve concentrations of 20-30% and is independent of airquality because water is used as the starting substrate.

Ozone 47

Special considerationsOzone cannot be stored and transported like other industrial gases (because it quickly decays into diatomic oxygen)and must therefore be produced on site. Available ozone generators vary in the arrangement and design of thehigh-voltage electrodes. At production capacities higher than 20 kg per hour, a gas/water tube heat-exchanger maybe utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime oftypical gas pressures is around 2 bar absolute in oxygen and 3 bar absolute in air. Several megawatts of electricalpower may be installed in large facilities, applied as one phase AC current at 50 to 8000 Hz and peak voltagesbetween 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled bycooling water temperature and/or gas velocity. The cooler the water, the better the ozone synthesis. The lower thegas velocity, the higher the concentration (but the lower the net ozone produced). At typical industrial conditions,almost 90% of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.Because of the high reactivity of ozone, only few materials may be used like stainless steel (quality 316L), titanium,aluminium (as long as no moisture is present), glass, polytetrafluorethylene, or polyvinylidene fluoride. Viton maybe used with the restriction of constant mechanical forces and absence of humidity (humidity limitations applydepending on the formulation). Hypalon may be used with the restriction that no water come in contact with it,except for normal atmospheric levels. Embrittlement or shrinkage is the common mode of failure of elastomers withexposure to ozone. Ozone cracking is the common mode of failure of elastomer seals like O-rings.Silicone rubbers are usually adequate for use as gaskets in ozone concentrations below 1 wt%, such as in equipmentfor accelerated aging of rubber samples.

Incidental productionOzone may be formed from O2 by electrical discharges and by action of high energy electromagnetic radiation.Certain electrical equipment generate significant levels of ozone. This is especially true of devices using highvoltages, such as ionic air purifiers, laser printers, photocopiers, tasers and arc welders. Electric motors using brushescan generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used byelevators or hydraulic pumps, will generate more ozone than smaller motors. Ozone is similarly formed in theCatatumbo lightning storms phenomenon on the Catatumbo River in Venezuela, which helps to replenish ozone inthe upper troposphere. It is the world's largest single natural generator of ozone, lending calls for it to be designated aUNESCO World Heritage Site.[59]

Laboratory productionIn the laboratory, ozone can be produced by electrolysis using a 9 volt battery, a pencil graphite rod cathode, aplatinum wire anode and a 3 molar sulfuric acid electrolyte.[60] The half cell reactions taking place are:

3 H2O → O3 + 6 H+ + 6 e− (ΔEo = −1.53 V)6 H+ + 6 e− → 3 H2 (ΔEo = 0 V)2 H2O → O2 + 4 H+ + 4 e− (ΔEo = −1.23 V)

In the net reaction, three equivalents of water are converted into one equivalent of ozone and three equivalents ofhydrogen. Oxygen formation is a competing reaction.It can also be prepared by applying 10,000-20,000 volts DC to dry O2. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top, with in and out spigots at the top and bottom of the outer tube. The inner core should have a length of metal foil inserted into it connected to one side of the power source. The other side of the power source should be connected to another piece of foil wrapped around the outer tube. Dry O2 should be run through the tube in one spigot. As the O2 is run through one spigot into the apparatus and 10,000-20,000 volts DC are applied to the foil leads, electricity will discharge between the dry dioxygen in the

Ozone 48

middle and form O3 and O2 out the other spigot. The reaction can be summarized as follows:[18]

3 O2 — electricity → 2 O3

Applications

IndustryThe largest use of ozone is in the preparation of pharmaceuticals, synthetic lubricants, and many other commerciallyuseful organic compounds, where it is used to sever carbon-carbon bonds.[18] It can also be used for bleachingsubstances and for killing microorganisms in air and water sources.[61] Many municipal drinking water systems killbacteria with ozone instead of the more common chlorine.[62] Ozone has a very high oxidation potential.[63] Ozonedoes not form organochlorine compounds, nor does it remain in the water after treatment. The Safe Drinking WaterAct mandates that these systems introduce an amount of chlorine to maintain a minimum of 0.2 μmol/mol residualfree chlorine in the pipes, based on results of regular testing. Where electrical power is abundant, ozone is acost-effective method of treating water, since it is produced on demand and does not require transportation andstorage of hazardous chemicals. Once it has decayed, it leaves no taste or odor in drinking water.Although low levels of ozone have been advertised to be of some disinfectant use in residential homes, theconcentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safelevels recommended by the U.S. Occupational Safety and Health Administration and Environmental ProtectionAgency. Humidity control can vastly improve both the killing power of the ozone and the rate at which it decaysback to oxygen (more humidity allows more effectiveness). Spore forms of most pathogens are very tolerant ofatmospheric ozone in concentrations where asthma patients start to have issues.Industrially, ozone is used to:• Disinfect laundry in hospitals, food factories, care homes etc.;[64]

• Disinfect water in place of chlorine[18]

• Deodorize air and objects, such as after a fire. This process is extensively used in fabric restoration• Kill bacteria on food or on contact surfaces;[65]

• Sanitize swimming pools and spas• Kill insects in stored grain[66]

• Scrub yeast and mold spores from the air in food processing plants;• Wash fresh fruits and vegetables to kill yeast, mold and bacteria;[65]

• Chemically attack contaminants in water (iron, arsenic, hydrogen sulfide, nitrites, and complex organics lumpedtogether as "colour");

• Provide an aid to flocculation (agglomeration of molecules, which aids in filtration, where the iron and arsenic areremoved);

• Manufacture chemical compounds via chemical synthesis[67]

• Clean and bleach fabrics (the former use is utilized in fabric restoration; the latter use is patented);• Assist in processing plastics to allow adhesion of inks;• Age rubber samples to determine the useful life of a batch of rubber;• Eradicate water borne parasites such as Giardia lamblia and Cryptosporidium in surface water treatment plants.Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of analkene to carbonyl compounds.Many hospitals in the U.S. and around the world use large ozone generators to decontaminate operating roomsbetween surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectivelykills or neutralizes all remaining bacteria.[68]

Ozone 49

Ozone is used as an alternative to chlorine or chlorine dioxide in the bleaching of wood pulp.[69] It is often used inconjunction with oxygen and hydrogen peroxide to eliminate the need for chlorine-containing compounds in themanufacture of high-quality, white paper.[70]

Ozone can be used to detoxify cyanide wastes (for example from gold and silver mining) by oxidizing cyanide tocyanate and eventually to carbon dioxide.[71]

ConsumersDevices generating high levels of ozone, some of which use ionization, are used to sanitize and deodorizeuninhabited buildings, rooms, ductwork, woodsheds, and boats and other vehicles.In the U.S., air purifiers emitting low levels of ozone have been sold. This kind of air purifier is sometimes claimedto imitate nature's way of purifying the air[72] without filters and to sanitize both it and household surfaces. TheUnited States Environmental Protection Agency (EPA) has declared that there is "evidence to show that atconcentrations that do not exceed public health standards, ozone is not effective at removing many odor-causingchemicals" or "viruses, bacteria, mold, or other biological pollutants." Furthermore, its report states that "results ofsome controlled studies show that concentrations of ozone considerably higher than these [human safety] standardsare possible even when a user follows the manufacturer’s operating instructions."[73] The government successfullysued one company in 1995, ordering it to stop repeating health claims without supporting scientific studies.Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. Accordingto the U.S. Food and Drug Administration (FDA), it is "amending the food additive regulations to provide for thesafe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry."Studies at California Polytechnic University demonstrated that 0.3 μmol/mol levels of ozone dissolved in filteredtapwater can produce a reduction of more than 99.99% in such food-borne microorganisms as salmonella, E. coli0157:H7, and Campylobacter. This quantity is 20,000 times the WHO recommended limits stated above.[65] [74]

Ozone can be used to remove pesticide residues from fruits and vegetables.[75] [76]

Ozone is used in homes and hot tubs to kill bacteria in the water and to reduce the amount of chlorine or brominerequired by reactivating them to their free state. Since ozone does not remain in the water long enough, ozone byitself is ineffective at preventing cross-contamination among bathers and must be used in conjunction with halogens.Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water.[77]

Ozone is also widely used in treatment of water in aquariums and fish ponds. Its use can minimize bacterial growth,control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water. Ozonemust not come in contact with fish's gill structures. Natural salt water (with life forms) provides enough"instantaneous demand" that controlled amounts of ozone activate bromide ion to hypobromous acid, and the ozoneentirely decays in a few seconds to minutes. If oxygen fed ozone is used, the water will be higher in dissolvedoxygen, fish's gill structures will atrophy and they will become dependent on higher dissolved oxygen levels.

AquacultureOzone can be used in aquaculture to facilitate organic breakdown. It is added to recirculating systems to reducenitrite levels[78] through conversion into nitrate. If nitrite levels in the water are high, nitrites will also accumulate inthe blood and tissues of fish, where it interferes with oxygen transport (it causes oxidation of the heme-group ofhaemoglobin from ferrous(Fe2+) to ferric (Fe3+), making haemoglobin unable to bind O2

[79] ). Despite these apparentpositive effects, ozone use in recirculation systems has been linked to reducing the level of bioavailable iodine in saltwater systems, resulting in iodine deficiency symptoms such as goitre and decreased growth in Senegalese sole(Solea senegalensis) larvae.[80]

Ozonate seawater is used for surface disinfection of haddock and Atlantic halibut eggs against nodavirus. Nodavirusis a lethal and vertically transmitted virus which causes severe mortality in fish. Haddock eggs should not be treatedwith high ozone level as eggs so treated did not hatch and died after 3–4 days.[81]

Ozone 50

AgricultureOzone application on freshly cut pineapple and banana shows increase in flavonoids and total phenol contents whenexposure is up to 20 minutes. Decrease in ascorbic acid content is observed but the positive effect on total phenolcontent and flavonoids can overcome the negative effect.[82] Tomatoes upon treatment with ozone shows an increasein β-carotene, lutein and lycopene.[83] However, ozone application on strawberries in pre-harvest period showsdecrease in ascorbic acid content.[84]

Ozone facilitates the extraction of some heavy metals from soil using EDTA. EDTA forms strong, water-solublecorrodination compounds with some heavy metals (Pb, Zn) thereby making it possible to dissolve them out fromcontaminated soil. If contaminated soil is pre-treated with ozone, the extraction efficacy of Pb, Am and Pu increasesby 11-28.9%,[85] 43.5%[86] and 50.7%[86] respectively.

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[70] Su, Yu-Chang; Chen, Horng-Tsai (2001). "Enzone Bleaching Sequence and Color Reversion of Ozone-Bleached Pulps" (http:/ / www. tfri.gov. tw/ enu/ pub_science_in. aspx?pid=339& catid0=37& catid1=64& pg0=& pg1=1). Taiwan Journal of Forest Science 16 (2): 93–102. .

[71] Bollyky, L. J. (1977). Ozone Treatment of Cyanide-Bearing Wastes, EPA Report 600/2-77-104. Research Triangle Park, N.C.: U.S.Environmental Protection Agency.

[72] "The Unknown Truth Regarding Ozone!" (http:/ / www. youtube. com/ watch?v=Ydb2_pyZeJk). . Retrieved 2006-09-16.[73] EPA report on consumer ozone air purifiers (http:/ / www. epa. gov/ iaq/ pubs/ ozonegen. html)[74] Long, Ron (2008). "POU Ozone Food Sanitation: A Viable Option for Consumers & the Food Service Industry" (http:/ / www. purityintl.

com/ Article POU. pdf) (pdf). . (report also shows tapwater removes 99.95% of pathogens from lettuce; samples were first inoculated withpathogens before treatment)

[75] Tersano Inc (2007). "lotus Sanitises Food without Chemicals" (http:/ / web. archive. org/ web/ 20070211025555/ http:/ / www. tersano. com/sanitizing_system_food. shtml). Archived from the original (http:/ / www. tersano. com/ sanitizing_system_food. shtml) on 2007-02-11. .Retrieved 2007-02-11.

[76] Jongen, W (2005). Improving the Safety of Fresh Fruit and Vegetables. Boca Raton: Woodhead Publishing Ltd. ISBN 1855739569.[77] "Alternative Disinfectants and Oxidant Guidance Manual" (http:/ / water. epa. gov/ lawsregs/ rulesregs/ sdwa/ mdbp/ upload/

2001_07_13_mdbp_alternative_disinfectants_guidance. pdf) (PDF). United States Environmental Protection Agency. April 1999. . Retrieved2008-01-14.

[78] Noble, A.C.; Summerfelt, S.T. (1996). "Diseases encountered in rainbow trout cultured in recirculating systems". Annual Review of FishDiseases 6: 65–92. doi:10.1016/S0959-8030(96)90006-X.

[79] Ferreira, O; de Costa, O.T.; Ferreira, Santos; Mendonca, F. (2004). "Susceptibility of the Amazonian fish, Colossoma macropomum(Serrasalminae), to short-term exposure to nitrite". Aquaculture 232: 627–636. doi:10.1016/S0044-8486(03)00524-6.

[80] Ribeiro, A.R.A.; Ribeiro, L.; Saele, Ø.; Hamre, K.; Dinis, M.T.; Moren, M. (2009). "Iodine-enriched rotifers andArtemiaprevent goitre inSenegalese sole (Solea senegalensis) larvae reared in a recirculation system". Aquaculture Nutrition 17 (3): 248–257.doi:10.1111/j.1365-2095.2009.00740.x.

[81] Buchan, K.; Martin-Robinchaud, D.; Benfey, T.J.; MacKinnon, A; Boston, L (2006). "The efficacy of ozonated seawater for surfacedisinfection of haddock (Melanogrammus aeglefinus) eggs against piscine nodavirus". Aquacultural Engineering 35: 102–107.doi:10.1016/j.aquaeng.2005.10.001.

[82] Alothman, M.; Kaur, B.; Fazilah, A.; Bhat, Rajeev; Karim, Alias A. (2010). "Ozone-induced changes of antioxidant capacity of fresh-cuttropical fruits". Innovative Food Science and Emerging Technologies 11 (4): 666–671. doi:10.1016/j.ifset.2010.08.008.

[83] Tzortzakis, N.; Borland, A.; Singleton, I.; Barnes, J (2007). "Impact of atmospheric ozone-enrichment on quality-related attributes of tomatofruit". Postharvest Biology and Technology 45 (3): 317–325. doi:10.1016/j.postharvbio.2007.03.004.

[84] Keutgen, A.J.; Pawelzik, E. (2008). "Influence of pre-harvest ozone exposure on quality of strawberry fruit under simulated retailconditions". Postharvest Biology and Technology 49: 10–18. doi:10.1016/j.postharvbio.2007.12.003.

[85] Lestan, D.; Hanc, A.; Finzgar, N. (2005). "Influence of ozonation on extractability of Pb and Zn from contaminated soils". Chemosphere 61(7): 1012–1019. doi:10.1016/j.chemosphere.2005.03.005. PMID 16257321.

[86] Plaue, J.W.; Czerwinski, K.R. (2003). "The influence of ozone on ligand-assisted extraction of 239Pu and 241Am from rocky flats soil".Radiochim. Acta 91 (6–2003): 309–313. doi:10.1524/ract.91.6.309.20026.

Ozone 53

Further reading• Greenwood, Norman N.; Earnshaw, Alan. (1997), Chemistry of the Elements (2nd ed.), Oxford:

Butterworth-Heinemann, ISBN 0080379419• Series in Plasma Physics: Non-Equilibrium Air Plasmas at Atmospheric Pressure. Edited by K.H. Becker, U.

Kogelschatz, K.H. Schoenbach, R.J. Barker; Bristol and Philadelphia: Institute of Physics Publishing Ltd; ISBN0-7503-0962-8; 2005

External links• International Ozone Association (http:/ / www. io3a. org/ )• European Environment Agency's near real-time ozone map (ozoneweb) (http:/ / www. eea. europa. eu/ maps/

ozone/ welcome)• NASA's Ozone Resource Page (http:/ / www. nasa. gov/ vision/ earth/ environment/ ozone_resource_page. html)• Paul Crutzen Interview (http:/ / www. vega. org. uk/ video/ programme/ 111) Freeview video of Paul Crutzen

Nobel Laureate for his work on decomposition of ozone talking to Harry Kroto Nobel Laureate by the VegaScience Trust.

• NASA's Earth Observatory article on Ozone (http:/ / earthobservatory. nasa. gov/ Library/ Ozone/ ozone. html)• International Day for the Preservation of the Ozone Layer (http:/ / internet12. eapps. com/ countme/

CMICampaigns. nsf/ UNIDs/ 70DD73E27B24746D85256D7F00331B23)• International Chemical Safety Card 0068 (http:/ / www. ilo. org/ public/ english/ protection/ safework/ cis/

products/ icsc/ dtasht/ _icsc00/ icsc0068. htm)• NIOSH Pocket Guide to Chemical Hazards (http:/ / www. cdc. gov/ niosh/ npg/ npgd0476. html)• National Institute of Environmental Health Sciences, Ozone Information (http:/ / www. niehs. nih. gov/ health/

topics/ agents/ ozone/ )• Ground-level Ozone Air Pollution (http:/ / www. greenfacts. org/ air-pollution/ ozone-o3/ index. htm)• NASA Study Links "Smog" to Arctic Warming (http:/ / www. giss. nasa. gov/ research/ news/ 20060314/ ) —

NASA Goddard Institute for Space Studies (GISS) study shows the warming effect of ozone in the Arctic duringwinter and spring.

• United States Environmental Protection Agency (EPA) report of effectiveness and safety of ozone generators soldas air cleaners (http:/ / www. epa. gov/ iaq/ pubs/ ozonegen. html)

• Pesticides Database; Ozone (http:/ / www. pesticideinfo. org/ Detail_Chemical. jsp?Rec_Id=PC39189)• Ground-level ozone information from the American Lung Association of New England (http:/ / ownyourair. org/

ozone. html)

Ultraviolet germicidal irradiation 54

Ultraviolet germicidal irradiation

A low pressure mercury vapor discharge tubefloods the inside of a hood with shortwave UV

light when not in use, sterilizing microbiologicalcontaminants from irradiated surfaces.

Ultraviolet germicidal irradiation (UVGI) is a sterilization methodthat uses ultraviolet (UV) light at sufficiently short wavelength tobreak down microorganisms.[1] It is used in a variety of applications,such as food, air and water purification. UV has been a knownmutagen at the cellular level for more than one-hundred years. The1903 Nobel Prize for Medicine was awarded to Niels Finsen for his useof UV against tuberculosis.[2]

UVGI utilises the short wavelength of UV that is harmful tomicroorganisms. It is effective in destroying the nucleic acids in theseorganisms so that their DNA is disrupted by the UV radiation. Thisremoves their reproductive capabilities and kills them.

The wavelength of UV that causes this effect is rare on Earth as itsatmosphere blocks it.[3] Using a UVGI device in certain environmentslike circulating air or water systems creates a deadly effect on micro-organisms such as pathogens, viruses and moldsthat are in these environments. Coupled with a filtration system, UVGI can remove harmful micro-organisms fromthese environments.

The application of UVGI to sterilization has been an accepted practice since the mid-20th century. It has been usedprimarily in medical sanitation and sterile work facilities. Increasingly it was employed to sterilize drinking andwastewater, as the holding facilities were enclosed and could be circulated to ensure a higher exposure to the UV. Inrecent years UVGI has found renewed application in air sanitization.

How UVGI WorksUltraviolet light is electromagnetic radiation with wavelengths shorter than visible light. UV can be separated intovarious ranges, with short range UV (UVC) considered “germicidal UV.” At certain wavelengths UV is mutagenic tobacteria, viruses and other micro-organisms. At a wavelength of 2,537 Angstroms (254 nm)[4] UV will break themolecular bonds within micro-organismal DNA, producing thymine dimers in their DNA thereby destroying them,rendering them harmless or prohibiting growth and reproduction. It is a process similar to the UV effect of longerwavelengths (UVB) on humans, such as sunburn or sun glare. Micro-organisms have less protection from UV andcannot survive prolonged exposure to it.A UVGI system is designed to expose environments such as water tanks, sealed rooms and forced air systems togermicidal UV. Exposure comes from germicidal lamps that emit germicidal UV electromagnetic radiation at thecorrect wavelength, thus irradiating the environment. The forced flow of air or water through this environmentensures the exposure.

Ultraviolet germicidal irradiation 55

EffectivenessUVGI is a highly effective method of destroying microorganisms. Since the Earth’s atmosphere absorbs most of theUV from the sun, germicidal UV is very rare in all circumstances. When concentrated in a closed environment suchas a water holding tank or duct system it is lethal over time to all micro-organisms.The effectiveness of germicidal UV in such an environment depends on a number of factors: the length of time amicro-organism is exposed to UV, power fluctuations of the UV source that impact the EM wavelength, the presenceof particles that can protect the micro-organisms from UV, and a micro-organism’s ability to withstand UV during itsexposure.In many systems redundancy in exposing micro-organisms to UV is achieved by circulating the air or waterrepeatedly. This ensures multiple passes so that the UV is effective against the highest number of micro-organismsand will irradiate resistant micro-organisms more than once to break them down.The effectiveness of this form of sterilization is also dependent on line-of-sight exposure of the micro-organisms tothe UV light. Environments where design creates obstacles that block the UV light are not as effective. In such anenvironment the effectiveness is then reliant on the placement of the UVGI system so that line-of-sight is optimumfor sterilization.A separate problem that will affect UVGI is dust or other film coating the bulb, which can lower UV output.Therefore bulbs require annual replacement and scheduled cleaning to ensure effectiveness. The lifetime ofgermicidal UV bulbs varies depending on design. Also the material that the bulb is made of can absorb some of thegermicidal rays.Lamp cooling under airflow can also lower UV output, thus care should be taken to shield lamps from direct airflowvia parabolic reflector. Or add additional lamps to compensate for the cooling effect.Increases in effectiveness and UV intensity can be achieved by using reflection. Aluminum has the highestreflectivity rate versus other metals and is recommended when using UV.

Creating UVGI

A 9 W germicidal lamp in a modern compactfluorescent lamp form factor

Germicidal UV is delivered by a mercury-vapor lamp that emits UV atthe germicidal wavelength. Mercury vapour emits at 254 nm. Manygermicidal UV bulbs use special transformers to ensure even electricalflow to the bulbs so the correct wavelength is maintained. Sincegermicidal UV has a narrow bandwidth, power fluctuations will renderintended irradiating environments ineffective. In some cases, UVGIelectrodeless lamps can be energised with microwaves, giving very long stable life and other advantages. This isknown as 'Microwave UV.'

There are several different types of germicidal lamps: - Low-pressure UV lamps offer high efficiencies (approx 35%UVC) but lower power, typically 1 W/cm³ power density. - Amalgam UV lamps are a high-power version oflow-pressure lamps. They operate at higher temperatures and have a lifetime of up to 16,000 hours. Their efficiencyis slightly lower than that of traditional low-pressure lamps (approx 33% UVC output) and power density is approx2-3 W/cm³. - Medium-pressure UV lamps have a broad and pronounced peak-line spectrum and a high radiationoutput but lower UVC efficiency of 10% or less. Typical power density is 30 W/cm³ or greater.Depending on the quartz glass used for the lamp body, low-pressure and amalgam UV lamps emit light at 254 nmand 185 nm (for oxidation).185 nm light is used to generate ozone.

Ultraviolet germicidal irradiation 56

Potential dangersAt certain wavelengths (including UVC) UV is harmful to humans and other forms of life. In most UVGI systemsthe lamps are shielded or are in environments that limit exposure, such as a closed water tank or closed aircirculation system, often with interlocks that automatically shut off the UV lamps if the system is opened for accessby human beings. Limited exposure mitigates the risk of danger.In human beings, skin exposure to germicidal wavelengths of UV light can produce sunburn and (in some cases) skincancer. Exposure of the eyes to this UV radiation can produce extremely painful inflammation of the cornea andtemporary or permanent vision impairment, up to and including blindness in some cases. UV can damage the retinaof the eye.Another potential danger is the UV production of ozone. UVC light from the sun is partly responsible for the earth’sozone layer in the stratosphere, but ozone at the atmospheric level can be harmful to a person’s health. The UnitedStates Environmental Protection Agency designated .05 parts per million (ppm) of ozone to be a safe level. Lampsdesigned to release UVC and higher frequencies are doped so that any UV light below 254 nm will not be released,thus ozone is not produced. A full spectrum lamp will release all UV wavelengths and will produce ozone as well asUVC, UVB, and UVA. (The ozone is produced when UVC hits oxygen (O2) molecules, and so is only producedwhen oxygen is present.)UV-C radiation is able to break down chemical bonds. This leads to rapid ageing of plastics (insulations, gasket) andother materials. Note that plastics sold to be "UV-resistant" are tested only for UV-B, as UV-C doesn't normallyreach the surface of the Earth. When UV is used near plastic, rubber, or insulations care should be taken to shieldsaid components; metal tape or aluminum foil will suffice.

Uses for UVGI

Air DisinfectionUVGI can be used to disinfect air with prolonged exposure. Disinfection is a function of UV concentration and time,CT. For this reason, it is not as effective on moving air, when the lamp is perpendicular to the flow, as exposuretimes are dramatically reduced. Air purification UVGI systems can be freestanding units with shielded UV lampsthat use a fan to force air past the UV light. Other systems are installed in forced air systems so that the circulationfor the premises moves micro-organisms past the lamps. Key to this form of sterilization is placement of the UVlamps and a good filtration system to remove the dead micro-organisms.[5] For example, forced air systems bydesign impede line-of-sight, thus creating areas of the environment that will be shaded from the UV light. However,a UV lamp placed at the coils and drainpan of cooling system will keep micro-organisms from forming in thesenaturally damp places.ASHRAE covers UVGI and its applications in IAQ and building maintenance in its 2008 Handbook, HVACSystems and Equipment in Chapter 16 titled Ultraviolet Lamp Systems.

Water sterilizationUVGI is commonly used for water sterilization in a variety of applications. Its use in wastewater treatment is replacing chlorination due to the chemical's toxic by-products. A disadvantage of the technique is that water treated by chlorination is resistant to reinfection, where UVGI water must be transported and delivered in such a way as to avoid contamination. Individual wastestreams to be treated by UVGI must be tested to ensure that the method will be effective due to potential interferences such as suspended solids, dyes or other substances that may block or absorb the UV radiation. "UV units to treat small batches (1 to several liters) or low flows (1 to several liters per minute) of water at the community level are estimated to have costs of 0.02 US$ per 1000 liters of water, including the cost of electricity

Ultraviolet germicidal irradiation 57

and consumables and the annualized capital cost of the unit." (WHO) [6]

Aquarium and pondUltraviolet sterilizers are often used in aquaria and ponds to help control unwanted microorganisms in the water.Continuous sterilization of the water neutralizes single-cell algae and thereby increases water clarity. UV radiationalso ensures that exposed pathogens cannot reproduce, thus decreasing the likelihood of a disease outbreak in anaquarium.Aquarium and pond sterilizers are typically small, with fittings for tubing that allows the water to flow through thesterilizer on its way to or from a separate external filter. Within the sterilizer, water flows near to the ultraviolet lightsource, usually through a baffle system that lengthens the time during which the water is exposed to the radiation.

Laboratory hygieneUVGI is often used to disinfect equipment such as safety goggles, instruments, pipettors, and other devices. Labpersonnel also disinfects glassware and plasticware this way. Microbiology laboratories use UVGI to disinfectsurfaces inside biological safety cabinets ("hoods") between uses.

Food and beverage protectionSince the FDA issued a rule in 2001 requiring that virtually all fruit and vegetable juice producers follow HACCPcontrols, and mandating a 5-log reduction in pathogens, UVGI has seen some use in sterilization of fresh juices suchas fresh-pressed apple cider.

Other uses

EPROM erasers

UVGI lamps are used to erase the stored information held in EPROMS (erasable programmable read only memory)in less than a minute. Longer wavelength UV-B or UV-A lamps can also be used, but the erase time is considerablygreater.

References[1] National Institute for Occupational Safety and Health. (2008, April). NIOSH eNews, 5(12). Retrieved September 10, 2008, from http:/ /

www. cdc. gov/ niosh/ enews/ enewsV5N12. html[2] "The Nobel Prize in Physiology or Medicine 1903" (http:/ / nobelprize. org/ nobel_prizes/ medicine/ laureates/ 1903/ ). Nobelprize.org. The

Nobel Foundation. . Retrieved 2006-09-09.[3] Lupu, Alexandra (2006-07-20). "UV Radiation – What UVA, UVB and UVC Rays Are and How They Affect Us" (http:/ / news. softpedia.

com/ news/ UV-Radiation-What-UVA-UVB-and-UVC-Rays-Are-and-How-They-Affect-Us-30345. shtml). Seasonal Discomforts. Softpedia.. Retrieved 2006-09-09.

[4] Kowalski W.J.; Bahnfleth W.P.; Witham D.L.; Severin B.F.; Whittam T.S. (October 2000). "Mathematical Modeling of UltravioletGermicidal Irradiation for Air Disinfection" (http:/ / www. ingentaconnect. com/ content/ klu/ quan/ 2000/ 00000002/ 00000003/ 00390528).Quantitative Microbiology (Springer) 2 (3): 249–270. doi:10.1023/A:1013951313398. ISSN 1388-3593. .

[5] "Environmental Analysis of Indoor Air Pollution" (http:/ / www. purifier. org/ snapcat/ iaq. pdf). CaluTech UV Air. . Retrieved 2006-12-05.[6] WHO (http:/ / www. who. int/ water_sanitation_health/ dwq/ wsh0207/ en/ index4. html)

Ultraviolet germicidal irradiation 58

External links• ASHRAE 2008 Handbook - Table of content (http:/ / www. ashrae. org/ publications/ page/ 1862)• Emission spectra of different germicidal lamp types (http:/ / www. heraeus-noblelight. com/ en/

uv-disinfection-oxidation/ information-for-disinfection-and-oxidation/ services-and-events/emission-spectra-of-germicidal-lamps. html)

• The mercury lamp (http:/ / www. lamptech. co. uk/ Documents/ M1 Introduction. htm)• Sanuvox (http:/ / www. sanuvox. com)• WHO (http:/ / www. who. int/ water_sanitation_health/ dwq/ wsh0207/ en/ index4. html)

Water treatment

A sewage treatment plant in northern Portugal.

Water treatment describes those processesused to make water more acceptable for adesired end-use. These can include use asdrinking water, industrial processes, medicaland many other uses. The goal of all watertreatment process is to remove existingcontaminants in the water, or reduce theconcentration of such contaminants so thewater becomes fit for its desired end-use.One such use is returning water that hasbeen used back into the natural environmentwithout adverse ecological impact.

The processes involved in treating water fordrinking purpose may be solids separationusing physical processes such as settling and filtration, and chemical processes such as disinfection and coagulation.Biological processes are also employed in the treatment of wastewater and these processes may include, for example,aerated lagoons, activated sludge or slow sand filters.

Potable water purification

Abandoned Water Purification Plant Springfield,Tennessee, United States

Water purification is the removal of contaminants from untreated waterto produce drinking water that is pure enough for the most critical of itsintended uses, usually for human consumption. Substances that areremoved during the process of drinking water treatment includesuspended solids, bacteria, algae, viruses, fungi, minerals such as iron,manganese and sulphur, and other chemical pollutants such asfertilisers.

Measures taken to ensure water quality not only relate to the treatmentof the water, but to its conveyance and distribution after treatment aswell. It is therefore common practice to have residual disinfectants inthe treated water in order to kill any bacteriological contamination during distribution.World Health Organisation (WHO) guidelines are generally followed throughout the world for drinking waterquality requirements. In addition of the WHO guidelines, each country or territory or water supply body can havetheir own guidelines in order for consumers to have access to safe drinking water.

Water treatment 59

Processes for drinking water

Empty aeration tank for iron precipitation

Tanks with sand filters to remove precipitatediron (not working at the time)

A combination selected from the following processes is used formunicipal drinking water treatment worldwide:• Pre-chlorination - for algae control and arresting any biological

growth• Aeration - along with pre-chlorination for removal of dissolved iron

and manganese• Coagulation - for flocculation• Coagulant aids, also known as polyelectrolytes - to improve

coagulation and for thicker floc formation• Sedimentation - for solids separation, that is, removal of suspended

solids trapped in the floc• Filtration - removing particles from water• Desalination - Process of removing salt from the water• Disinfection - for killing bacteria.There is no unique solution (selection of processes) for any type ofwater. Also, it is difficult to standardise the solution in the form ofprocesses for water from different sources. Treatability studies for eachsource of water in different seasons need to be carried out to arrive atmost appropriate processes.The above mentioned technologies are well developed and generaliseddesigns are available which are used by many water utilities (public orprivate). In addition to the generalised solutions, a number of privatecompanies provide solutions by patenting their technologies.

Sewage treatment

Sewage treatment is the process that removes the majority of thecontaminants from wastewater or sewage and produces both a liquideffluent suitable for disposal to the natural environment and a sludge.To be effective, sewage must be conveyed to a treatment plant by appropriate pipes and infrastructure and theprocess itself must be subject to regulation and controls. Some wastewaters require different and sometimesspecialized treatment methods. At the simplest level, treatment of sewage and most wastewaters is carried outthrough separation of solids from liquids, usually by sedimentation. By progressively converting dissolved materialinto solids, usually a biological floc which is then settled out, an effluent stream of increasing purity is produced.

In developing countriesAs of 2006, waterborne diseases are estimated to have caused 1.8 million deaths each year. These deaths areattributable to inadequate public sanitation systems and in these cases, proper sewerage (or other options assmall-scale wastewater treatment) need to be installed.[1]

Appropriate technology options in water treatment include both community-scale and household-scale point-of-use(POU) designs.[2] Military surplus water treatment units like the ERDLator are still seen in developing countries.Newer military style Reverse Osmosis Water Purification Units (ROWPU) are portable, self-contained watertreatment plants are becoming more available for public use. [3]

Water treatment 60

In order for the decrease of waterborne diseases to have long term effects, water treatment programs implemented byresearch and development groups in developing countries must be sustainable by their citizens. This can ensure theefficiency of such programs after the departure of the research team as monitoring is difficult because of theremoteness of many locations.

References[1] "Safe Water System" (http:/ / www. cdc. gov/ safewater/ publications_pages/ fact_sheets/ WW4. pdf), US Centers for Disease Control and

Prevention Fact Sheet, June 2006.[2] "Household Water Treatment Guide" (http:/ / www. cawst. org/ en/ resources/ pubs), Centre for Affordable Water and Sanitation Technology,

March 2008.[3] Lindsten, Don C. (September 1984). "Technology transfer: Water purification, U.S. Army to the civilian community". The Journal of

Technology Transfer 9 (1): 57–59. doi:10.1007/BF02189057.

Articles• Water management technologies: The high tide of hi-tech (http:/ / www. tlvinsider. com/ tlvinsider/ nl3/

interviews?name=nl3_interview2) TLVInsider] from TLVInsider (http:/ / www. tlvinsider. com), Issue 3, IsraelH2O, November 2009.

• Water Talk: Deep in the well of Israeli Innovation (http:/ / www. tlvinsider. com/ tlvinsider/ nl3/ forum) fromTLVInsider (http:/ / www. tlvinsider. com), Issue 3, Israel H2O, November 2009.

Further reading• Clescerel, Lenore S.; Greenburg, Arnold E.; Eaton, Andrew D. (1999), Standard Methods for the Examination of

Water and Wastewater, American Public Health Association; 20th edition.

External links• International Water Association (http:/ / www. iwahq. org) Professional / research organization• Nanotechnology-enabled Water Treatment (http:/ / cohesion. rice. edu/ centersandinst/ cben/ events.

cfm?doc_id=10142)-Project NeWT, Center for Biological and Environmental Nanotechnology (CBEN), RiceUniversity (http:/ / cben. rice. edu)

• NSF International (http:/ / www. nsf. org) - Independent non-profit standards organization• Photo essay of water treatment system (http:/ / www. marcsteinmetz. com/ pages/ wasser/ ewasser_minis. html) -

by photographer Marc Steinmetz• Transnational Ecological Project (http:/ / www. hydropark. ru/ index. en. htm) - Industrial wastewater treatment

(Russia)• Water Environment Federation (http:/ / www. wef. org) - Professional association focusing on wastewater

treatment• WHO.int (http:/ / www. who. int/ water_sanitation_health/ dwq/ gdwq3rev/ en/ index. html), WHO Guidelines

Settling 61

Settling

Settling pond for iron particles at water works

Settling is the process by which particulates settle to the bottom of aliquid and form a sediment. Particles that experience a force, either dueto gravity or due to centrifugal motion will tend to move in a uniformmanner in the direction exerted by that force. For gravity settling, thismeans that the particles will tend to fall to the bottom of the vessel,forming a slurry at the vessel base.

Settling is an important operation in many applications, such asmining, wastewater treatment, biological science, space propellantreignition,[1] and particle mechanics.

Physics

Creeping flow past a sphere: streamlines, dragforce Fd and force by gravity Fg.

For settling particles that are considered individually, i.e. dilute particlesolutions, there are two main forces enacting upon any particle. Theprimary force is an applied force, such as gravity, and a drag force thatis due to the motion of the particle through the fluid. The applied forceis usually not affected by the particle's velocity, whereas the drag forceis a function of the particle velocity.

For a particle at rest no drag force will exhibited, which causes theparticle to accelerate due to the applied force. When the particleaccelerates, the drag force acts in the direction opposite to the particle'smotion, retarding further acceleration, in the absence of other forcesdrag directly opposes the applied force. As the particle increases invelocity eventually the drag force and the applied force willapproximately equate, causing no further change in the particle'svelocity. This velocity is known as the terminal velocity, settlingvelocity or fall velocity of the particle. This is readily measurable byexamining the rate of fall of individual particles.

The terminal velocity of the particle is affected by many parameters,i.e. anything that will alter the particle's drag. Hence the terminalvelocity is most notably dependent upon grain size, the shape (roundness and sphericity) and density of the grains, aswell as to the viscosity and density of the fluid.

Settling 62

Single particle drag

Stokes' drag

Dimensionless force versus Reynolds number forspherical particles

For dilute suspensions, Stokes' law predicts the settling velocity ofsmall spheres in fluid, either air or water. This originates due tothe strength of viscous forces at the surface of the particleproviding the majority of the retarding force. Stokes' law findsmany applications in the natural sciences, and is given by:

where w is the settling velocity, ρ is density (the subscripts p and f indicate particle and fluid respectively), g is theacceleration due to gravity, r is the radius of the particle and μ is the dynamic viscosity of the fluid.Stokes' law applies when the Reynolds number, Re, of the particle is less than 0.1. Experimentally Stokes' law isfound to hold within 1% for , within 3% for and within 9% .[2] With increasingReynolds numbers, Stokes law begins to break down due to fluid inertia, requiring the use of empirical solutions tocalculate drag forces.

Newtonian drag

Defining a drag coefficient, , as the ratio of the force experienced by the particle divided by the impact pressureof the fluid, a coefficient that can be considered as the transfer of available fluid force into drag is established. In thisregion the inertia of the impacting fluid is responsible for the majority of force transfer to the particle.

For a spherical particle in the Stokes regime this value is not constant, however in the Newtonian drag regime thedrag on a sphere can be approximated by a constant, 0.44. This constant value implies that the efficiency of transferof energy from the fluid to the particle is not a function of fluid velocity.As such the terminal velocity of a particle in a Newtonian regime can again be obtained by equating the drag force tothe applied force, resulting in the following expression

Settling 63

Transitional drag

In the intermediate region between Stokes drag and Newtonian drag, there exists a transitional regime, where theanalytical solution to the problem of a falling sphere becomes problematic. To solve this, empirical expressions areused to calculate drag in this region. One such empirical equation is that of Schiller and Naumann, and may be validfor :[3]

Hindered settlingStokes, transitional and Newtonian settling describe the behaviour of a single spherical particle in an infinite fluid,known as free settling. However this model has limitations in practical application. Alternate considerations, such asthe interaction of particles in the fluid, or the interaction of the particles with the container walls can modify thesettling behaviour. Settling that has these forces in appreciable magnitude is known as hindered settling.Subsequently semi-analytic or empirical solutions may be used to perform meaningful hindered settling calculations.

ApplicationsSettleable solids are the particulates that settle out of a still fluid. Settleable solids can be quantified for a suspensionusing an Imhoff tank or cone.The solid-gas flow systems are present in many industrial applications, as dry, catalytic reactors, settling tanks,pneumatic conveying of solids, among others. Obviously, in industrial operations the drag rule isn’t simple as asingle sphere settling in a stationary fluid. However, this knowledge indicates how drag behaves in more complexsystems, which are designed and studied by engineers applying empirical and more sophisticated tools.For example, Settling tanks are used for separating solids and/or oil from another liquid. In food processing, thevegetable is crushed and placed inside of a settling tank with water. The oil floats the top of the water then iscollected. In water and waste water treatment a flocculant is often added prior to settling to form larger particles thatsettle out quickly in a settling tank leaving the water with a lower turbidity.In winemaking, the French term for this process is débourbage. This step usually occurs in white wine productionbefore the start of fermentation.[4]

References[1] Zegler, Frank; Bernard Kutter (2010-09-02). "Evolving to a Depot-Based Space Transportation Architecture" (http:/ / www. ulalaunch. com/

site/ docs/ publications/ DepotBasedTransportationArchitecture2010. pdf). AIAA SPACE 2010 Conference & Exposition. AIAA. . Retrieved2011-01-25. "It consumes waste hydrogen and oxygen to produce power, generate settling and attitude control thrust."

[2] Martin Rhodes. Introduction to Particle Technology.[3] Chemical Engineering, Volume 2.. Pergamon press. 1955.[4] J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 223 Oxford University Press 2006 ISBN 0198609906

External links• Settleable solids methodology (http:/ / www. ne-wea. org/ LabManual/ settleable_solids. htm)• Imhoff cone (http:/ / www. vincentcorp. com/ tech_papers/ issue140. html)• Stokes Law terminal velocity calculator (http:/ / www. ajdesigner. com/ phpstokeslaw/

stokes_law_terminal_velocity. php)

Flocculation 64

FlocculationFlocculation, in the field of chemistry, is a process where colloids come out of suspension in the form of floc orflakes. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquidand not actually dissolved in a solution. In the flocculated system there is no formation of a cake since all the flocsare in the suspension.

Term definitionAccording to the IUPAC definition, flocculation is "a process of contact and adhesion whereby the particles of adispersion form larger-size clusters." Flocculation is synonymous with agglomeration, aggregation, and coagulation /coalescence.[1] [2]

Applications

Surface chemistryIn colloid chemistry, flocculation refers to the process by which fine particulates are caused to clump together into afloc. The floc may then float to the top of the liquid, settle to the bottom of the liquid, or can be readily filtered fromthe liquid.

Physical chemistryFor emulsions, flocculation describes clustering of individual dispersed droplets together, whereby the individualdroplets do not lose their identity.[3] Flocculation is thus the initial step leading to further aging of the emulsion(droplet coalescence and the ultimate separation of the phases).

Civil engineering/earth sciencesIn civil engineering, and in the earth sciences, flocculation is a condition in which clays, polymers or other smallcharged particles become attached and form a fragile structure, a floc. In dispersed clay slurries, flocculation occursafter mechanical agitation ceases and the dispersed clay platelets spontaneously form flocs because of attractionsbetween negative face charges and positive edge charges.

BiologyIn biology, the process is used to refer to the asexual aggregation of microorganisms.

Cheese production

Flocculation is widely employed to measure the progress of curd formation while in the initial stages of makingmany cheeses to determine how long the curds must set.[4] [5] The reaction involving the rennet micelles are modeledby Smoluchowski Kinetics.[4]

Brewing

Flocculation is used to measure the progress of brewing yeast at the end of fermentation.

Water treatmentFlocculation and sedimentation are widely employed in the purification of drinking water as well as sewagetreatment, stormwater treatment and treatment of other industrial wastewater streams.

Flocculation 65

FlocculantsParticles finer than 0.1 µm (10-7m) in water remain continuously in motion due to electrostatic charge (oftennegative) which causes them to repel each other. Once their electrostatic charge is neutralized by the use ofcoagulant chemical, the finer particles start to collide and agglomerate (combine together) under the influence of Vander Waals's forces. These larger and heavier particles are called flocs.Flocculants, or flocculating agents (also known as flocking agents), are chemicals that promote flocculation bycausing colloids and other suspended particles in liquids to aggregate, forming a floc. Flocculants are used in watertreatment processes to improve the sedimentation or filterability of small particles. For example, a flocculant may beused in swimming pool or drinking water filtration to aid removal of microscopic particles which would otherwisecause the water to be turbid (cloudy) and which would be difficult or impossible to remove by filtration alone.Many flocculants are multivalent cations such as aluminium, iron, calcium or magnesium.[6] These positivelycharged molecules interact with negatively charged particles and molecules to reduce the barriers to aggregation. Inaddition, many of these chemicals, under appropriate pH and other conditions such as temperature and salinity, reactwith water to form insoluble hydroxides which, upon precipitating, link together to form long chains or meshes,physically trapping small particles into the larger floc.Long-chain polymer flocculants, such as modified polyacrylamides, are manufactured and sold by the flocculantproducing business. These can be supplied in dry or liquid form for use in the flocculation process. The mostcommon liquid polyacrylamide is supplied as an emulsion with 10-40% actives and the rest is a carrier fluid,surfactants and latex. Emulsion polymers require activation to invert the emulsion and allow the electrolyte groups tobe exposed.The following chemicals are used as flocculants:• alum• aluminium chlorohydrate• aluminium sulfate• calcium oxide• calcium hydroxide• iron(II) sulfate• iron(III) chloride• polyacrylamide• polyDADMAC• sodium aluminate• sodium silicateThe following natural products are used as flocculants:[7]

• Chitosan• Isinglass• Moringa oleifera seeds (Horseradish Tree)• Gelatin• Strychnos potatorum seeds (Nirmali nut tree)• Guar gum• Alginates (brown seaweed extracts)

Flocculation 66

DeflocculationA deflocculant is a chemical additive to prevent a colloid from coming out of suspension or to thin suspensions orslurries. It is used to reduce viscosity or prevent flocculation and is sometimes incorrectly called a "dispersant." Mostdeflocculants are low-molecular weight anionic polymers that neutralize positive charges on suspended particles,particularly clays and aryl-alkyl derivative of sulfonic acid. Examples include polyphosphates, lignosulfonates,quebracho tannins and various water-soluble synthetic polymers.Deflocculation is also used to describe the undesired effect in an activated sludge basin if the sludge is subjected tohigh-speed mixing. Generally, deflocculation can be prevented or reduced by applying gentle mixing (e.g., by usingsubmersible propeller mixers that utilize large/wide propeller blades and operate at low rotational speed).

References[1] IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) " flocculation (http:/ /

goldbook. iupac. org/ F02429. html)".[2] Hubbard, Arthur T. (2004). Encyclopedia of Surface and Colloid Science (http:/ / books. google. com/ ?id=vnb2X7Q8_cYC& pg=PA4230&

lpg=PA4230& dq=ostwald+ ripening+ emulsion+ polymerization). CRC Press. p. 4230. ISBN 0824707591. . Retrieved 2007-11-13.[3] Adamson A.W. and Gast A.P. (1997) "Physical Chemistry of Surfaces", John Wiley and Sons.[4] Fox, Patrick F. (1999). Cheese Volume 1: Chemistry, Physics, and Microbiology (2nd ed.). Gaithersburg, Maryland: Aspen Publishers.

pp. 144–150. ISBN 0412 53500.[5] Journal of Scientific and Industrial Research 57: 680–681. 1998.[6] Global Health and Education Foundation (2007). "Conventional Coagulation-Flocculation-Sedimentation" (http:/ / drinking-water. org/ html/

en/ Treatment/ Coagulation-Flocculation-technologies. html). Safe Drinking Water is Essential. National Academy of Sciences. . Retrieved2007-12-01.

[7] P. Somasundaran "Encyclopedia of surface and colloid science, Volume 7, pp 4980-4982."

Further reading• John Gregory (2006), Particles in water: properties and processes, Taylor & Francis, ISBN 1-58716-085-4

Activated sludge 67

Activated sludgeActivated sludge is a process for treating sewage and industrial wastewaters using air and a biological floccomposed of bacteria and protozoans.

PurposeIn a sewage (or industrial wastewater) treatment plant, the activated sludge process can be used for one or several ofthe following purposes:• oxidizing carbonaceous matter: biological matter.• oxidizing nitrogeneous matter: mainly ammonium and nitrogen in biological materials.• removing phosphate.• driving off entrained gases carbon dioxide, ammonia, nitrogen, etc.• generating a biological floc that is easy to settle.• generating a liquor that is low in dissolved or suspended material.

The processThe process involves air or oxygen being introduced into a mixture of primary treated or screened sewage orindustrial wastewater (called wastewater from now on) combined with organisms to develop a biological floc whichreduces the organic content of the sewage. This material, which in healthy sludge is a brown floc, is largelycomposed of saprotrophic bacteria but also has an important protozoan flora mainly composed of amoebae,Spirotrichs, Peritrichs including Vorticellids and a range of other filter feeding species. Other important constituentsinclude motile and sedentary Rotifers. In poorly managed activated sludge, a range of mucilaginous filamentousbacteria can develop including Sphaerotilus natans which produces a sludge that is difficult to settle and can result inthe sludge blanket decanting over the weirs in the settlement tank to severely contaminate the final effluent quality.This material is often described as sewage fungus but true fungal communities are relatively uncommon.The combination of wastewater and biological mass is commonly known as mixed liquor. In all activated sludgeplants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanksand the treated supernatant is run off to undergo further treatment before discharge. Part of the settled material, thesludge, is returned to the head of the aeration system to re-seed the new wastewater entering the tank. This fractionof the floc is called return activated sludge (R.A.S.). Excess sludge is called surplus activated sludge(S.A.S.) orwaste activated sludge(W.A.S). S.A.S is removed from the treatment process to keep the ratio of biomass to foodsupplied in the wastewater in balance. S.A.S is stored in sludge tanks and is further treated by digestion, either underanaerobic or aerobic conditions prior to disposal.Many sewage treatment plants use axial flow pumps to transfer nitrified mixed liquor from the aeration zone to theanoxic zone for denitrification. These pumps are often referred to as internal mixed liquor recycle pumps (IMLRpumps). The raw sewage, the RAS, and the nitrified mixed liquor are mixed by submersible mixers in the anoxiczones in order to achieve denitrification.Activated sludge is also the name given to the active biological material produced by activated sludge plants.

Activated sludge 68

HistoryThe activated sludge process was discovered in 1913 in the UK by two engineers, Edward Ardern and W.T. Lockett,[1] conducting research for the Manchester Corporation Rivers Department at Davyhulme Sewage Works.Experiments on treating sewage in a draw-and-fill reactor (the precursor to today's sequencing batch reactor)produced a highly treated effluent. Believing that the sludge had been activated (in a similar manner to activatedcarbon) the process was named activated sludge. Not until much later was it realized that what had actually occurredwas a means to concentrate biological organisms, decoupling the liquid retention time (ideally, low, for a compacttreatment system) from the solids retention time (ideally, fairly high, for an effluent low in BOD5 and ammonia.)

A generalized, schematic diagram of an activated sludge process.

Arrangement

The general arrangement of an activatedsludge process for removing carbonaceouspollution includes the following items:• Aeration tank where air (or oxygen) is

injected in the mixed liquor.• Settling tank (usually referred to as "final

clarifier" or "secondary settling tank") toallow the biological flocs to settle, thusseparating the biological sludge from theclear treated water.

Treatment of nitrogenous matter orphosphate involves additional steps wherethe mixed liquor is left in anoxic condition(meaning that there is no residual dissolvedoxygen).

Types of plants

Activated sludge system in China

There are a variety of types of activated sludge plants.[1] These include:

Package plants

There are a wide range of other types of plants, often serving smallcommunities or industrial plants that may use hybrid treatment processesoften involving the use of aerobic sludge to treat the incoming sewage. Insuch plants the primary settlement stage of treatment may be omitted. In theseplants, a biotic floc is created which provides the required substrate.Package plants are commonly variants of extended aeration, to promote the'fit & forget' approach required for small communities without dedicatedoperational staff. There are various standards to assist with their design.[2] [3]

[4]

Activated sludge 69

Oxidation ditchIn some areas, where more land is available, sewage is treated in large round or oval ditches with one or morehorizontal aerators typically called brush or disc aerators which drive the mixed liquor around the ditch and provideaeration.[1] These are oxidation ditches, often referred to by manufacturer's trade names such as Pasveer, Orbal, orCarrousel. They have the advantage that they are relatively easy to maintain and are resilient to shock loads thatoften occur in smaller communities (i.e. at breakfast time and in the evening).Oxidation ditches are installed commonly as 'fit & forget' technology, with typical design parameters of a hydraulicretention time of 24 - 48 hours, and a sludge age of 12 - 20 days. This compares with nitrifying activated sludgeplants having a retention time of 8 hours, and a sludge age of 8 - 12 days.

Deep ShaftWhere land is in short supply sewage may be treated by injection of oxygen into a pressured return sludge streamwhich is injected into the base of a deep columnar tank buried in the ground. Such shafts may be up to 100 metresdeep and are filled with sewage liquor. As the sewage rises the oxygen forced into solution by the pressure at thebase of the shaft breaks out as molecular oxygen providing a highly efficient source of oxygen for the activatedsludge biota. The rising oxygen and injected return sludge provide the physical mechanism for mixing of the sewageand sludge. Mixed sludge and sewage is decanted at the surface and separated into supernatant and sludgecomponents. The efficiency of deep shaft treatment can be high.Surface aerators are commonly quoted as having an aeration efficiency of 0.5 - 1.5 kg O2/kWh, diffused aeration as1.5 - 2.5 kg O2/KWh. Deep Shaft claims 5 - 8 kg O2/kWh.However, the costs of construction are high. Deep Shaft has seen greatest uptake in Japan, because of the land areaissues. Deep Shaft was developed by ICI, as a spin-off from their Pruteen process. In the UK it is found at three sites:Tilbury, Anglian water, treating a wastewater with a high industrial contribution;[5] Southport, United Utilities,because of land space issues; and Billingham, ICI, again treating industrial effluent, and built (after the Tilburyshafts) by ICI to help the agent sell more.DeepShaft is a patented, licensed, process. The licensee has changed several times and, currently (2007), it is AkerKvaerner Engineering Services.[6]

Surface-aerated Basins/Lagoons

A Typical Surface-Aerated Basing (using motor-driven floating aerators)

Most biological oxidation processes fortreating industrial wastewaters have incommon the use of oxygen (or air) andmicrobial action. Surface-aerated basinsachieve 80 to 90% removal of BODwith retention times of 1 to 10 days.[7]

The basins may range in depth from 1.5to 5.0 metres and utilize motor-drivenaerators floating on the surface of thewastewater.[7]

In an aerated basin system, the aeratorsprovide two functions: they transfer airinto the basins required by the biologicaloxidation reactions, and they provide the

Activated sludge 70

mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes).Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kWh.However, they do not provide as good mixing as is normally achieved in activated sludge systems and thereforeaerated basins do not achieve the same performance level as activated sludge units.[7]

Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of biologicalreactions increase with temperature. Most surface aerated vessels operate at between 4 °C and 32 °C.[7]

Aeration methods

Diffused Aeration

Fine bubble membrane diffusers in an aeration tank

Sewage liquor is run into deep tanks with diffuser grid aerationsystems that are attached to the floor. These are like the diffusedairstone used in tropical fish tanks but on a much larger scale. Airis pumped through the blocks and the curtain of bubbles formedboth oxygenates the liquor and also provide the necessary mixingaction. Where capacity is limited or the sewage is unusually strongor difficult to treat, oxygen may be used instead of air. Typically,the air is generated by some type of blower or compressor.

Jet Aerators

A jet aerator works through aspirating technology and thus doesnot require any external air source (i.e. compressor), except for thesurrounding atmosphere. Jet aerators can be installed either assubmersible units or piped through the tank wall using an externaldry-installed chopper pump to feed the aspirating ejector(s).

Surface aerators (cones)Vertically mounted tubes of up to 1 metre diameter extending from just above the base of a deep concrete tank to justbelow the surface of the sewage liquor. A typical shaft might be 10 metres high. At the surface end the tube isformed into a cone with helical vanes attached to the inner surface. When the tube is rotated, the vanes spin liquor upand out of the cones drawing new sewage liquor from the base of the tank. In many works each cone is located in aseparate cell that can be isolated from the remaining cells if required for maintenance. Some works may have twocones to a cell and some large works may have 4 cones per cell.

Activated sludge 71

References[1] Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons Ltd. LCCN 67019834.[2] Code of Practice, Flows and Loads-2, British Water (http:/ / www. britishwater. co. uk/ Document/ Download.

aspx?uid=3d63842c-eb86-48b1-be17-12eebf7487a5)[3] Review of UK and international standards (http:/ / products. ihs. com/ cis/ Doc. aspx?AuthCode=& DocNum=252510)[4] British Standard BS 6297:1983 (http:/ / www. standardsdirect. org/ standards/ standards4/ StandardsCatalogue24_view_5158. html)[5] Tilbury construction (http:/ / cat. inist. fr/ ?aModele=afficheN& cpsidt=8054833)[6] Deep Shaft Process Technology (http:/ / www. akerkvaerner. com/ NR/ rdonlyres/ 3EA1C953-F85D-4B29-80E3-516F0908D86E/ 12470/

DeepShaftProcessJuly2005. pdf)[7] Beychok, M.R. (1971). "Performance of surface-aerated basins". Chemical Engineering Progress Symposium Series 67 (107): 322–339.

Available at CSA Illumina website (http:/ / md1. csa. com/ partners/ viewrecord. php?requester=gs& collection=ENV& recid=7112203& q=&uid=788301038& setcookie=yes)

External links• Modelling using ASM1 (http:/ / www. iea. lth. se/ sbr/ iawq/ iawq. pdf)• Steady-state activated sludge model (http:/ / www. ce. utexas. edu/ prof/ speitel/ steady/ steady. htm)• Various PhD theses on modelling activated sludge systems (http:/ / biomath. ugent. be/ publications/ download)• Detailed algorithms for ASM1 and Takacs settling tank model (http:/ / www. benchmarkwwtp. org/ )• Metabolic activated sludge model (http:/ / www. darenet. nl/ nl/ page/ repository. item/

show?saharaIdentifier=tuddare:oai:tudelft. nl:161787)• Aerated, Partial Mix Lagoons (http:/ / www. epa. gov/ owm/ mtb/ apartlag. pdf) (Wastewater Technology Fact

Sheet by the U.S. Environmental Protection Agency‎)• Aerated Lagoon Technology (http:/ / www. ces. clemson. edu/ ees/ rich/ technotes/ index. html) (Linvil G. Rich,

Professor Emeritus, Department of Environmental Engineering and Science, Clemson University)• Modeling activated sludge (http:/ / fr. calameo. com/ books/ 00014533379b0de2c0fb2) (Y. Heymann, 2010)• Why surface aeration? Comparison between surface aerators or low-speed turbines and fine bubble bottom

aeration. (http:/ / www. aerator. be)

Slow sand filter 72

Slow sand filter

Slow filter in "Filtry Lindleya", Warsaw

Slow sand filters are used in water purification for treating raw waterto produce a potable product. They are typically 1 to 2 metres deep,can be rectangular or cylindrical in cross section and are used primarilyto treat surface water. The length and breadth of the tanks aredetermined by the flow rate desired by the filters, which typically havea loading rate of 0.1 to 0.2 metres per hour (or cubic metres per squaremetre per hour). Although they are often the preferred technology inmany developing countries, they are also used to treat water in some ofthe most developed countries such as the UK where they are used totreat water supplied to London.

Features

Typical configuration of a housed slow slow sandfilter system

Artificial infiltration works on the principles ofslow sand filters

Slow sand filters have a number of unique qualities:1. Unlike other filtration methods, slow sand filters use biological

processes to clean the water, and are non-pressurized systems. Slowsand filters do not require chemicals or electricity to operate.

2. Cleaning is traditionally by use of a mechanical scraper, which isusually driven into the filter bed once it has been dried out.However, some slow sand filter operators use a method called "wetharrowing", where the sand is scraped while still under water, andthe water used for cleaning is drained to waste;

3. For municipal systems there usually is a certain degree ofredundancy, it is desirable for the maximum required throughput ofwater to be achievable with one or more beds out of service;

4. Slow sand filters require relatively low turbidity levels to operateefficiently. In summer conditions and in conditions when the rawwater is turbid, blinding of the filters occurs more quickly andpre-treatment is recommended.

5. Unlike other water filtration technologies that produce water ondemand, slow sand filters produce water at a slow, constant flowrate and are usually used in conjunction with a storage tank for peakusage. This slow rate is necessary for healthy development of thebiological processes in the filter.[1] :38-41 [2]

While many municipal water treatment works will have 12 or more beds in use at any one time, smaller communitiesor households may only have one or two filter beds.In the base of each bed is a series of herringbone drains that are covered with a layer of pebbles which in turn iscovered with coarse gravel. Further layers of sand are placed on top followed by a thick layer of fine sand. Thewhole depth of filter material may be more than 1 metre in depth, the majority of which will be fine sand material.On top of the sand bed sits a supernatant layer of raw, unfiltered water.

Slow sand filter 73

How it worksSlow sand filters work through the formation of a gelatinous layer (or biofilm) called the hypogeal layer orSchmutzdecke in the top few millimetres of the fine sand layer. The Schmutzdecke is formed in the first 10–20 daysof operation[3] and consists of bacteria, fungi, protozoa, rotifera and a range of aquatic insect larvae. As aSchmutzdecke ages, more algae tend to develop and larger aquatic organisms may be present including somebryozoa, snails and Annelid worms. The Schmutzdecke is the layer that provides the effective purification in potablewater treatment, the underlying sand providing the support medium for this biological treatment layer. As waterpasses through the Schmutzdecke, particles of foreign matter are trapped in the mucilaginous matrix and dissolvedorganic material is adsorbed and metabolised by the bacteria, fungi and protozoa. The water produced from awell-managed slow sand filter can be of exceptionally good quality with 90-99% bacterial reduction.[4]

Slow sand filters slowly lose their performance as the Schmutzdecke grows and thereby reduces the rate of flowthrough the filter. Eventually it is necessary to refurbish the filter. Two methods are commonly used to do this. In thefirst, the top few millimetres of fine sand is scraped off to expose a new layer of clean sand. Water is then decantedback into the filter and re-circulated for a few hours to allow a new Schmutzdecke to develop. The filter is then filledto full depth and brought back into service.[4] The second method, sometimes called wet harrowing, involveslowering the water level to just above the Schmutzdecke, stirring the sand and thereby suspending any solids held inthat layer and then running the water to waste. The filter is then filled to full depth and brought back into service.Wet harrowing can allow the filter to be brought back into service more quickly. [3]

Advantages• As they require little or no mechanical power, chemicals or replaceable parts, and they require minimal operator

training and only periodic maintenance, they are often an appropriate technology for poor and isolated areas.• Slow sand filters, due to their simple design, may be created diy. DIY-slow sand filters have been used in

Afghanistan and other countries to aid the poor.[5]

• Slow sand filters are recognized by the World Health Organization [6], Oxfam, United Nations [7] and the UnitedStates Environmental Protection Agency [8] as being superior technology for the treatment of surface watersources. According to the World Health Organization, "Under suitable circumstances, slow sand filtration may benot only the cheapest and simplest but also the most efficient method of water treatment."

Disadvantages• Due to the low filtration rate, slow sand filters require extensive land area for a large municipal system.[1] :38-39

Many municipal systems in the U.S. initially used slow sand filters, but as cities have grown they subsequentlyinstalled rapid sand filters, due to increased demand for drinking water.

References[1] United States Environmental Protection Agency (EPA)(1990). Cincinnati, OH. "Technologies for Upgrading Existing or Designing New

Drinking Water Treatment Facilities." (http:/ / nepis. epa. gov/ Exe/ ZyPURL. cgi?Dockey=300048WU. txt) Document no.EPA/625/4-89/023.

[2] HDR Engineering (2001). Handbook of Public Water Systems (http:/ / books. google. com/ books?id=ieg7lvujyt0C& ). New York: JohnWiley and Sons. p. 353. ISBN 9780471292111. . Retrieved 2010-03-28.

[3] Centre for Affordable Water and Sanitation Technology, Biosand Filter Manual: Design, Construction, & Installation," July 2007.[4] National Drinking Water Clearinghouse (U.S.), Morgantown, WV. "Slow Sand Filtration." (http:/ / www. nesc. wvu. edu/ pdf/ DW/

publications/ ontap/ tech_brief/ TB15_SlowSand. pdf) Tech Brief Fourteen, June 2000.[5] DIY slow sand filter (http:/ / tilz. tearfund. org/ Publications/ Footsteps+ 31-40/ Footsteps+ 35/ The+ household+ slow+ sand+ filter. htm)[6] http:/ / www. who. int/ water_sanitation_health/ publications/ ssf/ en/ index. html[7] http:/ / www. the-ecentre. net/ resources/ e_library/ doc/ 11-WATER. pdf[8] http:/ / www. epa. gov/ safewater/ regs/ swtrsms. pdf

Slow sand filter 74

• "Learn More: Water (slow sand filter)" (http:/ / web. archive. org/ web/ 20070728135100/ http:/ / www.refugeecamp. org/ learnmore/ water/ slow_sand_filter. htm). Refugee Camp Project -. Doctors Without Borders.Archived from the original (http:/ / www. refugeecamp. org/ learnmore/ water/ slow_sand_filter. htm) on2007-07-28. Retrieved 2007-03-27.

• "Slow Sand Filtration" (http:/ / www. who. int/ water_sanitation_health/ publications/ ssf/ en/ index. html), WorldHealth Organization, 1974 ISBN 92-4-154037-0

• "UN High Commissioner for Refugees (UNHCR) Water Manual for Refugee Situations" (http:/ / www.the-ecentre. net/ resources/ e_library/ doc/ 11-WATER. pdf), Geneva, November 1992. Slow sand filtersrecommendations listed on page 38.

• "Small System Compliance Technology List for The Surface Water Treatment Rule" (http:/ / www. epa. gov/safewater/ regs/ swtrsms. pdf), United States Environmental Protection Agency, EPA 815-R-97-002 August 1997.Slow sand filtration is listed on page 24.

• http:/ / www. manzwaterinfo. ca Descriptive content, academic papers, and good links.• http:/ / www. biosandfilter. org• http:/ / www. slowsandfilter. com Links to academic papers and international slow sand filtration standards,

further explanations of how slow sand filtration works.• "Home-made biological filter" (http:/ / www. cms-uk. org/ water)• Modelling slow sand filters - PhD Thesis, Luiza Campos, Imperial College, 2002. PDF in 5 parts: Part 1 (http:/ /

tede. ibict. br/ tde_busca/ arquivo. php?codArquivo=205), Part 2 (http:/ / tede. ibict. br/ tde_busca/ arquivo.php?codArquivo=206), Part 3 (http:/ / tede. ibict. br/ tde_busca/ arquivo. php?codArquivo=207), Part 4 (http:/ /tede. ibict. br/ tde_busca/ arquivo. php?codArquivo=208), Part 5 (http:/ / tede. ibict. br/ tde_busca/ arquivo.php?codArquivo=209)

External links•  "Filter bed". Collier's New Encyclopedia. 1921.

Aerated lagoon 75

Aerated lagoon

Aerated lagoon used to treat wastewater from a hogfarm. Courtesy ofEnvironmental Dynamics Inc.

An aerated lagoon or aerated basin is a holdingand/or treatment pond provided with artificialaeration to promote the biological oxidation ofwastewaters.[1] [2] [3] There are many otherbiological processes for treatment of wastewaters,for example activated sludge, trickling filters,rotating biological contactors and biofilters. Theyall have in common the use of oxygen (or air) andmicrobial action to biotreat the pollutants inwastewaters.

Types of aerated lagoons or basins

• Suspension mixed lagoons, where there is lessenergy provided by the aeration equipment tokeep the sludge in suspension. [4]

• Faculative lagoons, where there is insufficientenergy provided by the aeration equipment tokeep the sludge in suspension and solids settle tothe lagoon floor. The biodegradable solids in thesettled sludge then degrade anaerobically.[4]

Suspension mixed lagoons

Suspension mixed lagoons flow through activatedsludge systems where the effluent has the samecomposition as the mixed liquor in the lagoon. Typically the sludge will have a residence time or sludge age of 1 to 5days. This means that the chemical oxygen demand(COD) removed is relatively little and the effluent is thereforeunacceptable for discharge into receiving waters.[4] The objective of the lagoon is therefore to act as a biologicallyassisted flocculator which converts the soluble biodegradable organics in the influent to a biomass which is able tosettle as a sludge.[4] Usually the effluent is then put in a second pond where the sludge can settle. The effluent canthen be removed from the top with a low COD, while the sludge accumulates on the floor and undergoes anaerobicstabilisation. .[4]

Methods of aerating lagoons or basinsThere are many methods for aerating a lagoon or basin:• Motor-driven floating surface aerators• Motor-driven submerged aerators• Motor-driven fixed-in-place surface aerators• Injection of compressed air through submerged diffusers

Aerated lagoon 76

Floating surface aerators

A Typical Surface-Aerated Basin (using motor-driven floating aerators)

Ponds or basins using floating surfaceaerators achieve 80 to 90% removal ofBOD with retention times of 1 to 10days.[5] The ponds or basins may rangein depth from 1.5 to 5.0 metres.[5]

In a surface-aerated system, the aeratorsprovide two functions: they transfer airinto the basins required by the biologicaloxidation reactions, and they provide themixing required for dispersing the airand for contacting the reactants (that is,oxygen, wastewater and microbes).Typically, the floating surface aeratorsare rated to deliver the amount of airequivalent to 1.8 to 2.7 kg O2/kWh.However, they do not provide as goodmixing as is normally achieved in activated sludge systems and therefore aerated basins do not achieve the sameperformance level as activated sludge units.[5]

Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of biologicalreactions increase with temperature. Most surface aerated vessels operate at between 4 °C and 32 °C.[5]

Submerged diffused aerationSubmerged diffused air is essentially a form of a diffuser grid inside a lagoon. There are two main types ofsubmerged diffused aeration systems for lagoon applications: floating lateral and submerged lateral. Both thesesystems utilize fine or medium bubble diffusers to provide aeration and mixing to the process water. The diffuserscan be suspended slightly above the lagoon floor or may rest on the bottom. Flexible airline or weighted air hosesupplies air to the diffuser unit from the air lateral (either floating or submerged). [6]

References[1] Middlebrooks, E.J., et al. (1982). Wastewater Stabilization Lagoon Design, Performance and Upgrading. McMillan Publishing.

ISBN 0-02-949500-8.[2] Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc. (4th

ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0.[3] Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834.[4] Henze, M., et al. (2008). Biological Wastewater Treatment. IWA Publishing. ISBN 1-84339-188-0.[5] Beychok, M.R. (1971). "Performance of surface-aerated basins". Chemical Engineering Progress Symposium Series 67 (107): 322–339.

Available at CSA Illumina website (http:/ / md1. csa. com/ partners/ viewrecord. php?requester=gs& collection=ENV& recid=7112203& q=&uid=788301038& setcookie=yes)

[6] http:/ / www. wastewater. com/ pdf/ 124. pdf

Aerated lagoon 77

External links• Wastewater Lagoon Systems in Maine (http:/ / www. lagoonsonline. com)• Aerated, Partial Mix Lagoons (http:/ / www. epa. gov/ owm/ mtb/ apartlag. pdf) (Wastewater Technology Fact

Sheet by the U.S. Environmental Protection Agency‎)• Aerated Lagoon Technology (http:/ / www. ces. clemson. edu/ ees/ rich/ technotes/ index. html) (Linvil G. Rich,

Professor Emeritus, Department of Environmental Engineering and Science, Clemson University)

Advanced oxidation processAdvanced Oxidation Processes (abbreviation: AOPs)refers to a set of chemical treatment procedures designed toremove organic and inorganic materials in waste water by oxidation. One such type of process is called In SituChemical Oxidation.Contaminants are oxidized by four different reagents: ozone, hydrogen peroxide, oxygen, and air, in precise,pre-programmed dosages, sequences, and combinations. These procedures may also be combined with UVirradiation and specific catalysts. This results in the development of hydroxyl radicals. A well known example ofAOP is the use of Fenton's reagent.The AOP procedure is particularly useful for cleaning biologically toxic or non-degradable materials such asaromatics, pesticides, petroleum constituents, and volatile organic compounds in waste water [1] . The contaminantmaterials are converted to a large extent into stable inorganic compounds such as water, carbon dioxide and salts, i.e.they undergo mineralization. A goal of the waste water purification by means of AOP procedures is the reduction ofthe chemical contaminants and the toxicity to such an extent that the cleaned waste water may be reintroduced intoreceiving streams or, at least, into a conventional sewage treatment.

References[1] Enric Brillasa, Eva Mur, Roser Sauleda, Laura Sànchez, José Peral, Xavier Domènech and Juan Casado (March 1998). "Aniline

mineralization by AOP's: anodic oxidation, photocatalysis, electro-Fenton and photoelectro-Fenton processes" (http:/ / www. sciencedirect.com/ science?_ob=ArticleURL& _udi=B6TF6-3VN03K2-3& _user=10& _rdoc=1& _fmt=& _orig=search& _sort=d& view=c&_version=1& _urlVersion=0& _userid=10& md5=d557b71411fdce56af991813ef191bf7). Applied Catalysis B: Environmental 16 (1): 31–42.doi:10.1016/S0926-3373(97)00059-3. .

• Michael OD Roth: Chemical oxidation: Technology for the Nineties, volume VI: Technologies for the Nineties: 6(Chemical oxidation) W. Wesley corner fields and John A. Roth, Technomic Publishing CO, Lancaster amongother things. 1997, ISBN 1566765978. (engl.)

• Oppenländer, Thomas (2003). Advanced Oxidation Processes (AOPs): Principles, Reaction Mechanisms, ReactorConcepts. Wiley VCH, Weinheim. ISBN 3-527-30563-7.

Aerobic treatment system 78

Aerobic treatment systemAn aerobic treatment system or ATS, often called (incorrectly) an aerobic septic system is a small scale sewagetreatment system similar to a septic tank system, but which uses an aerobic process for digestion rather than just theanaerobic process used in septic systems. These systems are commonly found in rural areas where public sewers arenot available, and may be used for a single residence or for a small group of homes.Unlike the traditional septic system, the aerobic treatment system produces a high quality secondary effluent, whichcan be sterilized and used for surface irrigation. This allows much greater flexibility in the placement of the leachfield, as well as cutting the required size of the leach field by as much as half.[1]

ProcessThe ATS process generally consists of the following phases:• Pre-treatment stage to remove large solids and other undesirable substances from the wastewater; this stage acts

much like a septic system, and an ATS may be added to an existing septic tank to further process the primaryeffluent.

• Aeration stage, where the aerobic bacteria digest the biological wastes in the wastewater.• Settling stage to allow any undigested solids to settle. This forms a sludge which must be periodically removed

from the system.• Disinfecting stage, where chlorine or similar disinfectant is mixed with the water, to produce an antiseptic output.The disinfecting stage is optional, and is used where a sterile effluent is required, such as cases where the effluent isdistributed above ground. The disinfectant typically used is tablets of calcium hypochlorite, which are speciallymade for waste treatment systems. Unlike the chlorine tablets used in swimming pools, which is stabilized forresistance to breakdown in ultraviolet light, the tablets used in waste treatment systems is intended to break downquickly in sunlight. Stabilized forms of chlorine will persist after the effluent is dispersed, and can kill off plants inthe leach field.[2]

Since the ATS contains a living ecosystem of microbes to digest the waste products in the water, excessive amountsof items such as bleach or antibiotics can damage the ATS environment and reduce treatment effectiveness.Non-digestible items should also be avoided, as they will build up in the system and require more frequent sludgeremoval.[3]

Types of aerobic treatment systemsSmall scale aerobic systems generally use one of two designs, fixed-film systems, or continuous flow, suspendedgrowth aerobic systems (CFSGAS). The pre-treatment and effluent handling are similar for both types of systems,and the difference lies in the aeration stage.[1]

Fixed film systemsFixed film systems use a porous medium which provides a bed to support the biomass film that digests the waste material in the wastewater. Designs for fixed film systems vary widely, but fall into two basic categories (though some systems may combine both methods). The first is a system where the media is moved relative to the wastewater, alternately immersing the film and exposing it to air, while the second uses a stationary media, and varies the wastewater flow so the film is alternately submerged and exposed to air. In both cases, the biomass must be exposed to both wastewater and air for the aerobic digestion to occur. The film itself may be made of any suitable porous material, such as formed plastic or peat moss. Simple systems use stationary media, and rely on intermittent, gravity driven wastewater flow to provide periodic exposure to air and wastewater. A common moving media system is the rotating biological contactor (RBC), which uses disks rotating slowly on a horizontal shaft. Approximately 40

Aerobic treatment system 79

percent of the disks are submerged at any given time, and the shaft rotates at a rate of one or two revolutions perminute.[1]

Continuous flow, suspended growth aerobic systemsCFSGAS systems, as the name imply, are designed to handle continuous flow, and do not provide a bed for abacterial film, relying rather on bacteria suspended in the wastewater. The suspension and aeration are typicallyprovided by an air pump, which pumps air through the aeration chamber, providing a constant stirring of thewastewater in addition to the oxygenation. A medium to promote fixed film bacterial growth may be added to somesystems designed to handle higher than normal levels of biomass in the wastewater.[1]

Retrofit or portable aerobic systemsAnother increasingly common use of aerobic treatment is for the remediation of failing or failed anaerobic septicsystems, by retrofitting an existing system with an aerobic feature. This class of product, known as aerobicremediation, is designed to remediate biologically failed and failing anaerobic distribution systems by significantlyreducing the BOD5 and TSS of the effluent. The reduction of the BOD5 and TSS reverses the developed bio-mat.Further, effluent with high dissolved oxygen and aerobic bacteria flow to the distribution component and digest thebio-mat.

Composting toiletsComposting toilets are designed to treat only toilet waste, rather than general residential waste water, and aretypically used with water-free toilets rather than the flush toilets associated with the above types of aerobic treatmentsystems. These systems treat the waste as a moist solid, rather than in liquid suspension, and therefore separate urinefrom feces during treatment to maintain the correct moisture content in the system. An example of a compostingtoilet is the clivus multrum (Latin for 'inclined chamber'), which consists of an inclined chamber that separates urineand feces and a fan to provide positive ventilation and prevent odors from escaping through the toilet. Within thechamber, the urine and feces are independently broken down not only by aerobic bacteria, but also by fungi,arthropods, and earthworms. Treatment times are very long, with a minimum time between removals of solid wasteof a year; during treatment the volume of the solid waste is decreased by 90%, with most being converted into watervapor and carbon dioxide. Pathogens are eliminated from the waste by the long durations in inhospitable conditionsin the treatment chamber.[4]

Comparison to traditional septic systemsThe aeration stage and the disinfecting stage are the primary differences from a traditional septic system; in fact, anaerobic treatment system can be used as a secondary treatment for septic tank effluent.[1] These stages increase theinitial cost of the aerobic system, and also the maintenance requirements over the passive septic system. Unlikemany other biofilters, aerobic treatment systems require a constant supply of electricity to drive the air pumpincreasing overall system costs. The disinfectant tablets must be periodically replaced, as well as the electricalcomponents (air compressor) and mechanical components (air diffusers). On the positive side, an aerobic systemproduces a higher quality effluent than a septic tank, and thus the leach field can be smaller than that of aconventional septic system, and the output can be discharged in areas too environmentally sensitive for septic systemoutput. Some aerobic systems recycle the effluent through a sprinkler system, using it to water the lawn whereregulations approve.

Aerobic treatment system 80

Effluent qualitySince the effluent from an ATS is often discharged onto the surface of the leach field, the quality is very important.A typical ATS will, when operating correctly, produce an effluent with less than 30 mg/liter biochemical oxygendemand, 25 mg/liter total suspended solids, and 10,000 cfu/mL fecal coliform bacteria. This is clean enough that itcannot support a biomat or "slime" layer like a septic tank.[5]

ATS effluent is relatively odorless; a properly operating system will produce effluent that smells musty, but not likesewage. Aerobic treatment is so effective at reducing odors, that it is the preferred method for reducing odor frommanure produced by farms.[6] [7] [8]

References[1] Office of Water, Office of Research and Development (February 2002). "Onsite Wastewater Treatment Systems Manual

(EPA/625/R-00/008)" (http:/ / www. epa. gov/ nrmrl/ pubs/ 625r00008/ html/ 625R00008. htm). U.S. Environmental Protection Agency. .[2] ATS chlorine tablets (http:/ / www. inspect-ny. com/ septic/ aerobicchlorine2. htm)[3] Items to Avoid (http:/ / www. hootsystems. com/ homeowners/ itemstoavoid. html), Hoot Aerobic Systems[4] Clivus Multrum, Inc.. "Composting Toilets and Greywater Systems Science & Technology" (http:/ / www. clivusmultrum. com/

science-technology. php). .[5] David M. Gustafson, James L. Anderson, Sara Heger Christopherson. "Aerobic Treatment Unit" (http:/ / www. extension. umn. edu/

distribution/ naturalresources/ DD7667. html). University of Minnesota Extension. .[6] Charles D. Fulhage and Donald L. Pfost. "Sewage Treatment Plants for Rural Homes" (http:/ / extension. missouri. edu/ explore/ envqual/

wq0403. htm). University of Missouri Extension. .[7] "LA-Hoot Homeowner's Manual" (http:/ / www. hootsystems. com/ homeowners/ pdffolder/ lahomeownersman. pdf). .[8] José R. Bicudo. "Frequently Asked Questions about Aerobic Treatment" (http:/ / www. bbe. umn. edu/ extens/ faq/ aerobicfaq. html).

University of Minnesota Extension. .

External links• Sewage Treatment Plants for Rural Homes (http:/ / muextension. missouri. edu/ explore/ envqual/ wq0403. htm)

at University of Missouri Extension• Aerobic Treatment Units (http:/ / www. cet. nau. edu/ Projects/ WDP/ resources/ treatmentsyst/ ATU. htm) at

Northern Arizona University• Diagram of SolarAir SATXN-500 (http:/ / www. solarair. net/ tank satxn. htm) 500 gallon per day ATS• Diagrams of Hoot Aerobic Systems (http:/ / www. hootsystems. com/ systems/ helpmechoose. html) different

ATS models• Features of Aerobic Treatment Systems (http:/ / www. norweco. com/ wiki/ ATS. htm) Norweco's ATS models• Retrofit Aerobic Treatment Systems (http:/ / www. septicairaid. com/ ) Convert Anaerobic System to Aerobic

system

Anaerobic digestion 81

Anaerobic digestion

Anaerobic digestion and regenerative thermaloxidiser component of Lübeck mechanical

biological treatment plant in Germany, 2007

Anaerobic digestion is a series of processes in which microorganismsbreak down biodegradable material in the absence of oxygen, used forindustrial or domestic purposes to manage waste and/or to releaseenergy.

The digestion process begins with bacterial hydrolysis of the inputmaterials in order to break down insoluble organic polymers such ascarbohydrates and make them available for other bacteria. Acidogenicbacteria then convert the sugars and amino acids into carbon dioxide,hydrogen, ammonia, and organic acids. Acetogenic bacteria thenconvert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide.Finally, methanogens convert these products to methane and carbon dioxide.[1]

It is used as part of the process to treat biodegradable waste and sewage sludge[2] As part of an integrated wastemanagement system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobicdigesters can also be fed with purpose grown energy crops such as maize.

Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, comprising ofmethane and carbon dioxide. This biogas can be used directly as cooking fuel, in combined heat and power gasengines[3] or upgraded to natural gas quality biomethane. The utilisation of biogas as a fuel helps to replace fossilfuels. The nutrient-rich digestate that is also produced can be used as fertilizer.

The technical expertise required to maintain industrial scale anaerobic digesters coupled with high capital costs andlow process efficiencies had limited the level of its industrial application as a waste treatment technology.[4]

Anaerobic digestion facilities have, however, been recognized by the United Nations Development Programme asone of the most useful decentralized sources of energy supply, as they are less capital intensive than large powerplants.[5]

Anaerobic digestion 82

History

Gas street lamp

Scientific interest in the manufacturing of gas produced by the naturaldecomposition of organic matter, was first reported in the seventeenth centuryby Robert Boyle and Stephen Hale, who noted that flammable gas was releasedby disturbing the sediment of streams and lakes.[6] In 1808, Sir Humphry Davydetermined that methane was present in the gases produced by cattle manure.[7]

[8] The first anaerobic digester was built by a leper colony in Bombay, India in1859. In 1895 the technology was developed in Exeter, England, where a septictank was used to generate gas for the sewer gas destructor lamp, a type of gaslighting. Also in England, in 1904, the first dual purpose tank for bothsedimentation and sludge treatment was installed in Hampton. In 1907, inGermany, a patent was issued for the Imhoff tank,[9] an early form of digester.

Through scientific research, anaerobic digestion gained academic recognition inthe 1930s. This research led to the discovery of anaerobic bacteria, themicroorganisms that facilitate the process. Further research was carried out toinvestigate the conditions under which methanogenic bacteria were able to growand reproduce.[10] This work was developed during World War II, during whichin both Germany and France there was an increase in the application ofanaerobic digestion for the treatment of manure.

Applications

Anaerobic digestion is particularly suited to organic material and is commonlyused for effluent and sewage treatment.[11] Anaerobic digestion is a simpleprocess that can greatly reduce the amount of organic matter, which mightotherwise be destined to be dumped at sea,[12] landfilled or burnt in an incinerator.[13]

Almost any organic material can be processed with anaerobic digestion.[14] [15] This includes biodegradable wastematerials such as waste paper, grass clippings, leftover food, sewage, and animal waste. The exception to this iswoody wastes that are largely unaffected by digestion, as most anaerobes are unable to degrade lignin. The exceptionbeing xylophalgeous anaerobes (lignin consumers), as used in the process for organic breakdown of cellulosicmaterial by a cellulosic ethanol start-up company in the U.S.[16] Anaerobic digesters can also be fed with speciallygrown energy crops such as silage for dedicated biogas production. In Germany and continental Europe, thesefacilities are referred to as biogas plants. A co-digestion or co-fermentation plant is typically an agriculturalanaerobic digester that accepts two or more input materials for simultaneous digestion.[17]

In developing countries, simple home and farm-based anaerobic digestion systems offer the potential for cheap,low-cost energy for cooking and lighting.[18] [19] [20] [21] Anaerobic digestion facilities have been recognized by theUnited Nations Development Programme as one of the most useful decentralized sources of energy supply.[5] From1975, China (See Bioenergy in China) and India have both had large government-backed schemes for adaptation ofsmall biogas plants for use in the household for cooking and lighting.[22] At the present time, projects for anaerobicdigestion in the developing world can gain financial support through the United Nations Clean DevelopmentMechanism if they are able to show that they provide reduced carbon emissions.[23]

Pressure from environmentally related legislation on solid waste disposal methods in developed countries has increased the application of anaerobic digestion as a process for reducing waste volumes and generating useful by-products. Anaerobic digestion may either be used to process the source separated fraction of municipal waste or alternatively combined with mechanical sorting systems, to process residual mixed municipal waste. These facilities

Anaerobic digestion 83

are called mechanical biological treatment plants.[24] [25] [26]

Utilising anaerobic digestion technologies can help to reduce the emission of greenhouse gasses in a number of keyways:• Replacement of fossil fuels• Reducing or eliminating the energy footprint of waste treatment plants• Reducing methane emission from landfills• Displacing industrially produced chemical fertilizers• Reducing vehicle movements• Reducing electrical grid transportation lossesMethane and power produced in anaerobic digestion facilities can be utilized to replace energy derived from fossilfuels, and hence reduce emissions of greenhouse gases.[27] This is due to the fact that the carbon in biodegradablematerial is part of a carbon cycle. The carbon released into the atmosphere from the combustion of biogas has beenremoved by plants in order for them to grow in the recent past. This can have occurred within the last decade, butmore typically within the last growing season. If the plants are re-grown, taking the carbon out of the atmosphereonce more, the system will be carbon neutral.[28] [29] This contrasts to carbon in fossil fuels that has been sequesteredin the earth for many millions of years, the combustion of which increases the overall levels of carbon dioxide in theatmosphere.If the putrescible waste processed in anaerobic digesters were disposed of in a landfill, it would break down naturallyand often anaerobically. In this case, the gas will eventually escape into the atmosphere. As methane is about twentytimes more potent as a greenhouse gas than carbon dioxide, this has significant negative environmental effects.[30]

Digester liquor can be used as a fertiliser supplying vital nutrients to soils. The solid, fibrous component of thedigested material can be used as a soil conditioner to increase the organic content of soils. The liquor can be usedinstead of chemical fertilisers that require large amounts of energy to produce and transport. The use ofmanufactured fertilisers is, therefore, more carbon-intensive than the use of anaerobic digester liquor fertiliser. Incountries, such as Spain where there are many organically depleted soils, the markets for the digested solids can beequally as important as the biogas.[31]

In countries that collect household waste, the utilization of local anaerobic digestion facilities can help to reduce theamount of waste that requires transportation to centralized landfill sites or incineration facilities. This reducedburden on transportation reduces carbon emissions from the collection vehicles. If localized anaerobic digestionfacilities are embedded within an electrical distribution network, they can help reduce the electrical losses that areassociated with transporting electricity over a national grid.[32]

In Oakland, California at the East Bay Municipal Utility District’s (EBMUD) Main Wastewater TreatmentPlant(MWWTP), food waste is currently co-digested with primary and secondary municipal wastewater solids andother high-strength wastes. Compared to municipal wastewater solids digestion, food waste digestion has manybenefits. Anaerobic digestion of food waste pulp from the EBMUD food waste process provides a higher normalizedenergy benefit, compared to municipal wastewater solids:• 730 to 1,300 kWh per dry ton of food waste applied.• 560 to 940 kWh per dry ton of municipal wastewater solids applied.[33] [34]

Power generationBiogas from sewage works is sometimes used to run a gas engine to produce electrical power; some or all of which can be used to run the sewage works.[35] Some waste heat from the engine is then used to heat the digester. It turns out that the waste heat is, in general, enough to heat the digester to the required temperatures. The power potential from sewage works is limited – in the UK there are about 80 MW total of such generation, with potential to increase to 150 MW, which is insignificant compared to the average power demand in the UK of about 35,000 MW. The

Anaerobic digestion 84

scope for biogas generation from non-sewage waste biological matter – energy crops, food waste, abattoir waste, etc.- is much higher, estimated to be capable of about 3,000 MW. Farm biogas plants using animal waste and energycrops are expected to contribute to reducing CO2 emissions and strengthen the grid while providing UK farmers withadditional revenues.[36]

Some countries offer incentives in the form of, for example, Feed-in Tariffs for feeding electricity onto the powergrid in order to subsidize green energy production.[37]

Grid injectionBiogas grid-injection is the injection of biogas into the natural gas grid.[38] As an alternative, the electricity and theheat can be used for on-site generation,[39] resulting in a reduction of losses in the transportation of energy. Typicalenergy losses in natural gas transmission systems range from 1–2%, whereas the current energy losses on a largeelectrical system range from 5–8%.[40]

In October 2010, Didcot Sewage Works became the first in the UK to produce biomethane gas supplied to thenational grid, for use in up to 200 homes in Oxfordshire.[41]

The processThere are many microorganisms that are involved in the process of anaerobic digestion including acetic acid-formingbacteria (acetogens) and methane-forming archaea (methanogens). These organisms feed upon the initial feedstock,which undergoes a number of different processes, converting it to intermediate molecules including sugars,hydrogen, and acetic acid, before finally being converted to biogas.[42]

Different species of bacteria are able to survive at different temperature ranges. Ones living optimally attemperatures between 35–40 °C are called mesophiles or mesophilic bacteria. Some of the bacteria can survive at thehotter and more hostile conditions of 55–60 °C; these are called thermophiles or thermophilic bacteria.[43]

Methanogens come from the domain of archaea. This family includes species that can grow in the hostile conditionsof hydrothermal vents. These species are more resistant to heat and can, therefore, operate at high temperatures, aproperty that is unique to thermophiles.[44]

As with aerobic systems, the bacteria in anaerobic systems the growing and reproducing microorganisms withinthem require a source of elemental oxygen to survive.[45] In an anaerobic system, there is an absence of gaseousoxygen. Gaseous oxygen is prevented from entering the system through physical containment in sealed tanks.Anaerobes access oxygen from sources other than the surrounding air. The oxygen source for these microorganismscan be the organic material itself or may be supplied by inorganic oxides from within the input material. When theoxygen source in an anaerobic system is derived from the organic material itself, the 'intermediate' end-products areprimarily alcohols, aldehydes, and organic acids plus carbon dioxide. In the presence of specialised methanogens, theintermediates are converted to the 'final' end-products of methane, carbon dioxide, and trace levels of hydrogensulfide.[46] [47] In an anaerobic system the majority of the chemical energy contained within the starting material isreleased by methanogenic bacteria as methane.[6]

Populations of anaerobic microorganisms typically take a significant period of time to establish themselves to befully effective. Therefore, it is common practice to introduce anaerobic microorganisms from materials with existingpopulations, a process known as "seeding" the digesters, and typically takes place with the addition of sewage sludgeor cattle slurry.[48]

Anaerobic digestion 85

Stages

The key process stages of anaerobic digestion

There are four key biological andchemical stages of anaerobicdigestion:[8]

1. Hydrolysis2. Acidogenesis3. Acetogenesis4. MethanogenesisIn most cases, biomass is made up of large organic polymers. In order for the bacteria in anaerobic digesters toaccess the energy potential of the material, these chains must first be broken down into their smaller constituentparts. These constituent parts or monomers such as sugars are readily available by other bacteria. The process ofbreaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore, hydrolysisof these high-molecular-weight polymeric components is the necessary first step in anaerobic digestion.[49] Throughhydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids.

Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules such asvolatile fatty acids (VFAs) with a chain length that is greater than that of acetate must first be catabolised intocompounds that can be directly utilised by methanogens.[50]

The biological process of acidogenesis is where there is further breakdown of the remaining components byacidogenic (fermentative) bacteria. Here, VFAs are created along with ammonia, carbon dioxide, and hydrogensulfide, as well as other by-products.[51] The process of acidogenesis is similar to the way that milk sours.The third stage of anaerobic digestion is acetogenesis. Here, simple molecules created through the acidogenesisphase are further digested by acetogens to produce largely acetic acid as well as carbon dioxide and hydrogen.[52]

The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here, methanogens utilise theintermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. It is thesecomponents that make up the majority of the biogas emitted from the system. Methanogenesis is sensitive to bothhigh and low pHs and occurs between pH 6.5 and pH 8.[53] The remaining, non-digestible material that the microbescannot feed upon, along with any dead bacterial remains, constitutes the digestate.A simplified generic chemical equation for the overall processes outlined above is as follows:

C6H12O6 → 3CO2 + 3CH4

Anaerobic digestion 86

Configuration

Farm-based maize silage digester located near Neumünster in Germany, 2007.Green inflatable biogas holder is shown on top of the digester

Anaerobic digesters can be designed andengineered to operate using a number ofdifferent process configurations:• Batch or continuous• Temperature: Mesophilic or thermophilic• Solids content: High solids or low solids• Complexity: Single stage or multistage

Batch or continuous

A batch system is the simplest form ofdigestion. Biomass is added to the reactor atthe start of the process in a batch and issealed for the duration of the process. Batchreactors suffer from odour issues that can bea severe problem when they are emptied. Ina typical scenario, biogas production will beformed with a normal distribution pattern over time. Operator can use this fact to determine when they believe theprocess of digestion of the organic matter has completed. As the batch digestion is simple and requires lessequipment and lower levels of design work, it is typically a cheaper form of digestion.[54]

In continuous digestion processes, organic matter is constantly added (continuous complete mixed) or added instages to the reactor (continuous plug flow; first in – first out). Here, the end-products are constantly or periodicallyremoved, resulting in constant production of biogas. A single or multiple digesters in sequence may be used.Examples of this form of anaerobic digestion include continuous stirred-tank reactors (CSTRs), Upflow anaerobicsludge blanket (UASB), Expanded granular sludge bed (EGSB) and Internal circulation reactors (IC).[55] [56]

TemperatureThere are two conventional operational temperature levels for anaerobic digesters, which are determined by thespecies of methanogens in the digesters:[57]

• Mesophilic, which takes place optimally around 30-38 °C or at ambient temperatures between 20-45 °C wheremesophiles are the primary microorganism present

• Thermophilic, which takes place optimally around 49-57 °C at elevated temperatures up to 70 °C wherethermophiles are the primary microorganisms present

A limit case has been reached in Bolivia, with anaerobic digestion in temperature working conditions less than10 °C. The anaerobic process is very slow, taking more than three times the normal mesophilic time process.[21] Inexperimental work at University of Alaska Fairbanks, a 1000 litre digester using psychrophiles harvested from "mudfrom a frozen lake in Alaska" has produced 200–300 litres of methane per day, about 20–30 % of the output fromdigesters in warmer climates.[58]

There are a greater number of species of mesophiles than thermophiles. These bacteria are also more tolerant tochanges in environmental conditions than thermophiles. Mesophilic systems are, therefore, considered to be morestable than thermophilic digestion systems.As mentioned above, thermophilic digestion systems are considered to be less stable, the energy input is higher, and more energy is removed from the organic matter. However, the increased temperatures facilitate faster reaction rates and, hence, faster gas yields. Operation at higher temperatures facilitates greater sterilization of the end-digestate. In countries where legislation, such as the Animal By-Products Regulations in the European Union, requires end

Anaerobic digestion 87

products to meet certain levels of reduction in the amount of bacteria in the output material, this may be a benefit.[59]

Certain processes shred the waste finely and use a short high-temperature and -pressure pre-treatment (pasteurization/ hygienisation) stage that significantly enhances the gas output of the following standard mesophilic stage. Thehygienisation process is also applied in order to reduce the pathogenic micro-organisms in the feedstock.Hygienisation/pasteurization may be achieved by using a Landia BioChop hygienisation unit [60] or similar methodof combined heat treatment and solids maceration.A drawback of operating at thermophilic temperatures is that more heat energy input is required to achieve thecorrect operational temperatures. This increase in energy may not be outweighed by the increase in the outputs ofbiogas from the systems. Therefore, it is important to consider an energy balance for these systems.

SolidsIn a typical scenario, there are three different operational parameters associated with the solids content of thefeedstock to the digesters:• High-solids (dry—stackable substrate)• High-solids (wet—pumpable substrate)• Low-solids (wet—pumpable substrate)High-solids (dry) digesters are designed to process materials with a high-solids content between ~25-40%. Unlikewet digesters that process pumpable slurries, high solids (dry – stackable substrate) digesters are designed to processsolid substrates without the addition of water. There are three primary styles of dry digesters: continuous verticalplug flow and batch tunnel horizontal digesters. Continuous vertical plug flow are upright, cylindrical tanks wherefeedstock is continuously fed to the top of the digester and flows downward by gravity during digestion. In batchtunnel digesters, the feedstock is deposited in tunnel-like chambers with a gas-tight door. Neither approach hasmixing inside the digester. The amount of pretreatment such as contaminant removal depends both upon the natureof the waste streams being processed and the desired quality of the digestate. Grinding for size reduction is beneficialin continuous vertical systems as it accelerates digestion, while batch systems avoid grinding and instead requirestructure (e.g. yard waste) to reduce compaction of the stacked pile. Continuous vertical dry digesters have a smallerfootprint due to the shorter effective retention time and vertical design.Wet digesters can be designed to operate in either a high-solids content, with a total suspended solids (TSS)concentration greater than ~20%, or a low-solids concentration less than ~15%.[61] [62]

High-solids (wet) digesters process a thick slurry that requires more energy input to move and process the feedstock.The thickness of the material may also lead to associated problems with abrasion. High-solids digesters will typicallyhave a lower land requirement due to the lower volumes associated with the moisture.High solids digesters requirecorrection of conventional performance calculations (e.g. gas production, retention time, kinetics, etc.) originallybased on very dilute sewage digestion concepts, since larger fractions of the feedstock mass are potentiallyconvertible to biogas.[63]

Low-solids (wet) digesters can transport material through the system using standard pumps that require significantlylower energy input. Low-solids digesters require a larger amount of land than high-solids due to the increasevolumes associated with the increased liquid-to-feedstock ratio of the digesters. There are benefits associated withoperation in a liquid environment, as it enables more thorough circulation of materials and contact between thebacteria and their food. This enables the bacteria to more readily access the substances they are feeding off andincreases the speed of gas yields.

Anaerobic digestion 88

Number of stages

Two-stage, low-solids, UASB digestion component of a mechanical biologicaltreatment system near Tel Aviv, process water is seen in balance tank and

sequencing batch reactor, 2005

Digestion systems can be configured withdifferent levels of complexity:[61]

• One-stage or single-stage• Two-stage or multistageA single-stage digestion system is one inwhich all of the biological reactions occurwithin a single sealed reactor or holdingtank. Utilising a single stage reducesconstruction costs, however facilitates lesscontrol of the reactions occurring within thesystem. Acidogenic bacteria, through theproduction of acids, reduce the pH of thetank. Methanogenic bacteria, as outlinedearlier, operate in a strictly defined pHrange.[64] Therefore, the biological reactionsof the different species in a single-stagereactor can be in direct competition witheach other. Another one-stage reaction system is an anaerobic lagoon. These lagoons are pond-like earthen basinsused for the treatment and long-term storage of manures.[65] Here the anaerobic reactions are contained within thenatural anaerobic sludge contained in the pool.

In a two-stage or multi-stage digestion system, different digestion vessels are optimised to bring maximum controlover the bacterial communities living within the digesters. Acidogenic bacteria produce organic acids and morequickly grow and reproduce than methanogenic bacteria. Methanogenic bacteria require stable pH and temperaturein order to optimise their performance.[66]

Under typical circumstances, hydrolysis, acetogenesis, and acidogenesis occur within the first reaction vessel. Theorganic material is then heated to the required operational temperature (either mesophilic or thermophilic) prior tobeing pumped into a methanogenic reactor. The initial hydrolysis or acidogenesis tanks prior to the methanogenicreactor can provide a buffer to the rate at which feedstock is added. Some European countries require a degree ofelevated heat treatment in order to kill harmful bacteria in the input waste.[67] In this instance, there may be apasteurisation or sterilisation stage prior to digestion or between the two digestion tanks. It should be noted that it isnot possible to completely isolate the different reaction phases, and often there is some biogas that is produced in thehydrolysis or acidogenesis tanks.

ResidenceThe residence time in a digester varies with the amount and type of feed material, the configuration of the digestionsystem, and whether it be one-stage or two-stage.In the case of single-stage thermophilic digestion, residence times may be in the region of 14 days, which, comparedto mesophilic digestion, is relatively fast. The plug-flow nature of some of these systems will mean that the fulldegradation of the material may not have been realised in this timescale. In this event, digestate exiting the systemwill be darker in colour and will typically have more odour.In two-stage mesophilic digestion, residence time may vary between 15 and 40 days.[68]

In the case of mesophilic UASB digestion, hydraulic residence times can be (1 hour to 1 day) and solid retentiontimes can be up to 90 days. In this manner, the UASB system is able to separate solid an hydraulic retention timeswith the utilisation of a sludge blanket.[69]

Anaerobic digestion 89

Continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mixthe contents enabling the bacteria and the food to be in contact. They also allow excess material to be continuouslyextracted to maintain a reasonably constant volume within the digestion tanks.

Feedstocks

Anaerobic lagoon & generators at the Cal Poly Dairy, United States 2003

The most important initial issue whenconsidering the application of anaerobicdigestion systems is the feedstock to theprocess. Digesters typically can accept anybiodegradable material; however, if biogasproduction is the aim, the level ofputrescibility is the key factor in itssuccessful application.[70] The moreputrescible (digestible) the material thehigher the gas yields possible from thesystem.

Substrate composition is a major factor indetermining the methane yield and methaneproduction rates from the digestion ofbiomass. Techniques to determine thecompositional characteristics of thefeedstock are available, while parameters such as solids, elemental, and organic analyses are important for digesterdesign and operation.[71]

Anaerobes can break down material with varying degrees of success from readily, in the case of short-chainhydrocarbons such as sugars, to over longer periods of time, in the case of cellulose and hemicellulose.[72] Anaerobicmicroorganisms are unable to break down long-chain woody molecules such as lignin.[73] Anaerobic digesters wereoriginally designed for operation using sewage sludge and manures. Sewage and manure are not, however, thematerial with the most potential for anaerobic digestion, as the biodegradable material has already had much of theenergy content taken out by the animal that produced it. Therefore, many digesters operate with co-digestion of twoor more types of feedstock. For example, in a farm-based digester that uses dairy manure as the primary feedstock,the gas production may be significantly increased by adding a second feedstock, e.g., grass and corn (typical on-sitefeedstock), or various organic byproducts, such as slaughterhouse waste, fats oils and grease from restaurants,organic household waste, etc. (typical off-site feedstock)..Digestors processing dedicated energy crops can achieve high levels of degradation and biogas production. [62] [74]

[75] Slurry-only systems are generally cheaper but generate far less energy than those using crops such as maize andgrass silage; by using a modest amount of crop material (30 per cent), an AD plant can increase energy outputtenfold for only three times the capital cost, relative to a slurry-only system.[76]

A second consideration related to the feedstock is moisture content. Dryer, stackable substrates, such as food- and yard-waste, are suitable for digestion in tunnel-like chambers. Tunnel-style systems typically have near-zero wastewater discharge as well, so this style of system has advantages where the discharge of digester liquids are a liability. The wetter the material the more suitable it will be to handling with standard pumps instead of energy intensive concrete pumps and physical means of movement. Also the wetter the material the more volume and area it takes up relative to the levels of gas that are produced. The moisture content of the target feedstock will also affect what type of system is applied to its treatment. In order to use a high-solids anaerobic digester for dilute feedstocks, bulking agents such as compost should be applied to increase the solid content of the input material.[77] Another key

Anaerobic digestion 90

consideration is the carbon:nitrogen ratio of the input material. This ratio is the balance of food a microbe requires inorder to grow. The optimal C:N ratio for the 'food' a microbe is 20–30:1.[78] Excess N can lead to ammoniainhibition of digestion.[74]

The level of contamination of the feedstock material is a key consideration. If the feedstock to the digesters hassignificant levels of physical contaminants such as plastic, glass, or metals, then pre-processing will be required inorder for the material to be used.[79] If it is not removed then the digesters can be blocked and will not functionefficiently. It is with this understanding that mechanical biological treatment plants are designed. The higher thelevel of pre-treatment a feedstock requires the more processing machinery will be required, and, hence, the projectwill have higher capital costs.[80]

After sorting or screening to remove any physical contaminants, such as metals, and plastics from the feedstock, thematerial is often shredded, minced, and mechanically or hydraulically pulped to increase the surface area available tomicrobes in the digesters and, hence, increase the speed of digestion. The maceration of solids can be achieved byusing a chopper pump to transfer the feedstock material into the airtight digester, where anaerobic treatment takesplace.

ProductsThere are three principal products of anaerobic digestion: biogas, digestate, and water.[61] [81] [82]

Biogas

Typical composition of biogas[83]

Matter %

Methane, CH4

50–75

Carbon dioxide, CO2

25–50

Nitrogen, N2

0–10

Hydrogen, H2

0–1

Hydrogen sulfide, H2S 0–3

Oxygen, O2

0–2

Biogas holder with lightning protection rods and back-up gas flare

Biogas is the ultimate waste product of thebacteria feeding off the input biodegradablefeedstock (the methanogenesis stage ofanaerobic digestion is performed by archaea- a micro-organism on a distinctly differentbranch of the phylogenetic tree of life tobacteria), and is mostly methane and carbondioxide,[84] [85] with a small amounthydrogen and trace hydrogen sulfide.(As-produced, biogas also contains watervapor, with the fractional water vaporvolume a function of biogastemperature).[63] Most of the biogas isproduced during the middle of the digestion,

Anaerobic digestion 91

Biogas carrying pipes

after the bacterial population has grown, and tapers off as theputrescible material is exhausted.[86] The gas is normally stored ontop of the digester in an inflatable gas bubble or extracted andstored next to the facility in a gas holder.

The methane in biogas can be burned to produce both heat andelectricity, usually with a reciprocating engine or microturbine[87]

often in a cogeneration arrangement where the electricity andwaste heat generated are used to warm the digesters or to heatbuildings. Excess electricity can be sold to suppliers or put into thelocal grid. Electricity produced by anaerobic digesters isconsidered to be renewable energy and may attract subsidies.[88]

Biogas does not contribute to increasing atmospheric carbondioxide concentrations because the gas is not released directly intothe atmosphere and the carbon dioxide comes from an organicsource with a short carbon cycle.

Biogas may require treatment or 'scrubbing' to refine it for use as afuel.[89] Hydrogen sulfide is a toxic product formed from sulfatesin the feedstock and is released as a trace component of the biogas.National environmental enforcement agencies such as the U.S. Environmental Protection Agency‎ or the English andWelsh Environment Agency put strict limits on the levels of gasses containing hydrogen sulfide, and, if the levels ofhydrogen sulfide in the gas are high, gas scrubbing and cleaning equipment (such as amine gas treating) will beneeded to process the biogas to within regionally accepted levels.[90] An alternative method to this is by the additionof ferrous chloride FeCl2 to the digestion tanks in order to inhibit hydrogen sulfide production.[91]

Volatile siloxanes can also contaminate the biogas; such compounds are frequently found in household waste andwastewater. In digestion facilities accepting these materials as a component of the feedstock, low-molecular-weightsiloxanes volatilise into biogas. When this gas is combusted in a gas engine, turbine, or boiler, siloxanes areconverted into silicon dioxide (SiO2), which deposits internally in the machine, increasing wear and tear.[92] [93]

Practical and cost-effective technologies to remove siloxanes and other biogas contaminants are available at thepresent time.[94] In certain applications, in situ treatment can be used to increase the methane purity by reducing theoffgas carbon dioxide content, purging the majority of it in a secondary reactor.[95]

In countries such as Switzerland, Germany, and Sweden, the methane in the biogas may be concentrated in order forit to be used as a vehicle transportation fuel or input directly into the gas mains.[96] In countries where the driver forthe utilisation of anaerobic digestion are renewable electricity subsidies, this route of treatment is less likely, asenergy is required in this processing stage and reduces the overall levels available to sell.[97]

DigestateDigestate is the solid remnants of the original input material to the digesters that the microbes cannot use. It alsoconsists of the mineralised remains of the dead bacteria from within the digesters. Digestate can come in three forms:fibrous, liquor, or a sludge-based combination of the two fractions. In two-stage systems, the different forms ofdigestate come from different digestion tanks. In single-stage digestion systems, the two fractions will be combinedand, if desired, separated by further processing.[98] [99]

Anaerobic digestion 92

Acidogenic anaerobic digestate

The second by-product (acidogenicdigestate) is a stable organic materialconsisting largely of lignin and cellulose,but also of a variety of mineral componentsin a matrix of dead bacterial cells; someplastic may be present. The materialresembles domestic compost and can beused as compost or to make low-gradebuilding products such as fibreboard.[100]

[101] The solid digestate can also be utilizedas feedstock for ethanol production.[102]

The third by-product is a liquid(methanogenic digestate) that is rich innutrients and can be used as a fertiliserdependent on the quality of the material being digested.[103] Levels of potentially toxic elements (PTEs) should bechemically assessed. This will be dependent upon the quality of the original feedstock. In the case of most clean andsource-separated biodegradable waste streams, the levels of PTEs will be low. In the case of wastes originating fromindustry, the levels of PTEs may be higher and will need to be taken into consideration when determining a suitableend use for the material.

Digestate typically contains elements such as lignin that cannot be broken down by the anaerobic microorganisms.Also the digestate may contain ammonia that is phytotoxic and will hamper the growth of plants if it is used as asoil-improving material. For these two reasons, a maturation or composting stage may be employed after digestion.Lignin and other materials are available for degradation by aerobic microorganisms such as fungi, helping reduce theoverall volume of the material for transport. During this maturation, the ammonia will be broken down into nitrates,improving the fertility of the material and making it more suitable as a soil improver. Large composting stages aretypically used by dry anaerobic digestion technologies.[104] [105]

WastewaterThe final output from anaerobic digestion systems is water. This water originates both from the moisture content ofthe original waste that was treated but also includes water produced during the microbial reactions in the digestionsystems. This water may be released from the dewatering of the digestate or may be implicitly separate from thedigestate.The wastewater exiting the anaerobic digestion facility will typically have elevated levels of biochemical oxygendemand (BOD) and chemical oxygen demand (COD). These are measures of the reactivity of the effluent and showan ability to pollute. Some of this material is termed 'hard COD', meaning that it cannot be accessed by the anaerobicbacteria for conversion into biogas. If this effluent were put directly into watercourses, it would negatively affectthem by causing eutrophication. As such, further treatment of the wastewater is often required. This treatment willtypically be an oxidation stage wherein air is passed through the water in a sequencing batch reactors or reverseosmosis unit.[106] [107] [108]

Anaerobic digestion 93

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Anaerobic digestion 95

[69] Finstein, M. S. (2006) ArrowBio process integrates preprocessing and advanced anaerobic digestion to recover recyclables and generateelectricity (http:/ / www. oaktech-environmental. com/ documents/ ProceedingsSWANA2006Nashville. pdf), oaktech-environmental.com.Retrieved 19.08.07.

[70] Anaerobic digestion feedstock classification (http:/ / www. wisbiorefine. org/ proc/ anaerobic. pdf), wisbiorefine.org. Retrieved 24.10.07.[71] Jerger, D. & Tsao, G. (2006) Feed composition in Anaerobic digestion of biomass, p65[72] Money for old rope? (http:/ / www. waste-management-world. com/ display_article/ 304404/ 123/ ARCHI/ none/ none/

Money-from-old-rope?/ ), waste-management-world.com. Retrieved 24.10.07.[73] Book Review: Biology of anaerobic microorganisms (http:/ / wwww. aslo. org/ lo/ toc/ vol_34/ issue_3/ 0647. pdf), aslo.org. Retrieved

24.10.07.[74]

This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia. org/wiki/ Template:cite_doi/ 10. 1016. 2f0961-9534. 2891. 2990036-c. 0a?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/editintro& action=edit)

[75] Richards, B. (1991). "High solids anaerobic methane fermentation of sorghum and cellulose". Biomass and Bioenergy 1: 47–53.doi:10.1016/0961-9534(91)90051-D.

[76] National Non-Food Crops Centre. Farm-Scale Anaerobic Digestion Plant Efficiency, NNFCC 11-015 (http:/ / www. nnfcc. co. uk/ tools/farm-scale-anaerobic-digestion-plant-efficiency-nnfcc-11-015)

[77] Management of Urban Biodegradable Waste (http:/ / books. google. com/ books?doi=XfhO-iP3em4C& pg=PA193& lpg=PA193&dq=high+ solids+ anaerobic+ digestion+ bulking& source=web& ots=fEDEUaGKK4&sig=B9CbvsX37xfMDLF4ILVhMKp2D-U#PPP1,M1), books.google.com. Retrieved 24.10.07.

[78] Anaerobic co-digestion of sewage sludge and rice straw (http:/ / www. bvsde. ops-oms. org/ bvsaar/ cdlodos/ pdf/ anaerobiccodigestion495.pdf), bvsde.ops-oms.org. Retrieved 24.10.07.

[79] Anaerobic digestion of classified municipal solid wastes (http:/ / www. seas. ucla. edu/ stenstro/ r/ r10), seas.ucla.edu. Retrieved 24.10.07.[80] National Non-Food Crops Centre. Economic Assessment of Anaerobic Digestion Technology & its Suitability to UK Farming & Waste

Systems (Report, 2nd Edition), NNFCC 10-010 (http:/ / www. nnfcc. co. uk/ tools/economic-assessment-of-anaerobic-digestion-technology-and-its-suitability-to-uk-farming-and-waste-systems-report-2nd-edition-nnfcc-10-010)

[81] Abstract from Operation of Municipal Wastewater Treatment Plants Manual of Practice-MOP 11 Fifth Edition (http:/ / www. e-wef. org/timssnet/ products/ tnt_products. cfm?primary_id=MOP1131& Action=LONG& subsystem=ORD), e-wef.org. Retrieved 19.08.07.

[82] Anaerobic digestion (http:/ / www. energy. ca. gov/ development/ biomass/ anaerobic. html), energy.ca.gov[83] Basic Information on Biogas (http:/ / www. kolumbus. fi/ suomen. biokaasukeskus/ en/ enperus. html), kolumbus.fi. Retrieved 2.11.07.[84] guide to biogas (http:/ / www. adelaide. edu. au/ biogas/ Beginners), adelaide.edu.au. Retrieved 19.08.07.[85] How Anaerobic Digestion (Methane Recovery) Works (http:/ / www. eere. energy. gov/ consumer/ your_workplace/ farms_ranches/ index.

cfm/ mytopic=30003), eere.energy.gov. Retrieved 19.08.07.[86] Anaerobic digestion briefing sheet (http:/ / www. foe. co. uk/ resource/ briefings/ anaerobic_digestion. pdf), foe.co.uk. Retrieved 24.10.07.[87] GE Energy – Jenbacher Gas Engines for Power Generation (http:/ / www. power-technology. com/ contractors/ cogeneration/ jenbacher/ ),

power-technology.com. Retrieved 19.08.07.[88] work3.pdf UK Biomass Strategy 2007 (http:/ / www. defra. gov. uk/ environment/ climatechange/ uk/ energy/ renewablefuel/ pdf/

ukbio0507-), defra.gov.uk , Retrieved 19.08.07.[89] What is anaerobic digestion? (http:/ / www. afbini. gov. uk/ index/ services/ specialist-advice/ renewable-energy/

re-anaerobic-digestion-intro/ re-anaerobic-digestion-what-is. htm), afbini.gov.uk. Retrieved 24.10.07.[90] Removal of hydrogen sulfide from anaerobic digester gas (http:/ / www. patentstorm. us/ patents/ 5976373-description. html), U.S. Patent,

patentstorm.us. Retrieved 17.08.07.[91] Abstract from Online Measurement of Dissolved and Gaseous-Hydrogen Sulfide in Anaerobic Biogas Reactors (http:/ / www. cheric. org/

research/ tech/ periodicals/ vol_view. php?seq=23639), cheric.org. Retrieved 24.10.07.[92] Wheles, E. & Pierece, E. (2004) Siloxanes in landfill and digester gas (http:/ / www. scsengineers. com/ Papers/

Pierce_2004Siloxanes_Update_Paper. pdf), scsengineers.com. Retrieved 17.08.07.[93] Biogas Upgrading and Utilisation, EEA Bioenergy (http:/ / www. iea-biogas. net/ Dokumente/ Biogas upgrading. pdf), iea-biogas.net.

Retrieved 25.10.07.[94] Tower, P.; Wetzel, J.; Lombard, X. (2006-03). "New Landfill Gas Treatment Technology Dramatically Lowers Energy Production Costs"

(http:/ / www. appliedfiltertechnology. com/ Userfiles/ Docs/ AFT_SWANA_2006_Paper_Rev1. pdf). Applied Filter Technology. . Retrieved2009-04-30. , appliedfiltertechnology.com

[95] Richards, B. (1994). "In situ methane enrichment in methanogenic energy crop digesters". Biomass and Bioenergy 6: 275–274.doi:10.1016/0961-9534(94)90067-1.

[96] Biogas as a road transport fuel (http:/ / www. nfuonline. com/ x9498. xml) nfuonline.com. Retrieved 24.10.07.[97] Haase biogas energy centre (http:/ / www. haase-energietechnik. de/ en/ Products_and_Services/ Waste_Treatment/ Biogas_Engineering/

FE-488-e_Biogas_CHP. pdf) haase-energietechnik.de. Retrieved 19.08.07.[98] Fact sheet on anaerobic digestion (http:/ / www. waste. nl/ page/ 248), waste.nl. Retrieved 19.08.07.[99] Biomass and biogas (http:/ / www. globalwarming101. com/ content/ view/ 595/ 88888958/ ) globalwarming101.com. Retrieved 19.08.07.

Anaerobic digestion 96

[100] Oaktech Consultation Response to UK Source Segregation Requirement (http:/ / www. alexmarshall. me. uk/ index_files/ documents/ResponsetoConsultationonthesourcesegregationrequirementinParagraph7AofSchedule3totheWasteMan. pdf), alexmarshall.me.uk. Retrieved19.08.07.

[101] UK Strategy for centralised anaerobic digestion (http:/ / www. ingentaconnect. com/ content/ els/ 09608524/ 1995/ 00000052/ 00000003/art00039), ingentaconnect.com. Retrieved 24.10.07.

[102] Solid Digestate to Ethanol (http:/ / onlinelibrary. wiley. com/ doi/ 10. 1002/ bit. 22627/ abstract), onlinelibrary.wiley.com Retrieved11.18.10

[103] Biomass and biogas (http:/ / www. globalwarming101. com/ content/ view/ 595/ 88888958/ ), globalwarming101.com. Retrieved 24.10.07.[104] Vitoria Plant Information (http:/ / www. ows. be/ pub/ Vitoria_InfoEnviro. mei07. pdf), ows.be. Retrieved 24.10.07.[105] Kompogas Homepage (http:/ / www. kompogas. ch/ en/ ), kompogas.ch. Retrieved 24.10.07.[106] Abstract: Modelling a sequencing batch reactor to treat the supernatant from anaerobic digestion of the organic fraction of municipal solid

waste (http:/ / www. ingentaconnect. com/ content/ jws/ jctb/ 2007/ 00000082/ 00000002/ art00006?crawler=true), ingentaconnect.com.Retrieved 24.10.07.

[107] Clarke Energy Reverse Osmosis Unit (http:/ / www. clarke-energy. co. uk/ clarke_waste/ water_treatment. htm), clarke-energy.co.uk.Retrieved 24.10.07.

[108] BOD Effluent Treatment (http:/ / web. archive. org/ web/ 20080524072038/ http:/ / www. virtualviz. com/ wastewater. htm),virtualviz.com. Retrieved 24.10.07.

External links• Official Website of the Anaerobic Digestion and Biogas Association (http:/ / www. adbiogas. co. uk/ ), Anaerobic

Digestion and Biogas Association (ADBA)• Online AD Cost Calculator (http:/ / www. nnfcc. co. uk/ tools/

economic-assessment-of-anaerobic-digestion-technology-and-its-suitability-to-uk-farming-and-waste-systems-ad-cost-model-tool-nnfcc-10-010),nnfcc.co.uk

• UK's Official Information Portal on Anaerobic Digestion and Biogas (http:/ / www. biogas-info. co. uk/ ),biogas-info.co.uk

• Glossary of Anaerobic Digestion terms (http:/ / www. bioplex. co. uk/ glossary. shtml), bioplex.co.uk• Anaerobic digestion forum (http:/ / listserv. repp. org/ pipermail/ digestion_listserv. repp. org/ ), listserv.repp.org• Anaerobic digestion website (http:/ / www. anaerobic-digestion. com/ ), anaerobic-digestion.com• US Government Information Sheet: Methane from anaerobic digesters (http:/ / web. archive. org/ web/

20041124201613/ www. eere. energy. gov/ consumerinfo/ factsheets/ ab5. html?print), web.archive.org• Anaerobic biodigester design for small tropical producers (http:/ / www. ruralcostarica. com/ biodigester. html),

ruralcostarica.com• Low cost biodigester, Vietnam (http:/ / www. cipav. org. co/ lrrd/ lrrd9/ 2/ an92. htm), cipav.org.co• Appropedia article on home biogas systems• Biogas Community on WikiSpaces (http:/ / www. biogas. wikispaces. com/ ), biogas.wikispaces.com• Online Anaerobic Digester Output Estimator (http:/ / www. bioplex. co. uk/ estimator. shtml), bioplex.co.uk• Biogas Forum (http:/ / forum. zorg-biogas. com/ ), forum.zorg-biogas.com• American Biogas Council (http:/ / www. americanbiogascouncil. org/ )• (http:/ / onlinelibrary. wiley. com/ doi/ 10. 1002/ bit. 22627/ abstract), Solid Digestate to Ethanol• Introduction to Biogas and Anaerobic Digestion (http:/ / www. extension. org/ pages/

Introduction_to_Biogas_and_Anaerobic_Digestion), information from eXtension's Livestock and PoultryEnvironmental Learning Center

• Harper Adams Energy Limited (http:/ / www. haenergy. co. uk), Information on the Anaerobic Digester at HarperAdams University College

Bioreactor 97

Bioreactor

Batch type bioreactor

General structure of batch type bioreactor

A bioreactor may refer to any manufactured or engineereddevice or system that supports a biologically activeenvironment.[1] In one case, a bioreactor is a vessel in which achemical process is carried out which involves organisms orbiochemically active substances derived from such organisms.This process can either be aerobic or anaerobic. Thesebioreactors are commonly cylindrical, ranging in size from litersto cubic meters, and are often made of stainless steel.

A bioreactor may also refer to a device or system meant to growcells or tissues in the context of cell culture. These devices arebeing developed for use in tissue engineering or biochemicalengineering.

On the basis of mode of operation, a bioreactor may beclassified as batch, fed batch or continuous (e.g. a continuousstirred-tank reactor model). An example of a continuousbioreactor is the chemostat.

Organisms growing in bioreactors may be suspended orimmobilized. A simple method, where cells are immobilized, isa Petri dish with agar gel. Large scale immobilized cellbioreactors are:

• moving media, also known as Moving Bed Biofilm Reactor(MBBR)

• packed bed• fibrous bed• membrane

Bioreactor 98

Bioreactor design

A closed bioreactor used in cellulosic ethanol research

Bioreactor design is a relatively complex engineeringtask, which is studied in the discipline of biochemicalengineering. Under optimum conditions, themicroorganisms or cells are able to perform theirdesired function with a 100 percent rate of success. Thebioreactor's environmental conditions like gas (i.e., air,oxygen, nitrogen, carbon dioxide) flow rates,temperature, pH and dissolved oxygen levels, andagitation speed/circulation rate need to be closelymonitored and controlled.

Most industrial bioreactor manufacturers use vessels,sensors and a control system networked together.

Fouling can harm the overall sterility and efficiency ofthe bioreactor, especially the heat exchangers. To avoidit, the bioreactor must be easily cleaned and as smoothas possible (therefore the round shape).

A heat exchanger is needed to maintain the bioprocessat a constant temperature. Biological fermentation is amajor source of heat, therefore in most casesbioreactors need refrigeration. They can be refrigeratedwith an external jacket or, for very large vessels, withinternal coils.

In an aerobic process, optimal oxygen transfer is perhaps the most difficult task to accomplish. Oxygen is poorlysoluble in water—even less in fermentation broths—and is relatively scarce in air (20.95%). Oxygen transfer isusually helped by agitation, which is also needed to mix nutrients and to keep the fermentation homogeneous. Thereare, however, limits to the speed of agitation, due both to high power consumption (which is proportional to the cubeof the speed of the electric motor) and to the damage to organisms caused by excessive tip speed. In practice,bioreactors are often pressurized; this increases the solubility of oxygen in water.

Photobioreactor

Moss photobioreactor withPhyscomitrella patens

A photobioreactor (PBR) is a bioreactor which incorporates some type of lightsource. Virtually any translucent container could be called a PBR, however theterm is more commonly used to define a closed system, as opposed to an opentank or pond. Photobioreactors are used to grow small phototrophic organismssuch as cyanobacteria, algae , or moss plants .[2] These organisms use lightthrough photosynthesis as their energy source and do not require sugars or lipidsas energy source. Consequently, risk of contamination with other organisms likebacteria or fungi is lower in photobioreactors when compared to bioreactors forheterotroph organisms.

Sewage treatment

Bioreactor 99

Bioreactors are also designed to treat sewage and wastewater. In the most efficient of these systems there is a supplyof free-flowing, chemically inert media that acts as a receptacle for the bacteria that breaks down the raw sewage.Examples of these bioreactors often have separate, sequential tanks and a mechanical separator or cyclone to speedthe division of water and biosolids. Aerators supply oxygen to the sewage and media further accelerating breakdown.Submersible mixers provide agitation in anoxic bioreactors to keep the solids in suspension and thereby ensure thatthe bacteria and the organic materials "meet". In the process, the liquids Biochemical Oxygen Demand (BOD) isreduced sufficiently to render the contaminated water fit for reuse. The biosolids can be collected for furtherprocessing or dried and used as fertilizer. An extremely simple version of a sewage bioreactor is a septic tankwhereby the sewage is left in situ, with or without additional media to house bacteria. In this instance, the biosludgeitself is the primary host (activated sludge) for the bacteria. Septic systems are best suited where there is sufficientlandmass and the system is not subject to flooding or overly saturated ground and where time and efficiency is not ofan essence.In bioreactors where the goal is to grow cells or tissues for experimental or therapeutic purposes, the design issignificantly different from industrial bioreactors. Many cells and tissues, especially mammalian ones, must have asurface or other structural support in order to grow, and agitated environments are often destructive to these celltypes and tissues. Higher organisms also need more complex growth media.Because they are the engine that drives biological wastewater treatment, it is critical to closely monitor the quantityand quality of microorganisms in bioreactors. One method for this is via 2nd Generation ATP tests.

NASA tissue cloning bioreactorNASA has developed a new type of bioreactor that artificially grows tissue in cell cultures. NASA's tissue bioreactorcan grow heart tissue, skeletal tissue, ligaments, cancer tissue for study, and other types of tissue.[3]For more information on artificial tissue culture, see tissue engineering.

References[1] IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) " bioreactor (http:/ /

goldbook. iupac. org/ B00662. html)".[2] Eva L. Decker und Ralf Reski (2008): Current achievements in the production of complex biopharmaceuticals with moss bioreactor.

Bioprocess and Biosystems Engineering 31, 3-9 (http:/ / www. springerlink. com/ content/ ux267q4q14736845/ fulltext. pdf)[3] http:/ / science. nasa. gov/ NEWHOME/ headlines/ msad05oct99_1. htm

External links• Photo-bioreactor (http:/ / inventgeek. com/ Projects/ photo-bio-reactor/ overview. aspx).

Carbon filtering 100

Carbon filteringCarbon filtering is a method of filtering that uses a piece of activated carbon to remove contaminants andimpurities, utilizing chemical adsorption.Each piece of carbon is designed to provide a large section of surface area, in order to allow contaminants the mostpossible exposure to the filter media. One pound (454g) of activated carbon contains a surface area of approximately100 acres.This carbon is generally activated with a positive charge and is designed to attract negatively charged watercontaminants. Carbon filtering is commonly used for water purification, but is also used in air purifiers.Carbon filters are most effective at removing chlorine, sediment, and volatile organic compounds (VOCs) fromwater. They are not effective at removing minerals, salts, and dissolved inorganic compounds.Typical particle sizes that can be removed by carbon filters range from 0.5 to 50 micrometres. The particle size willbe used as part of the filter description. The efficacy of a carbon filter is also based upon the flow rate regulation.When the water is allowed to flow through the filter at a slower rate, the contaminants are exposed to the filter mediafor a longer amount of time.

Types of carbon filters

Carbon filtering is usually used in water filtrationsystems. In this illustration, the activated carbon is in the

fourth level (counted from bottom).

There are two predominant types of carbon filters used in thefiltration industry: powdered block filters and granular activatedfilters. In general, carbon block filters are more effective atremoving a larger number of contaminants, based upon theincreased surface area of carbon. Many carbon filters also usesecondary media, such as silver or KDF-55, to prevent bacteriagrowth within the filter.

History of carbon filters

Carbon filters have been used for several hundred years and areconsidered one of the oldest means of water purification.Historians have shown evidence that carbon filtration may havebeen used in ancient Egyptian cultures for both air and watersanitization. 2000 B.C. Sanskrit text refers to filtering waterthrough charcoal (1905 translation of "Sushruta Samhita" byFrancis Evelyn Place). The first modern use of a carbon filter topurify potable water occurred in 1862. Carbon filtration wasfurther advanced in the mid 1970's by H. Allen Rice and AlvinE. Rice when they first manufactured a porous carbon block fordrinking water use.

Currently, carbon filters are used in individual homes aspoint-of-use water filters and, occasionally, in municipal watertreatment facilities. They are also used as pre-treatment devicesfor reverse osmosis systems and as specialized filters designed to remove chlorine-resistant cysts, such as giardia andcryptosporidium.

Carbon filtering 101

Hydrogen productionFor small scale production of hydrogen water purifiers are installed to prevent formation of minerals on the surfaceof the electrodes and to remove organics and chlorine from utility water. First the water passes through a 20micrometer interference (mesh or screen filter) filter to remove sand and dust particles, second, a charcoal filter(activated carbon) to remove organics and chlorine, third stage, a de-ionizing filter to remove metallic ions. A testcan be done before and after the filter for proper functioning on barium, calcium, potassium, magnesium, sodium andsilicon.

Radiation or nuclear medicineCarbon filters, along with HEPA filters, are widely used in the construction of hot cells. This allows the room toexhaust air that contains infinitesimal quantities of radioactivity and contaminants.

References

Constructed wetland

Vertical Flow type of Constructed Wetlands

A constructed wetland or wetpark isan artificial wetland, marsh or swampcreated as a new or restored habitat fornative and migratory wildlife, foranthropogenic discharge such aswastewater, stormwater runoff, orsewage treatment, for land reclamationafter mining, refineries, or otherecological disturbances such asrequired mitigation for naturalwetlands lost to a development.

Natural wetlands act as a biofilter,removing sediments and pollutantssuch as heavy metals from the water,and constructed wetlands can bedesigned to emulate these features.

Biofiltration

Vegetation in a wetland provides a substrate (roots, stems, and leaves) upon which microorganisms can grow as theybreak down organic materials. This community of microorganisms is known as the periphyton. The periphyton andnatural chemical processes are responsible for approximately 90 percent of pollutant removal and waste breakdown.The plants remove about seven to ten percent of pollutants, and act as a carbon source for the microbes when theydecay. Different species of aquatic plants have different rates of heavy metal uptake, a consideration for plantselection in a constructed wetland used for water treatment. Constructed wetlands are of two basic types:subsurface-flow and surface-flow wetlands.

Constructed wetland 102

Newly planted Constructed Wetland. Same Constructed Wetland, two years later.

Wetlands types

Natural wetlands

• Ramsar Classification System for Wetland Type - Ramsar Convention

Subsurface-flow wetlandsSubsurface-flow wetlands can be further classified as horizontal flow and vertical flow constructed wetlands.Subsurface-flow wetlands move effluent (agricultural or mining runoff, tannery or meat processing wastes,wastewater from sewage or storm drains, or other water to be cleansed) through a gravel lavastone or sand mediumon which plants are rooted. In subsurface-flow systems, the effluent may move either horizontally, parallel to thesurface, or vertically, from the planted layer down through the substrate and out. Subsurface horizontal-flowwetlands are less hospitable to mosquitoes, whose populations can be a problem in constructed wetlands.Carnivorous plants have been used to address this problem. Subsurface-flow systems have the advantage of requiringless land area for water treatment, but are not generally as suitable for wildlife habitat as are surface-flow constructedwetlands.

Surface-flow wetlandsSurface-flow wetlands move effluent above the soil in a planted marsh or swamp, and thus can be supported by awider variety of soil types including bay mud and other silty clays.Plantings of reedbeds are popular in European constructed wetlands, and plants such as cattails (Typha spp.), sedges,Water Hyacinth (Eichhornia crassipes) and Pontederia spp. are used worldwide. Recent research in use ofconstructed wetlands for subarctic regions has shown that buckbeans (Menyanthes trifoliata) and pendant grass(Arctophila fulva) are also useful for metals uptake.

Tidal Flow wetlandsTidal Flow wetlands are the latest evolution of wetland technology, used to treat domestic, agricultural & industrialwastewater, including heavy load. In this system, organic carbon is primarily oxidized with nitrate, which isproduced through a series of flood and drain cycles, from one side of the wetland to the other. This process holds anumber of benefits over traditional subsurface & surface-flow wetlands including, reduced land requirements andincreased de-nitrification capabilities for the treatment of heavy load.

Constructed wetland 103

General contaminants removalPhysical, chemical, and biological processes combine in wetlands to remove contaminants from wastewater. Anunderstanding of these processes is fundamental not only to designing wetland systems but to understanding the fateof chemicals once they enter the wetland. Theoretically, wastewater treatment within a constructed wetland occurs asit passes through the wetland medium and the plant rhizosphere. A thin film around each root hair is aerobic due tothe leakage of oxygen from the rhizomes, roots, and rootlets.[1] Aerobic and anaerobic micro-organisms facilitatedecomposition of organic matter. Microbial nitrification and subsequent denitrification releases nitrogen as gas to theatmosphere. Phosphorus is coprecipitated with iron, aluminium, and calcium compounds located in the root-bedmedium.[2] [3] [4] [5] [6] Suspended solids filter out as they settle in the water column in surface flow wetlands or arephysically filtered out by the medium within subsurface flow wetland cells. Harmful bacteria and viruses are reducedby filtration and adsorption by biofilms on the rock media in subsurface flow and vertical flow systems.

Specific contaminants removal

Nitrogen removalThe dominant forms of nitrogen in wetlands that are of importance to wastewater treatment include organic nitrogen,ammonia, ammonium, nitrate, nitrite, and nitrogen gases. Inorganic forms are essential to plant growth in aquaticsystems but if scarce can limit or control plant productivity.[7] Total Nitrogen refers to all nitrogen species.Wastewater nitrogen removal is important because of ammonia’s toxicity to fish if discharged into watercourses.Excessive nitrates in drinking water is thought to cause methemoglobinemia in infants, which decreases the blood'soxygen transport ability. The UK has experienced a significant increase in nitrate concentration in groundwater andrivers.[8]

Organic nitrogen

Mitsch & Gosselink define nitrogen mineralisation as "the biological transformation of organically combinednitrogen to ammonium nitrogen during organic matter degradation".[9] This can be both an aerobic and anaerobicprocess and is often referred to as ammonification. Mineralisation of organically combined nitrogen releasesinorganic nitrogen as nitrates, nitrites, ammonia and ammonium, making it available for plants, fungi and bacteria.[9]

Mineralisation rates may be affected by oxygen levels in a wetland.[6]

Ammonia removal

(NH3) and ammonium (NH )

The formation of ammonia (NH3) occurs via the mineralisation or ammonification of organic matter under eitheranaerobic or aerobic conditions.[10] The ammonium ion (NH ) is the primary form of mineralized nitrogen in mostflooded wetland soils. This ion forms when ammonia combines with water as follows:NH3 + H2O ⇌ NH + OH −[9]

Upon formation, several pathways are available to the ammonium ion. It can be absorbed by plants and algae andconverted back into organic matter, or the ammonium ion can be electrostatically held on negatively chargedsurfaces of soil particles.[9] At this point, the ammonium ion can be prevented from further oxidation because of theanaerobic nature of wetland soils. Under these conditions the ammonium ion is stable and it is in this form thatnitrogen predominates in anaerobic sediments typical of wetlands.[6] [11]

Most wetland soils have a thin aerobic layer at the surface. As an ammonium ion from the anaerobic sedimentsdiffuses upward into this layer it converts to nitrite or nitrified.[12] An increase in the thickness of this aerobic layerresults in an increase in nitrification.[6] This diffusion of the ammonium ion sets up a concentration gradient acrossthe aerobic-anaerobic soil layers resulting in further nitrification reactions.[6] [12]

Constructed wetland 104

Nitrification is the biological conversion of organic and inorganic nitrogenous compounds from a reduced state to amore oxidized state.[13] Nitrification is strictly an aerobic process in which the end product is nitrate (NO); thisprocess is limited when anaerobic conditions prevail.[6] Nitrification will occur readily down to 0.3 ppm dissolvedoxygen.[10] The process of nitrification (1) oxidizes ammonium (from the sediment) to nitrite (NO), and then (2)nitrite is oxidized to nitrate (NO). The overall nitrification reactions are as follows:

(1) 2NH + 3O2 ⇌ 4H+ + 2H2O + 2NO(2) 2NO + O2 ⇌ 2NO(Davies & Hart, 1990)Two different bacteria are required to complete this oxidation of ammonium to nitrate. Nitrosomonas sp. oxidizesammonium to nitrite via reaction (1), and Nitrobacter sp. oxidizes nitrite to nitrate via reaction (2).[10]

Denitrification is the biochemical reduction of oxidized nitrogen anions, nitrate (NO) and nitrite (NO) to producethe gaseous products nitric oxide (NO), nitrous oxide (N2O) and nitrogen gas (N2), with concomitant oxidation oforganic matter.[13] The general sequence is as follows:NO → NO → NO → N2O → N2The end products, N2O and N2 are gases that re-enter the atmosphere. Denitrification occurs intensely in anaerobicenvironments but also in aerobic conditions.[14] Oxygen deficiency causes certain bacteria to use nitrate in place ofoxygen as an electron acceptor for the reduction of organic matter.[6] Denitrification is restricted to a narrow zone inthe sediment immediately below the aerobic-anaerobic soil interface.[9] [15] Denitrification is considered to be thepredominant microbial process that modifies the chemical composition of nitrogen in a wetland system and themajor process whereby elemental nitrogen returns to the atmosphere.[6] [16] To summarize, the nitrogen cycle iscompleted as follows: ammonia in water, at or near neutral pH is converted to ammonium ions; the aerobicbacterium Nitrosomonas sp. oxidizes ammonium to nitrite; Nitrobacter sp. then converts nitrite to nitrate. Underanaerobic conditions, nitrate is reduced to relatively harmless nitrogen gas that enters the atmosphere.

Domestic sewage - ammonia

In a review of 19 surface flow wetlands it was found that nearly all reduced total nitrogen.[17] A review of bothsurface flow and subsurface flow wetlands concluded that effluent nitrate concentration is dependent on maintaininganoxic conditions within the wetland so that denitrification can occur and that subsurface flow wetlands weresuperior to surface flow wetlands for nitrate removal. The 20 surface flow wetlands reviewed reported effluentnitrate levels below 5 mg/L; the 12 subsurface flow wetlands reviewed reported effluent nitrate ranging from <1 to <10 mg/L.[18] Results obtained from the Niagara-On-The-Lake vertical flow systems show a significant reduction inboth total nitrogen and ammonia (> 97%) when primary treated effluent was applied at a rate of 60L/m²/day.Calculations showed that over 50% of the total nitrogen going into the system was converted to nitrogen gas.Effective removal of nitrate from the sewage lagoon influent was dependent on medium type used within the verticalcell as well as water table level within the cell.[19]

Mine water - ammonia

Constructed wetlands have been used to remove ammonia from mine drainage. In Ontario, Canada, drainage from the polishing pond at the Campbell Mine flows by gravity through a 9.3 hectare surface flow constructed wetland during the ice-free season.[20] Ammonia is removed by approximately 95% on inflows of up to unknown operator: u',' cubic metres (unknown operator: u'strong'unknown operator: u','cu ft)/day during the summer months, while removal rates decrease to 50-70% removal during cold months. This ammonia was oxidized to nitrate, which was immediately and quantitatively removed in the wetland. Surprisingly, and contrary to Reed (see above), anoxic conditions were not necessary for nitrate removal, which occurred as readily on leaf and detritus biofilm as it did in sediments. Other contaminants, including copper, are also removed in the wetland, such that the final discharge is fully detoxified. Campbell became one of the first gold mines in Ontario to produce a completely non-toxic

Constructed wetland 105

discharge, as determined by acute and chronic toxicity tests. At the Ranger Uranium Mine, in Australia, ammonia isremoved in "enhanced" natural wetlands (rather than fully engineered constructed wetlands), along with manganese,uranium and other metals.Other mines use natural or constructed wetlands to remove nitrogenous compounds from contaminated mine water,including cyanide (at the Jolu and Star Lake Mines, using natural muskeg and wetlands) and nitrate (demonstrated atthe Quinsam Coal Mine). Wetlands were also proposed to remove nitrogenous compounds (present as blastingresidues) from diamond mines in Northern Canada. However, land application is equally effective and is easier toimplement than a constructed wetland.

Phosphorus removalPhosphorus occurs naturally in both organic and inorganic forms. The analytical measure of biologically availableorthophosphates is referred to as soluble reactive phosphorus (SR-P). Dissolved organic phosphorus and insolubleforms of organic and inorganic phosphorus are generally not biologically available until transformed into solubleinorganic forms.[9]

In freshwater aquatic ecosystems phosphorus is typically the major limiting nutrient. Under undisturbed naturalconditions, phosphorus is in short supply. The natural scarcity of phosphorus is demonstrated by the explosivegrowth of algae in water receiving heavy discharges of phosphorus-rich wastes. Because phosphorus does not havean atmospheric component, unlike nitrogen, the phosphorus cycle can be characterized as closed. The removal andstorage of phosphorus from wastewater can only occur within the constructed wetland itself. Phosphorus may besequestered within a wetland system by:1. The binding of phosphorus in organic matter as a result of incorporation into living biomass,2. Precipitation of insoluble phosphates with ferric iron, calcium, and aluminium found in wetland soils.[9]

Biomass plants incorporation - phosphorus

Higher plants in wetland systems may be viewed as transient nutrient storage compartments absorbing nutrientsduring the growing season and releasing them at senescence.[21] [22] Generally, plants in nutrient-rich habitatsaccumulate more nutrients than those in nutrient-poor habitats, a phenomenon referred to as luxury uptake ofnutrients.[21] Aquatic vegetation may play an important role in phosphorus removal and, if harvested, extend the lifeof a system by postponing phosphorus saturation of the sediments.[21] [23] [24] Vascular plants may account for only asmall amount of phosphorus uptake with only 5 to 20% of the nutrients detained in a natural wetland being stored inharvestable plant material. Bernard and Solsky also reported relatively low phosphorus retention, estimating that asedge (Carex sp.) wetland retained 1.9 g of phosphorus per square meter of wetland.[22] [25] Bulrushes (Scirpus sp.)in a constructed wetland system receiving secondarily treated domestic wastes contained 40.5% of the totalphosphorus influent. The remaining 59.0% was found to be stored in the gravel substratum.[25] Phosphorus removalin a surface flow wetland treatment system planted with one of Scirpus sp., Phragmites sp. or Typha sp. wasinvestigated by Finlayson and Chick (1983).Phosphorus removal of 60%, 28%, and 46% were found for Scirpus sp., Phragmites sp. and Typha sp. respectively.This may prove to be a low estimate. Vascular plants are a major phosphorus storage compartment accounting for67.3% of the influent phosphorus.[23] Plant adsorption may reach 80% phosphorus removal.[26]

Only a small proportion (<20%) of phosphate removal by constructed wetlands can be attributed to nutritionaluptake by bacteria, fungi and algae.[27] The lack of seasonal fluctuation in phosphorus removal rates suggests that theprimary mechanism is bacterial and alga fixation.[28] However, this mechanism may be temporary, because themicrobial pool is small and quickly becomes saturated at which point the soil medium takes over as the majorcontributor to phosphate removal.[29]

Plants create a unique environment at the biofilm's attachment surface. Certain plants transport oxygen which is released at the biofilm/root interface, perhaps adding oxygen to the wetland system.[30] Plants also increase soil or

Constructed wetland 106

other root-bed medium hydraulic conductivity. As roots and rhizomes grow they are thought to disturb and loosenthe medium, increasing its porosity, which may allow more effective fluid movement in the rhizosphere. When rootsdecay they leave behind ports and channels known as macropores which are effective in channeling water throughthe soil.[31]

Whether or not wetland systems act as a phosphorus sink or source seems to depend on system characteristics suchas sediment and hydrology. There seems to be a net movement of phosphorus into the sediment in many lakes.[32] InLake Erie as much as 80% of the total phosphorus is removed from the waters by natural processes and ispresumably stored in the sediment. Marsh sediments high in organic matter act as sinks.[12] Phosphorus release froma marsh exhibits a cyclical pattern. Much of the spring phosphorus release comes from high phosphorusconcentrations locked up in the winter ice covering the marsh; in summer the marsh acts as a phosphorus sponge.[12]

Phosphorus is exported from the system following dieback of vascular plants.[33] Phosphorus concentrations in waterare reduced during the growing season due to plant uptake but decomposition and subsequent mineralisation oforganic matter releases phosphorus over the winter and accounts for the higher winter phosphorus concentrations inthe marsh.[9] [12]

Retention by soils or root-bed media - phosphorus

Two types of phosphate retention mechanisms may occur in soils or root-bed media: chemical adsorption onto themedium[34] and physical precipitation of the phosphate ion.[35] Both result from the attraction between phosphate ionand ions of Al, Fe or Ca [34] [36] and terminates with formation of various iron phosphates (Fe-P), aluminumphosphates (Al-P) or calcium phosphates (Ca-P).[4]

Oxidation-reduction potential (ORP, formally reported as Eh) of soil or water is a measure of its ability to reduce oroxidize chemical substances and may range between -350 and +600 millivolts (mV). Though redox potential doesnot affect phosphorus' oxidation state, redox potential is indirectly important because of its effect on iron solubility(through reduction of ferric oxides). Severely reduced conditions in the sediments may result in phosphorusrelease,[37] Typical wetland soils may have an Eh of -200 mV.[38] Under these reduced conditions Fe (Ferric iron)in insoluble ferric oxides may be reduced to soluble Fe (Ferrous iron). Any phosphate ion bound to the ferricoxide may be released back into solution as it dissolves[5] [35] However, the Fe diffusing in the water column maybe re-oxidized to Fe and re-precipitated as an iron oxide when it encounters oxygenated surface water. Thisprecipitation reaction may remove phosphate from the water column and deposit it back on the surface of sediments.[13] Thus, there can be a dynamic uptake and release of phosphorus in sediments that is governed by the amount ofoxygen in the water column. A well documented occurrence in the hypolimnion of lakes is the release of solublephosphorus when conditions become anaerobic.[39] [40] This phenomenon also occurs in natural wetlands. Oxygenconcentrations of less than 2.0 mg/l result in the release of phosphorus from sediments.[41] [42]

Domestic sewage - phosphorus

Adsorption to binding sites within sediments was the major phosphorus removal mechanism in the surface flowconstructed wetland system at Port Perry, Ontario[43] Release of phosphorus from the sediments occurred whenanaerobic conditions prevailed. The lowest wetland effluent phosphorus levels occurred when oxygen levels of theoverlying water column were above 1.0 mg / L. Removal efficiencies for total phosphorus were 54-59% with meaneffluent levels of 0.38 mg P/L. Wetland effluent phosphorus concentration was higher than influent levels during thewinter months.The phosphorus removed in a VF wetland in Australia over a short term was stored in the following wetlandcomponents in order of decreasing importance: substratum> macrophyte >biofilm, but over the long termphosphorus storage was located in macrophyte> substratum>biofilm components. Medium iron-oxide adsorptionprovides additional removal for some years.[44]

Constructed wetland 107

A comparison of phosphorus removal efficiency of two large-scale, surface flow wetland systems in Australia whichhad a gravel substratum to laboratory phosphorus adsorption indicated that for the first two months of wetlandoperation, the mean phosphorus removal efficiency of system 1 and 2 was 38% and 22%, respectively. Over the firstyear a decline in removal efficiencies occurred. During the second year of operation more phosphorus came out thanwas put in. This release was attributed to the saturation of phosphorus binding sites. Close agreement was foundbetween the phosphorus adsorption capacity of the gravel as determined in the laboratory and the adsorption capacityrecorded in the field.The phosphorus adsorption capacity of a subsurface flow constructed wetland system containing a predominantlyquartz gravel in the laboratory using the Langmuir adsorption isotherm was 25 mg P/g gravel.[23] Close agreementbetween calculated and realized phosphorus adsorption was found. The poor adsorption capacity of the quartz gravelimplied that plant uptake and subsequent harvesting were the major phosphorus removal mechanism.[45]

Metals removalConstructed wetlands have been used extensively for the removal of dissolved metals and metalloids. Although thesecontaminants are prevalent in mine drainage, they are also found in stormwater, landfill leachate and other sources(e.g., leachate or FDG washwater at coal-fired power plants), for which treatment wetlands have been constructed formines,[46] and other applications.[47]

Mine water - Acid drainage removalA seminal publication was a 1994 report from the US Bureau of Mines [48] described the design of wetlands fortreatment of acid mine drainage from coal mines. This report replaced the existing trial-and-error process with astrong scientific approach. This legitimized this technology and was followed in treating other contaminated waters.

Combined treatment ponds - commercial systems

The 3 treatment set-ups mostly employed in combined treatment ponds

Three types, using reed beds, are used. All thesesystems are used commercially, usually togetherwith septic tanks.[49] An other way is thecombination Constructed wetland- Compostingtoilet.

System types are:• Surface flow (SF) reed beds• Sub Surface Flow (SSF) reed beds• Vertical Flow (VF) reed bedsAll three types are placed in a closed basin with asubstrate. Also, for most commercialundertakings (e.g. agricultural enterprises), thebottom is covered with a rubber foil (tocompletely waterproof the whole, which isessential in urban areas). The substrate can beeither gravel, sand or lavastone.

Constructed wetland 108

Design characteristics - commercial systems

A commercial water-purifying pond, planted with Iris pseudacorus

• Surface flow reed beds - characterized by thehorizontal flow of wastewater across the rootsof the plants. They are no longer used as muchdue to the large land-area requirements topurify water—20 square metres (220 sq ft) perperson—and the increased smell and poorpurification in winter.[49]

• Subsurface flow reed beds - the flow ofwastewater occurs between the roots of theplants (and not at the water surface). As aresult the system is more efficient, lessodorous and less sensitive to winter conditions.Also, less area is needed to purifywater—5–10 square metres (54–110 sq ft). Adownside to the system are the intakes, which can clog easily.[49]

• Vertical flow reed beds - these are very similar to subsurface flow reed beds (subsurface wastewater flow ispresent here as well), according comparable advantages in efficiency and winter hardiness. The wastewater isdivided at the bottom with the assistance of a pump. Unlike the 2 previous systems, this system makes almostexclusive use of fine sand to increase bacteria counts. Intake of oxygen into the water is also better, and pumpingis pulsed to reduce obstructions within the intakes. The increased efficiency requires only 3 square metres(32 sq ft) of space per person.[49]

Plants and other organisms - commercial systems

Plants

In North America, cattails (Typha latifolia) are common in constructed wetlands because of their widespreadabundance, ability to grow at different water depths, ease of transport and transplantation, and broad tolerance ofwater composition (including pH, salinity, dissolved oxygen and contaminant concentrations). Elsewhere, CommonReed (Phragmites australis) are common (e.g. in greywater treatment systems to purify wastewater). Inself-purifying water reservoirs (used to purify rainwater) however, certain other plants are used as well. Thesereservoirs firstly need to be dimensioned to be filled with 1/4 of lavastone and water-purifying plants to purify acertain water quantity.[50]

They include a wide variety of plants, depending on the local climate and location. Plants are usually indigenous inthat location for ecological reasons and optimum workings. Plants that supply oxygen and shade are also added in tocomplete the ecosystem.

The plants used (placed on an area 1/4 of the water mass) are divided in 4 separate water depth-zones:1. 0–20 cm; Yellow Iris (Iris pseudacorus), Simplestem Bur-reed (Sparganium erectum), ... may be placed here

(temperate climates)2. 40–60 cm; Water Soldier (Stratiotes aloides), European Frogbit (Hydrocharis morsus-ranae), ... may be placed

here (temperate climates)3. 60–120 cm; European White Waterlily (Nymphaea alba), ... my be placed here (temperate climates)4. Below 120 cm; Eurasian Water-milfoil (Myriophyllum spicatum), may be placed here (temperate climates)The plants are usually grown on Coco Peat.[51] At the time of implantation to water-purifying ponds, de-nutrified soilis used to prevent unwanted algae and other organisms from taking over.

Constructed wetland 109

Fish and bacteria

Finally, locally grown bacteria and non-predatory fish are added to eliminate or reduce pests, such as mosquitos. Thebacteria are usually grown locally by submerging straw to support bacteria arriving from the surroundings.Three types of (non-predatory) fish are chosen to ensure that the fish can coexist: 1. surface; 2. middle-groundswimmers, and 3. bottom.

Examples of three types (for temperate climates) are:

A hybrid system using Flowforms in a treatment pond, in Norway.

1. Surface swimming fish: Common dace(Leuciscus leuciscus), Ide (Leuciscusidus), common rudd (Scardiniuserythrophthalmus), ...

2. Middle-swimmers: Common roach(Rutilus rutilus), ...

3. Bottom-swimming fish: Tench (Tincatinca), ...

Hybrid systems

Hybrid systems for example aerate the waterafter it exits the final reedbed using cascadessuch as Flowforms before holding the waterin a shallow pond.[52] Also, primarytreatments as septic tanks, and different types of pumps as grinder pumps may also be added.[53]

References

Literature citations

• Bernard, J.M.; Solsky, B.A. (1976). "Nutrient cycling in a Carex lacustris wetland". Canadian Journal of Botany55: 630–638. doi:10.1139/b77-077.

• Bhamidimarri, R; Shilton, A.; Armstrong, I.; Jacobsen, P.; Scarlet, D. (1991). "Constructed wetlands forwastewater treatment: the New Zealand experience.". Water Science Technology 24: 247–253.

• Bowmer, K.H. (1987). "Nutrient removal from effluents by an artificial wetland: influence of rhizosphere aerationand preferential flow studied using bromide and dye tracers". Water Research: 591–599.

• Breen, P.F. (1990). "A mass balance method for assessing the potential of artificial wetlands for wastewatertreatment". Water Research 24: 689–697. doi:10.1016/0043-1354(90)90024-Z.

• Brix, Hans (1994). "Use of constructed wetlands in water pollution control: Historical development, presentstatus, and future perspectives". Water Science & Technology 30 (8): 209–223.

• Burgoon, P.S.; Reddy, T.A. DeBusk. "Domestic wastewater treatment using emergent plants cultured in graveland plastic substrates". In Hammer 1989, pp. 536–541

• Burgoon, P.S.; Reddy, K.R.; DeBusk, T.A. (1991). "Vegetated submerged beds with artificial substrates II: N andP removal". Journal of Environmental Engineering 117 (4): 408–422.doi:10.1061/(ASCE)0733-9372(1991)117:4(408).

• Cole, C.V.; Olsen, S.R.; Scott, C.O. (1953). "The nature of phosphate sorption by calcium carbonate". SoilScience Society of America Proceedings 410.

• Cole, Stephen (1998). "The emergence of treatment wetlands". Environmental Science & Technology 32 (9):218–223.

• Conway, T.E.; Murtha, J. M. (1989). The Iselin marsh pond meadow In Hammer 1989, pp. 139–140.

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• Davies, T.H.; Hart, B.T. (1990). "Use of aeration to promote nitrification in reed beds treating wastewater".Advanced Water Pollution Control 11: 77–84.

• Finlayson, M.C.; Chick, A.J. (1983). "Testing the significance of aquatic plants to treat abattoir effluent". WaterResearch 17: 15–422.

• Fried, M.; Dean, L.A. (1955). "Phosphate retention by iron and aluminum in cation exchange systems". SoilScience Society American Proceedings: 143–47.

• Good, R.E.; Whigham, D.F.; Simpson, R.L., eds (1978). Freshwater wetlands, ecological processes andmanagement potential. New York: Academic Press.

• Gelt, Joe (1997). "Constructed Wetlands: Using Human Ingenuity, Natural Processes to Treat Water, BuildHabitat" [54]. ARROYO 9 (4).

• Guntensbergen, G.R.; Stearns, F.; Kadlec, J.A. (1989). "Wetland vegetation". In Hammer 1989, pp. 73–88• Hammer, D.A. (1992). Creating freshwater wetlands Lewis Publishers. Chelsea, MI.• Hammer, D.A., ed (1989). Constructed wetlands for wastewater treatment. Chelsea, Michigan: Lewis publishers.• Hammer, D.A.; Bastion, R.K. (1989). Wetlands ecosystems: Natural water purifiers?. In Hammer 1989, pp. 5–20• Hedin, R.S.; Nairn, R.W.; Kleinmann, R.L.P. (1994). "Passive treatment of coal mine drainage". Information

Circular (Pittsburgh, PA.: U.S. Bureau of Mines).• Herskowitz, J. (1986). Listowell artificial marsh project report. Ontario Ministry of the Environment project. p.

253.• Hsu, P.H. (1964). "Adsorption of phosphate by aluminum and iron in soils". Soil Science Society Proceedings 9:

474–478.• Jenssen, P.D., T.; Maehlum, T. Zhu; Warner, W.S. (1992). Cold-climate constructed wetlands. Aas, Norway:

JORDFORSK Centre for Soil and Environmental Research, N-1432.• Kadlec, R.H. (1989). Hydrologic factors in wetland water treatment. In Hammer 1989, pp. 21– 40• Kadlec, R. H. (1995). Wetland treatment at Listowel (revisited) unpublished.• Klopatek, J.M. (1978). Nutrient dynamics of Freshwater Riverine marshes and the role of emergent macrophytes.

In Good, Whigham & Simpson 1978, pp. 195–217• Kotz, J.C.; Purcell, K.F. (1987). Chemistry and chemical reactivity. New York, N.Y.: CBS College Publishing.• Kramer, J.R.; Allen, H.E., eds (1972). Nutrients in natural waters John. Toronto: Wiley and Sons.• Lantzke, I.R.; Mitchell, D.S.; Heritage, A.D.; Sharma, K.P. (1999). "A model controlling orthophosphate removal

in planted vertical flow wetlands". Ecological Engineering 12: 93–105. doi:10.1016/S0925-8574(98)00056-1.• Lemon, E. R.; Smith, I.D. (October 1993 Unpublished). Sewage waste amendment marsh process (SWAMP)

Interim report,.• Lemon, E.R., G.; Bis.; Braybrook, T.; Rozema, L.; Smith, I. (1997). Sewage waste amendment marsh process

(SWAMP) Final report.• Mann, R.A. (1990). "Phosphorus removal by constructed wetlands: substratum adsorption". Advanced Water

Pollution Control 11 h.• Mitsch, J.W.; Gosselink, J.G. (1986). Wetlands. New York: Van Nostrand Reinhold Company.• Moss, B. (1988). Ecology of freshwater Blackball Scientific Publishers. London.• Nichols, D.S.; Boelter, D.H. (1982). "Treatment of secondary sewage with a peat-sand filter bed". Journal

Environmental Quality 11 (1).• Niering, W.A. (1988). Wetlands: Audubon society nature guide.. Toronto: Random House of Canada Limited. p.

638.• Ontario Ministry of the Environment (1994). 23, Part 8, Sewage Systems. "Storm water management practices

planning and design manual". O.B.C.-Ontario Building Code Act (Queen's Printer for Ontario): 8–14.• Patrick, W.H., Jr.; Reddy, K.R. (1976). "Nitrification-denitrification in flooded soils and water bottoms:

dependence on oxygen supply and ammonium diffusion". Journal of Environmental Quality 5.

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• Reddy, K.R.; DeBusk, W.F. (1987). Reddy and, K.R.; Smith, W.H.. eds. "Nutrient storage capabilities of aquaticand wetland plants". Aquatic plants for water treatment and resource recovery (Magnolia Publishing Inc).

• Reed, S.C. (1986). "Wetlands as effluent treatment systems". Tech Press (Halifax, N.S): 207–219.• Reed, S.C. (1991). "Constructed Wetlands for Wastewater Treatment". BioCycle (January): 44–49.• Reed, S.C. (1995). Natural systems for waste management and treatment. McGraw Hill, Inc.• Reed, S.C.; Brown, D. (1995). "Subsurface flow wetlands-a performance evaluation". Water Environmental

Research 67: 244–248. doi:10.2175/106143095X131420.• Rogers, K.H.; Breen, P.F.; Chick, A.J. (1991). "Nitrogen removal in experimental wetland treatment systems:

evidence for the role of aquatic plants". Research Journal Water Political Control Fed 63: 934–941.• Rozema, L.R. , G.N.; Bis, T. Braybrook, E, R, Lemon; Smith, I. (1996). Retention of phosphorus in a Sub-surface

flow constructed wetland Presented at: The 31st central Canadian symposium on water pollution research,Burlington, Ontario.

• Sah, R.N.; Mikkelson, D. (1986). "Transformations of inorganic phosphorus during the flooding and drainingcycles of soil". American Journal Soil Science 50: 62–67. doi:10.2136/sssaj1986.03615995005000010012x.

• Smith, I.; Bis, G.N.; Lemon, E.R.; Rozema, L.R. (1997). "A thermal analysis of a vertical flow constructedwetland". Water Science Technology 35: 55–62. doi:10.1016/S0273-1223(97)00052-8.

• Snell, D. (1990). Port Perry artificial marsh sewage treatment system unpublished report.• Steiner, R.S.; Freeman Configuration and substrate design considerations for constructed wetlands wastewater

treatment, R.J.. In Hammer 1989, pp. 363–377• Tanner, C. C., J. S.; Clayton; Upsdell, M.P. (199). "Effect of loading rate and planting on treatment of dairy farm

wastewater’s in constructed wetlands-II removal of nitrogen phosphorus". Water Research 29: 27–34.doi:10.1016/0043-1354(94)00140-3.

• Thut, N.R. (1989). Utilisation of artificial marshes for treatment of pulp mill effluents. In Hammer 1989,pp. 239–251

• United States environmental protection agency. (1988). Design manual: constructed wetlands and aquatic plantsystems for municipal wastewater treatment EPA/625/1- 88/022. p. 83.

• van Oirschot, Dion; Zaakvoerder; Rietland; Poppel (2002) (in German). Certificering vanplantenwaterzuiveringssystemen [55]. Retrieved 2008-06-18.</ref>

• Watson, J.T.; Reed, S.C.; Kadlec, R.H.; Knight, R.L.; Whitehouse, A.E.. Performance expectations and loadingrates for constructed wetlands. In Hammer 1989, pp. 319–353

• Weber, L.R. (1990). Ontario soils Physical, chemical and biological properties and soil managementpractices—A reprint of Ontario Soils. Guelph, Ontario: Faculty and Staff of the Department of Land ResourcesScience Ontario Agricultural College University of Guelph.

• Wetzel, R.G. (1983). Limnology. Orlando, Florida: Saunders college publishing.• University of Alaska Agriculture and Forestry Station (2005). "Wetlands and wastewater treatment in Alaska".

Agroborealis 36 (2).

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Footnotes

[1] Hammer 1989[2] Hammer 1989, pp. 565–573 Brix, H.; Schierup, H.. "Danish experience with sewage treatment in constructed wetlands".[3] Davies & Hart 1990[4] Fried & Dean 1955[5] Sah & Mikkelson 1986[6] Patrick & Reddy 1976, pp. 469–472[7] Mitsch & Gosselink 1993[8] Gray, N.F. (1989). Biology of wastewater treatment. New York: Oxford University Press. p. 828.[9] Mitsch & Gosselink 1986, p. 536[10] Keeney 1973[11] Brock & Madigan 1991[12] Klopatek 1978[13] Wetzel 1983, pp. 255–297[14] Bandurski 1965[15] Nielson et al. 1990[16] Richardson, et. al. 1978[17] US EPA, 1988[18] Reed 1995[19] Smith et al. 1997[20] (http:/ / www. oma. on. ca/ environment/ resources/ oma_towards_greener_footprints. pdf)[21] Guntensbergen, Stearns & Kadlec 1989[22] Bernard & Solsky 1976[23] Breen 1990[24] Rogers, Breen & Chick 1991[25] Sloey, W.E.; Spangler, F.L.; Fetter, Jr, C.W.. "Management of freshwater wetlands For nutrient assimilation". pp. 321–340. In Good,

Whigham & Simpson 1978[26] Thut 1989[27] Moss 1988[28] Swindell 1990[29] Richardson 1985[30] Pride et al. 1990[31] Conley et al. 1991[32] Kramer, J.R.; Herbes, S.E.; Allen, H.E. (1972). Phosphorus: analysis of water, biomass, and sediment. In Kramer & Allen 1972, pp. 51–101[33] Simpson, R.L.; Whigham, D.F.. Seasonal patterns of nutrient movement in a freshwater tidal marsh. In Good, Whigham & Simpson 1978,

pp. 243–257[34] Hsu 1964[35] Faulkner, S.P.; Richardson, C.J. (1989). Physical and chemical characteristics of freshwater wetland soils. In Hammer 1989, pp. 41–131[36] Cole, Olsen & Scott 1953, pp. 352–356[37] Mann 1990, pp. 97–105[38] Hammer 1992, pp. 298[39] Burns, N.M.; Ross, C. (1972). Oxygen-nutrient relationships within the central basin of lake Erie. In Kramer & Allen 1972, pp. 193–250[40] Williams, J.D.H.; Mayer, T. (1972). Effects of sediment diagenesis and regeneration of phosphorus with special reference to lakes Erie and

Ontario. In Kramer & Allen 1972, pp. 281–315[41] Gosselink, J.G.; Turner, R.E.. The role of hydrology in freshwater wetland ecosystems. In Good, Whigham & Simpson 1978, pp. 63–78[42] Kramer & Allen 1972[43] Snell 1990[44] Lantzke et al. 1999[45] Lloyd R. Rozema, M.Sc. (excerpt from Master of Science thesis, Brock University, St. Catharines, ON, 2000)[46] (http:/ / technology. infomine. com/ enviromine/ wetlands/ Welcome. htm)[47] (http:/ / www. natural-resources. org/ minerals/ europe/ docs/ PIRAMID_Guidelines_v1. 0. pdf)[48] Hedin, Nairn & Kleinmann 1994[49] van Oirschot et al. 2002[50] "LavFilters" (http:/ / www. stowa-selectedtechnologies. nl/ Sheets/ Sheets/ Lava. Filters. html). . Retrieved 2008-06-18.[51] Coconut growing medium used for water purifying plants (http:/ / www. lukmertens. be/ kwekerij. html)[52] (http:/ / www. sheepdrove. com/ article. asp?art_id=115) Reedbed and Flowform cascade polishing, Sheepdrove Organic Farm, England[53] Pictures of hybrid reed bed systems (http:/ / www. pure-milieutechniek. be/ Page22. htm)[54] http:/ / ag. arizona. edu/ AZWATER/ arroyo/ 094wet. html[55] http:/ / www. certipro. be/ docs/ Certificering%20van%20plantenwaterzuiveringssystemen. pdf

Constructed wetland 113

External links• American Society of Professional Wetland Engineers website (http:/ / www. aspwe. org) - a wetland restoration

for habitat and treatment 'wiki'• On-line constructed wetlands workshop (http:/ / www. olawai. org) moderated by Greg Gearheart and Bob

Gearheart.• U.S.EPA: Constructed Wetlands resources website (http:/ / www. epa. gov/ owow/ wetlands/ watersheds/

cwetlands. html) - United States Environmental Protection Agency• Google Books: "Creating Freshwater Wetlands" - by Donald A. Hammer (http:/ / books. google. com/

books?id=t9WDhaX__KYC& printsec=frontcover)• Constructed wetlands in Lake Macquarie, Australia (http:/ / www. lakemac. com. au/ page. aspx?pid=109&

vid=10& fid=196& ftype=True)• Federal Park Wetlands, Australia (http:/ / www. ramin. com. au/ annandale/ wetlands. shtml)• Whites Creek Wetland, Australia (http:/ / www. ramin. com. au/ creekcare/ whitescreek. shtml)• Wetpark: Water treatment systems website (http:/ / www. holon. se/ folke/ projects/ vatpark/ concept. shtml)• WATER REPORT: Compost Toilets and Constructed Wetlands (http:/ / www. a-spi. org/ tp/ tp58. htm)

Dissolved air flotationDissolved air flotation (DAF) is a water treatment process that clarifies wastewaters (or other waters) by theremoval of suspended matter such as oil or solids. The removal is achieved by dissolving air in the water orwastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank or basin. The releasedair forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface ofthe water where it may then be removed by a skimming device.[1] [2] [3]

Dissolved air flotation is very widely used in treating the industrial wastewater effluents from oil refineries,petrochemical and chemical plants, natural gas processing plants, paper mills, general water treatment and similarindustrial facilities. A very similar process known as induced gas flotation is also used for wastewater treatment.Froth flotation is commonly used in the processing of mineral ores.In the oil industry, dissolved gas flotation (DGF) units do not use air as the flotation medium due to the explosionrisk. Natural gas is used instead to create the bubbles.

Dissolved air flotation 114

Process description

A typical dissolved air flotation unit (DAF)

Modern DAF units using parallel plate technology arequite compact.

Picture shows a 225 m³/h DAF.

The feed water to the DAF float tank isoften (but not always) dosed with acoagulant (such as ferric chloride oraluminum sulfate) to flocculate thesuspended matter.

A portion of the clarified effluentwater leaving the DAF tank is pumpedinto a small pressure vessel (called theair drum) into which compressed air isalso introduced. This results insaturating the pressurized effluentwater with air. The air-saturated waterstream is recycled to the front of thefloat tank and flows through a pressurereduction valve just as it enters thefront of the float tank, which results inthe air being released in the form oftiny bubbles. The bubbles adhere to thesuspended matter, causing thesuspended matter to float to the surfaceand form a froth layer which is thenremoved by a skimmer. The froth-freewater exits the float tank as theclarified effluent from the DAF unit.[1]

Some DAF unit designs utilize parallelplate packing material, lamellas, toprovide more separation surface and therefore to enhance the separation efficiency of the unit.

References[1] Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834.[2] Lawrence K. Wang, Yung-Tse Hung, Howard H. Lo and Constantine Yapijakis (2004). Handbook of Industrial and Hazardous Wastes

Treatment (2nd ed.). CRC Press. ISBN 0-8247-4114-5.[3] Kiuru, H.; Vahala, R., eds (2000). "Dissolved air flotation in water and waste water treatment". International conference on DAF in water

and waste water treatment No. 4, Helsinki, Finland. IWA Publishing, London. ISBN 1-900222-81-7.

External links• Treatment and Disposal of Ship-Generated Solid and Liquid Wastes (http:/ / www. rempec. org/ admin/ store/

wyswigImg/ file/ Information resources/ Other Meetings-Activities/ Port reception facilities/ Technical Reports/Activity B - Final Report Consolidated. pdf) (REMPEC Regional Marine Pollution Emergency Response Centrefor the Mediterranean Sea, Project MED.B4.4100.97.0415.8, April 2004)

• htm http:/ / www. kroftaswiss. com (http:/ / www. kroftaswiss. com)

Desalination 115

Desalination

Water desalination

Methods

• Distillation

• Multi-stage flash distillation (MSF)• Multiple-effect distillation (MED|ME)• Vapor-compression (VC)

• Ion exchange• Membrane processes

• Electrodialysis reversal (EDR)• Reverse osmosis (RO)• Nanofiltration (NF)• Membrane distillation (MD)

• Freezing desalination• Geothermal desalination• Solar desalination

• Solar humidification-Dehumidification (HDH)• Multiple-effect humidification (MEH)

• Methane hydrate crystallization• High grade water recycling• Seawater greenhouse

Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of saltand other minerals from water. More generally, desalination may also refer to the removal of salts and minerals,[1] asin soil desalination.[2]

Water is desalinated in order to convert salt water to fresh water so it is suitable for human consumption or irrigation.Sometimes the process produces table salt as a by-product. Desalination is used on many seagoing ships andsubmarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providingfresh water for human use in regions where the availability of fresh water is, or is becoming, limited.Large-scale desalination typically uses extremely large amounts of energy as well as specialized, expensiveinfrastructure, making it very costly compared to the use of fresh water from rivers or groundwater.[3]

However, along with recycled water this is one of the only non-rainfall dependent water sources particularly relevantto countries like Australia which traditionally have relied on rainfall in dams to provide their drinking water supplies.The world's largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in the United Arab Emirates. It isa dual-purpose facility that uses multi-stage flash distillation and is capable of producing 300 million cubic metres ofwater per year. By comparison the largest desalination plant in the United States is located in Tampa Bay, Floridaand operated by Tampa Bay Water, which began desalinating 34.7 million cubic meters of water per year inDecember 2007.[4] The Tampa Bay plant runs at around 12% the output of the Jebel Ali Desalination Plants. AJanuary 17, 2008, article in the Wall Street Journal states, "World-wide, 13,080 desalination plants produce morethan 12 billion gallons of water a day, according to the International Desalination Association."[5]

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Schematic of a multi-stage flash desalinatorA – Steam in

B – Seawater inC – Potable water out

D – Waste outE – Steam out

F – Heat exchangeG – Condensation collection

H – Brine heater

Plan of a typical reverse osmosis desalination plant

Methods

The traditional process used in theseoperations is vacuumdistillation—essentially the boiling ofwater at less than atmospheric pressureand thus a much lower temperaturethan normal. This is because theboiling of a liquid occurs when thevapor pressure equals the ambientpressure and vapor pressure increaseswith temperature. Thus, because of thereduced temperature, energy is saved.A leading distillation method ismulti-stage flash distillationaccounting for 85% of productionworldwide in 2004.[6]

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Reverse osmosis desalination plant in Barcelona,Spain

The principal competing processes use membranes to desalinate,principally applying reverse osmosis technology.[7] Membraneprocesses use semi-permeable membranes and pressure to separatesalts from water. Reverse osmosis plant membrane systems typicallyuse less energy than thermal distillation, which has led to a reduction inoverall desalination costs over the past decade. Desalination remainsenergy intensive, however, and future costs will continue to depend onthe price of both energy and desalination technology.

Considerations and criticism

Cogeneration

Cogeneration is the process of using excess heat from powerproduction to accomplish another task. For desalination, cogenerationis the production of potable water from seawater or brackishgroundwater in an integrated, or "dual-purpose", facility in which apower plant is used as the source of energy for the desalinationprocess. The facility’s energy production may be dedicated entirely to the production of potable water (a stand-alonefacility), or excess energy may be produced and incorporated into the energy grid (a true cogeneration facility).There are various forms of cogeneration, and theoretically any form of energy production could be used. However,the majority of current and planned cogeneration desalination plants use either fossil fuels or nuclear power as theirsource of energy. Most plants are located in the Middle East or North Africa, due to their petroleum resources andsubsidies. The advantage of dual-purpose facilities is that they can be more efficient in energy consumption, thusmaking desalination a more viable option for drinking water in areas of scarce water resources.[8] [9]

Shevchenko BN350, a nuclear-heated desalination unit

In a December 26, 2007, opinion column inthe The Atlanta Journal-Constitution, NolanHertel, a professor of nuclear andradiological engineering at Georgia Tech,wrote, "... nuclear reactors can be used ... toproduce large amounts of potable water. Theprocess is already in use in a number ofplaces around the world, from India to Japanand Russia. Eight nuclear reactors coupledto desalination plants are operating in Japanalone ... nuclear desalination plants could bea source of large amounts of potable watertransported by pipelines hundreds of milesinland..."[10]

Additionally, the current trend indual-purpose facilities is hybrid configurations, in which the permeate from an RO desalination component is mixedwith distillate from thermal desalination. Basically, two or more desalination processes are combined along withpower production. Such facilities have already been implemented in Saudi Arabia at Jeddah and Yanbu.[11]

A typical aircraft carrier in the U.S. military uses nuclear power to desalinate 400000 US gallons ( l;  imp gal) ofwater per day.[12]

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EconomicsA number of factors determine the capital and operating costs for desalination: capacity and type of facility, location,feed water, labor, energy, financing, and concentrate disposal. Desalination stills now control pressure, temperatureand brine concentrations to optimize the water extraction efficiency. Nuclear-powered desalination might beeconomical on a large scale.[13] [14]

While noting that costs are falling, and generally positive about the technology for affluent areas that are proximateto oceans, one study argues that "Desalinated water may be a solution for some water-stress regions, but not forplaces that are poor, deep in the interior of a continent, or at high elevation. Unfortunately, that includes some of theplaces with biggest water problems." and "Indeed, one needs to lift the water by 2000 metres (6600 ft), or transport itover more than 1600 kilometres (990 mi) to get transport costs equal to the desalination costs. Thus, it may be moreeconomical to transport fresh water from somewhere else than to desalinate it. In places far from the sea, like NewDelhi, or in high places, like Mexico City, high transport costs would add to the high desalination costs. Desalinatedwater is also expensive in places that are both somewhat far from the sea and somewhat high, such as Riyadh andHarare. In many places, the dominant cost is desalination, not transport; the process would therefore be relativelyless expensive in places like Beijing, Bangkok, Zaragoza, Phoenix, and, of course, coastal cities like Tripoli."[15]

After being desalinated at Jubail, Saudi Arabia, water is pumped 200 miles (320 km) inland through a pipeline to thecapital city of Riyadh.[16] For cities on the coast, desalination is being increasingly viewed as an untapped andunlimited water source.Desalination makes sense only after less expensive options are exhausted, including recycling water and fixingbroken infrastructure. Water is reused in Las Vegas NV, Fountain Valley CA, Fairfax VA, El Paso TX andScottsdale AZ. Compared to desalinated sea water, recycling requires 50% less energy due to the significantly lowersalt content and produces new water at 30% less cost to the consumer, without the damage to marine life andecosystems common to desalination plants.Israel is now desalinating water at a cost of US$0.53 per cubic meter.[17] Singapore is desalinating water forUS$0.49 per cubic meter.[18] Many large coastal cities in developed countries are considering the feasibility ofseawater desalination, due to its cost effectiveness compared with other water supply options, which can includemandatory installation of rainwater tanks or stormwater harvesting infrastructure. Studies have shown that thedesalination option is more cost-effective than large-scale recycled water for drinking, and more cost-effective inSydney than the vastly expensive option of mandatory installation of rainwater tanks or stormwater harvestinginfrastructure. The city of Perth has been successfully [19] operating a reverse osmosis seawater desalination plantsince 2006, and the Western Australian government have announced that a second plant will be built to serve thecity's needs. A desalination plant is now operating in Australia's largest city of Sydney,[20] and the Wonthaggidesalination plant under construction in Wonthaggi, Victoria.The Perth desalination plant is powered partially by renewable energy from the Emu Downs Wind Farm.[21] A windfarm at Bungendore in NSW has been purpose-built to generate enough renewable energy to offset the energy use ofthe Sydney plant,[22] mitigating concerns about harmful greenhouse gas emissions, a common argument used againstseawater desalination due to the energy requirements of the technology. The purchase or production of renewableenergy to power desalination plants naturally adds to the capital and/or operating costs of desalination. However,recent experience in Perth and Sydney indicates that the additional cost is acceptable to communities, as a city maythen augment its water supply without doing environmental harm to the atmosphere. The Queensland stategovernment also purchased renewable energy certificates on behalf of its Gold Coast plant which will see the plantoffset its carbon emissions for the initial 18 to 20 months of operations, bringing its environmental footprint down, inline with the other major plants that will be operating around the same time, in Perth and Sydney.In December 2007, the South Australian government announced that it would build a seawater desalination plant for the city of Adelaide, Australia, located at Port Stanvac. The desalination plant is to be funded by raising water rates to achieve full cost recovery.[23] [24] An online, unscientific poll showed that nearly 60% of votes cast were in favor

Desalination 119

of raising water rates to pay for desalination.[25]

A January 17, 2008, article in the Wall Street Journal states, "In November, Connecticut-based Poseidon ResourcesCorp. won a key regulatory approval to build the US$300 million water-desalination plant in Carlsbad, north of SanDiego. The facility would produce 50000000 US gallons ( l;  imp gal) of drinking water per day, enough to supplyabout 100,000 homes ... Improved technology has cut the cost of desalination in half in the past decade, making itmore competitive ... Poseidon plans to sell the water for about US $950 per acre-foot [1200 cubic metres (42000cu ft)]. That compares with an average US$700 an acre-foot [1200 m³] that local agencies now pay for water." [26]

$1,000 per acre-foot works out to $3.06 for 1,000 gallons, or $.81 for 1 cubic meter, which is the unit of watermeasurement that residential water users are accustomed to being billed in.[27]

While this regulatory hurdle was met, Poseidon Resources is not able to break ground until the final approval of amitigation project for the damage done to marine life through the intake pipe, as is required by California law.Poseidon Resources has made progress in Carlsbad, CA, despite its unsuccessful attempt to complete construction ofTampa Bay Desal, a desalination plant in Tampa Bay, FL, in 2001. The Board of Directors of Tampa Bay Waterwere forced to buy Tampa Bay Desal from Poseidon Resources in 2001 to prevent a third failure of the project.Tampa Bay Water faced five years of engineering problems and operation at 20% capacity due to marine life andgrowth captured and stuck to reverse osmosis filters prior to fully utilizing this facility in 2007.[28]

According to a May 9, 2008, article in Forbes, a San Leandro, California, company called Energy Recovery Inc. hasbeen desalinating water for US $0.46 per cubic meter.[29]

According to a June 5, 2008, article in the Globe and Mail, a Jordanian-born chemical engineering doctoral studentat the University of Ottawa, named Mohammed Rasool Qtaisha, has invented a new desalination technology that isalleged to be between 600% and 700% more water output per square meter of membrane than current technology.According to the article, General Electric is looking into similar technology, and the U.S. National ScienceFoundation announced a grant to the University of Michigan to study it as well. Because the patents were still beingworked out, the article was very vague about the details of this alleged technology.[30]

While desalinating 1000 US gallons (3800 l; 830 imp gal) of water can cost as much as $3, the same amount ofbottled water costs $7,945.[31]

Environmental

Intake

One of the main environmental considerations of ocean water desalination plants is the impact of the open oceanwater intakes, especially when co-located with power plants. Many proposed ocean desalination plants' initial plansrelied on these intakes despite perpetuating ongoing impacts on marine life. In the United States, due to a recentcourt ruling under the Clean Water Act, these intakes are no longer viable without reducing mortality, by 90%, of thelife in the ocean; the plankton, fish eggs and fish larvae.[32] There are alternatives, including beach wells thateliminate this concern, but require more energy and higher costs while limiting output.[33] Other environmentalconcerns include air pollution and greenhouse gas emissions from the power plants.

Outflow

To limit the environmental impact of returning the brine to the ocean, it can be diluted with another stream of water entering the ocean, such as the outfall of a waste water treatment plant or power plant. While seawater power plant cooling water outfalls are not freshwater like waste water treatment plant outfalls, the salinity of the brine will still be reduced. If the power plant is medium- to large-sized and the desalination plant is not enormous, the flow of the power plant's cooling water is likely to be at least several times larger than that of the desalination plant. Another method to reduce the increase in salinity is to spread the brine via a diffuser to mix in a mixing zone so that there is only a slight increase in salinity. For example, once the pipeline containing the brine reaches the sea floor, it can split

Desalination 120

off into many branches, each one releasing the brine gradually along its length through small holes. This method canbe used in combination with the joining of the brine with power plant or waste water plant outfalls.There are methods of desalination, particularly in combination with open pond evaporation (solar desalination), thatdo not discharge brine back into the ocean at all.The concentrated seawater has the potential to harm ecosystems, especially marine environments in regions with lowturbidity and high evaporation that already have elevated salinity. Examples of such locations are the Persian Gulf,the Red Sea and, in particular, coral lagoons of atolls and other tropical islands around the world.The UAE, Qatar, Bahrain, Saudi Arabia, Kuwait and Iran have 120 desalination plants between them. These plantsflush nearly 24 tons of chlorine, 65 tons of algae-harming antiscalants used to descale pipes, and around 300 kg ofcopper into the Persian Gulf every day. [34]

Because the brine is denser than the surrounding sea water due to the higher solute concentration, discharge intowater bodies means that the ecosystems on the bed of the water body are most at risk because the brine sinks andremains there long enough to damage the ecosystems. Careful re-introduction can minimize this problem. Forexample, for the desalination plant and ocean outlet structures to be built in Sydney from late 2007, the waterauthority states that the ocean outlets will be placed in locations at the seabed that will maximize the dispersal of theconcentrated seawater, such that it will be indistinguishable from normal seawater between 50 and 75 metres (160and 246 ft) from the outlet points. Sydney is fortunate to have typical oceanographic conditions off the coast thatallow for such rapid dilution of the concentrated byproduct, thereby minimizing harm to the environment.In Perth, Australia, in 2007, the Kwinana Desalination Plant was opened. The water is sucked in from the ocean atonly 0.1 metres per second (0.33 ft/s), which is slow enough to let fish escape. The plant provides nearly140000 cubic metres ( cu ft) of clean water per day.[35] This is the same at Queensland's Gold Coast DesalinationPlant and Sydney's Desalination Plant.

Desalination compared to other water supply options

Increased water conservation and water use efficiency remain the most cost-effective priorities in areas of the worldwhere there is a large potential to improve the efficiency of water use practices.[36] While comparing ocean waterdesalination to waste water reclamation for drinking water shows desalination as the first option, using reclamationfor irrigation and industrial use provides multiple benefits.[37] Urban runoff and storm water capture also providebenefits in treating, restoring and recharging groundwater.[38] A proposed alternative to desalinization in the state ofCalifornia and other areas in the American Southwest is the commercial importation of bulk water either by verylarge crude carriers converted to water carriers, or via pipelines. The idea is politically unpopular in Canada, wheregovernments have been scrambling to impose trade barriers to bulk water exports as a result of a claim filed in 1999under Chapter 11 of the North American Free Trade Agreement (NAFTA) by Sun Belt Water Inc. a companyestablished in 1990 in Santa Barbara, California, to address pressing local needs due to a severe drought in that area.Sun Belt maintains a web site where documents relating to their dispute are posted online.[39]

Experimental techniques and other developmentsIn the past, many novel desalination techniques have been researched with varying degrees of success.One such process which has recently been commercialised by Modern Water plc is a forward osmosis based processfor desalinated water, with a number of plants reported in operation.[40] [41] Other techniques have also attractedresearch funding. For example, to offset the energy requirements of desalination, the U.S. government is working todevelop practical solar desalination.As an example of newer theoretical approaches for desalination, focusing specifically on maximizing energyefficiency and cost effectiveness, the Passarell Process may be considered.[42]

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Other approaches involve the use of geothermal energy. From an environmental and economic point of view, in mostlocations geothermal desalination can be preferable to using fossil groundwater or surface water for human needs, asin many regions the available surface and groundwater resources already have long been under severe stress.Recent research in the U.S. indicates that nanotube membranes may prove to be extremely effective for waterfiltration and may produce a viable water desalination process that would require substantially less energy thanreverse osmosis.[43]

Another method being looked into for water desalination is the use of biomimetic membranes [44]

On June 23, 2008, it was reported that Siemens Water Technologies had developed a new technology, based onapplying electric field on seawater, that desalinates one cubic meter of water while using only 1.5 kWh of energy,which, according to the report, is one half the energy that other processes use.[45]

Fresh water can also be produced by freezing seawater, as happens naturally in the polar regions, and is known asfreeze-thaw desalination.According to MSNBC, a report by Lux Research estimated that the worldwide desalinated water supply will triplebetween 2008 and 2020.[46]

Low-temperature thermal desalinationLow-temperature thermal desalination (LTTD) takes advantage of the fact that water boils at low pressures, even aslow as ambient temperature. The system uses vacuum pumps to create a low pressure, low-temperature environmentin which water boils at a temperature gradient of 8 to 10 °C between two volumes of water. Cooling water issupplied from sea depths of as much as 600 metres (2000 ft). This cold water is pumped through coils to condensethe evaporated water vapor. The resulting condensate is purified water. The LTTD process may also take advantageof the temperature gradient available at power plants, where large quantities of warm waste water are dischargedfrom the plant, reducing the energy input needed to create a temperature gradient.[47]

The principle of LTTD is known for a long time, originally stemming from ocean thermal energy conversionresearch. Some experiments were conducted in U.S. and Japan to test the low-temperature driven desalinationtechnology. In Japan, a spray flash evaporation system was tested by Saga University.[48] In US, at Hawaii Islands,the National Energy Laboratory tested an open-cycle OTEC plant with fresh water and power production using atemperature of 20 °C between surface water and water at a depth of around 500 m. LTTD was studied by India'sNational Institute of Ocean Technology (NIOT) from 2004. Their first LTTD plant was opened in 2005 at Kavarattiin the Lakshadweep islands. The plant's capacity is 100000 litres (22000 imp gal; 26000 US gal)/day, at a capitalcost of INR 50 million (€922,000). The plant uses deep water at a temperature of 7 to 15 °C (45 to 59 °F).[49] In2007, NIOT opened an experimental floating LTTD plant off the coast of Chennai with a capacity of 1000000 litres( imp gal;  US gal)/day. A smaller plant was established in 2009 at the North Chennai Thermal Power Station toprove the LTTD application where power plant cooling water is available.[47] [50] [51]

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Thermo-ionic processIn October 2009, Saltworks Technologies, a Canadian firm, announced a process that uses solar or other thermal heatto drive an ionic current that empties all the sodium and chlorine ions from the water.[52]

Existing facilities and facilities under construction

Abu Dhabi, United Arab Emirates• Taweelah A1 Power and Desalination Plant has an output 385000000 litres ( imp gal;  US gal) per day of clean

water.• Umm Al Nar Desalination Plant has an output of 394000000 litres ( imp gal;  US gal) per day of clean water.• Fujairah F2 is to be completed by July 2010 will have a water production capacity of 492000000 litres ( imp gal;

 US gal) per day.[53]

ArubaThe island of Aruba has a large (world’s largest at the time of its inauguration) desalination plant with the totalinstalled capacity of 42,000 metric tons (11.1 million gallons or 42 × 103 m3) per day.[54]

AustraliaA combination of increased water usage and lower rainfall/drought in Australia has caused State governments tobuild a number of desalination plants, including the recently commissioned Kurnell Desalination Plant serving theSydney area. While desalination has been adopted by state governments to secure water supply, it is highly energyintensive (~$140 energy demand/ML) and has a high carbon footprint due to continued reliance on Australia'scoal-based energy generation.

Bahrain• The Al Hidd Desalination Plant on Muharraq island treats seawater through a multistage flash process, and

produces 30 million gallons per day. This project was completed in 2000. The Al Hidd distillate forwardingstation, comprises of a 410 million litres distillate water storage in 45 million litres steel tanks. A 135 millionlitres/day forwarding pumping station sends flows to the Hidd blending station, Muharraq blending station, Hoorablending station, Sanabis blending station and Seef blending station and which has an option for gravity supplyfor low flows to blending pumps and pumps which forward to Janusan, Budiya and Saar. [55]

• When completed in three phases, the Durrat Al Bahrain sea water reverse osmosis (SWRO) desalination plantwill have a capacity of 36,000 cubic meters of potable water per day which will serve the irrigation needs of theentire Durrat Al Bahrain development.[56] The Bahrain-based utility company, Energy Central Co (ECC) willprovide the plant a 25-year design, build and operate contract.[57]

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ChinaChina operates the Beijiang Desalination Plant in Tianjin, a combination desalination and coal-fired power plantdesigned to alleviate Tianjin's critical water shortage. Though the facility has the capacity to produce 200,000 cubicmeters of potable water per day, it has never operated at more than one quarter capacity due to difficulties with localutility companies and an inadequate local infrastructure.[58]

CyprusThere are also desalination plants in Cyprus, like the one near the town of Larnaca.[59] This is called the DhekeliaDesalination Plant, which utilises the reverse osmosis system.[60]

GibraltarThe fresh water supply in Gibraltar is supplied by a number of reverse osmosis and multi-stage flash desalinationplants.[61] . There is also a demonstation forward osmosis desalination plant operational.[62]

IsraelThe Hadera seawater reverse osmosis (SWRO) desalination plant in Israel is the largest of its kind in the world.[63]

[64] The project was developed as a Build-Operate-Transfer (BOT) by a consortium of three international companies:Veolia water, IDE Technologies and Elran.[65]

Existing Israeli water desalination facilities[66]

Location Opened Capacity(mln m3/year)

Cost ofwater

(per m3)

Notes

Ashkelon August 2005 120 (as of 2010) NIS 2.60 [67]

Palmachim May 2007 45 NIS 2.90 [68]

Hadera December 2009 127 NIS 2.60 [69]

Israeli water desalination facilities under construction

Location Opening Capacity(mln m3/year)

Cost of water(per m3)

Notes

Ashdod 2012 100 (expansion up to 150 possible) NIS 2.40 [70] [71]

Soreq 2013 150 (expansion up to 300 approved) NIS 2.01 – 2.19 [72]

MaldivesMaldives is a small island nation and most of the islands depend on desalination as a source of water.

Saudi ArabiaThe Saline Water Conversion Corporation of Saudi Arabia provides 50% of the municipal water in the Kingdom, operates a number of desalination plants, and has contracted $1892 million [73] to a Japanese-South Korean consortium to build one capable of producing a billion litres a day, opening at the end of 2013. They currently operate approximately 14 plants in the Kingdom;[74] one example at Shoaiba cost $1060 million and produces 450

Desalination 124

million litres a day.

United Kingdom

Beckton Desalination Plant

The first large scale water desalination plant in the United Kingdom, the Thames Water Desalination Plant,[75] hasbeen built in Beckton, east London for Thames Water by Acciona Agua

United States

El Paso (Texas) Desalination Plant

Brackish groundwater has been treated at the El Paso plant since around 2004. Producing 27500000 US gallons ( l; imp gal) of fresh water daily (about 25% of total freshwater deliveries) by reverse osmosis, it is a crucialcontribution to water supplies in this water-stressed city.[76]

Tampa Bay Water Desalination Project

The Tampa Bay Water Desalination project was originally a private venture led by Poseidon Resources. This projectwas delayed by the bankruptcy of Poseidon Resources' successive partners in the venture, Stone & Webster, thenCovanta (formerly Ogden) and its principal subcontractor Hydranautics. Poseidon's relationship with Stone &Webster through S & W Water LLC ended in June 2000 when Stone & Webster declared bankruptcy and PoseidonResources purchased Stone & Webster's stake in S & W Water LLC. Poseidon Resources partnered with Covantaand Hydranautics in 2001, changing the consortium name to Tampa Bay Desal. Through the inability of Covanta tocomplete construction bonding of the project, the Tampa Bay Water agency was forced to purchase the project fromPoseidon on May 15, 2002, and underwrite the project financing under its own credit rating. Tampa Bay Water thencontracted with Covanta Tampa Construction, which produced a project that did not meet required performancetests. Covanta Tampa Construction's parent company filed bankruptcy in October 2003 to prevent losing the contractwith Tampa Bay Water. Then, Covanta Tampa Construction filed bankruptcy prior to performing renovations thatwould have satisfied contractual agreements. This resulted in nearly six months of litigation between Covanta TampaConstruction and Tampa Bay Water. In 2004, Tampa Bay Water hired a renovation team, American Water/AccionaAqua, to bring the plant to its original, anticipated design. The plant was deemed fully operational in 2007[28] and isdesigned to run at a maximum capacity of 25 million gallons per day.[77] Nevertheless, the plant continues to be setwith problems limiting it to producing only about half that amount (14 million gallons per day or 42 af/day in2009.[78]

Yuma Desalting Plant (Arizona)

The Yuma Desalting Plant was constructed under authority of the Colorado River Basin Salinity Control Act of 1974to treat saline agricultural return flows from the Wellton-Mohawk Irrigation and Drainage District. The treated wateris intended for inclusion in water deliveries to Mexico thereby preserving the like amount of water in Lake Mead.Construction of the plant was completed in 1992 and it has operated on two occasions since then. The plant has beenmaintained, but largely not operated due to surplus and then normal water supply conditions on the ColoradoRiver.[79] An agreement was reached in April 2010 between the Southern Nevada Water Authority, the MetropolitanWater District of Southern California, the Central Arizona Project and the U.S. Bureau of Reclamation to underwritethe cost of running the plant in a year long pilot project.[80]

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Trinidad and TobagoThe Republic of Trinidad and Tobago is using desalination to free up more of the island's water supply for drinkingpurposes. The desalination facility, opened in March 2003, is considered to be the first of its kind. It is the largestdesalination facility in the Americas and will process 28800000 US gallons ( l;  imp gal) of water a day and sellwater at the price of $2.67 per 1000 US gallons (3800 l; 830 imp gal).[81] This facility will be located at Trinidad'sPoint Lisas Industrial Estate, a park of more than 12 companies in various manufacturing and processing functionsand will allow for easy access to water for both factories and residents in the country.[82]

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People's Daily Online, 2001-08-03, via english.people.com.cn. Retrieved on 2007-08-19.[3] Fischetti, Mark (September 2007). "Fresh from the Sea". Scientific American 297 (3): 118–119. doi:10.1038/scientificamerican0907-118.

PMID 17784633.[4] Applause, At Last, For Desalination Plant (http:/ / www2. tbo. com/ content/ 2007/ dec/ 22/ na-applause-at-last-for-desalination-plant/ ), The

Tampa Tribune, December 22, 2007[5] Kathryn Kranhold, Water, Water, Everywhere... (http:/ / online. wsj. com/ article/ SB120053698876396483. html?mod=googlenews_wsj),

The Wall Street Journal, January 17, 2008[6] Shoaiba Desalination Plant (http:/ / www. water-technology. net/ projects/ shuaiba/ ). Water Technology. Retrieved on 2011-03-20.[7] Fritzmann, C; Lowenberg, J; Wintgens, T; Melin, T (2007). "State-of-the-art of reverse osmosis desalination". Desalination 216: 1–76.

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[32] UNITED STATES COURT OF APPEALS FOR THE SECOND CIRCUIT August Term, 2005 (http:/ / www. desalresponsegroup. org/files/ RiverkeepervEPA1-25-07_decision. pdf). (PDF) . Retrieved on 2011-05-29.

[33] Heather Cooley, Peter H. Gleick, and Gary Wolff DESALINATION, WITH A GRAIN OF SALT. A California Perspective (http:/ / www.pacinst. org/ reports/ desalination/ desalination_report. pdf), Pacific Institute for Studies in Development, Environment, and Security, June2006 ISBN 1-893790-13-4

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[36] Gleick, Peter H., Dana Haasz, Christine Henges-Jeck, Veena Srinivasan, Gary Wolff, Katherine Kao Cushing, and Amardip Mann.(November 2003.) "Waste not, want not: The potential for urban water conservation in California." (http:/ / www. pacinst. org/ reports/urban_usage/ waste_not_want_not_full_report. pdf) (Website). Pacific Institute. Retrieved on 2007-09-20.

[37] Cooley, Heather, Peter H. Gleick, and Gary Wolff. (June 2006.) "Desalination, With a Grain of Salt – A California Perspective." (http:/ /www. pacinst. org/ reports/ desalination/ index. htm) (Website). Pacific Institute. Retrieved on 2007-09-20.

[38] Gleick, Peter H., Heather Cooley, David Groves. (September 2005.) "California water 2030: An efficient future." (http:/ / pacinst. org/reports/ california_water_2030/ ca_water_2030. pdf). Pacific Institute. Retrieved on 2007-09-20.

[39] Sun Belt Inc. Legal Documents (http:/ / www. sunbeltwater. com/ docs. shtml). Sunbeltwater.com. Retrieved on 2011-05-29.[40] "FO plant completes 1-year of operation" (http:/ / www. modernwater. co. uk/ files/ files/ WDR - 44. pdf). Water Desalination Report: 2–3.

15 Nov. 2010. . Retrieved 28 May 2011.[41] "Modern Water taps demand in Middle East" (http:/ / www. modernwater. co. uk/ files/ files/ demand_mdeast_n. pdf). The Independent. 23

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(http:/ / www. llnl. gov/ pao/ news/ news_releases/ 2006/ NR-06-05-06. html). Press release. . Retrieved 2007-09-07.[44] Sandia National Labs: Desalination and Water Purification: Research and Development (http:/ / www. sandia. gov/ water/ desal/

research-dev/ membrane-tech. html). Sandia.gov. Retrieved on 2011-03-20.[45] Team wins $4m grant for breakthrough technology in seawater desalination (http:/ / news. asiaone. com/ News/ AsiaOne+ News/ Singapore/

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ISOPE_OMS/ OMS 2009/ papers/ M09-83Sistla. pdf). International Society of Offshore and Polar Engineers. . Retrieved 22 June 2010.[48] Haruo Uehara and Tsutomu Nakaoka Development and Prospective of Ocean Thermal Energy Conversion and Spray Flash Evaporator

Desalination (http:/ / www. ioes. saga-u. ac. jp/ VWF/ general-review_e. html)[49] Desalination: India opens world’s first low temperature thermal desalination plant – IRC International Water and Sanitation Centre (http:/ /

www. irc. nl/ page/ 24010). Irc.nl (2005-05-31). Retrieved on 2011-03-20.[50] Floating plant, India (http:/ / www. headlinesindia. com/ archive_html/ 18April2007_35210. html). Headlinesindia.com (2007-04-18).

Retrieved on 2011-05-29.[51] Tamil Nadu / Chennai News : Low temperature thermal desalination plants mooted (http:/ / www. hindu. com/ 2007/ 04/ 21/ stories/

2007042109200400. htm). The Hindu (2007-04-21). Retrieved on 2011-03-20.[52] Current thinking (http:/ / www. economist. com/ sciencetechnology/ displayStory. cfm?story_id=14743791), Oct 29th 2009, The Economist[53] Abu Dhabi to Build Three Power and Water Desalination Plants by 2016 to Meet Demand (http:/ / www. industrialinfo. com/ showAbstract.

jsp?newsitemID=152606). industrialinfo.com (2009-11-18). Retrieved on 2011-03-20.[54] W.E.B. Aruba N.V. – Water Plant (http:/ / www. webaruba. com/ index. php?option=com_content& task=view& id=44& Itemid=159).

Webaruba.com. Retrieved on 2011-05-29.[55] Al Hidd Desalination Plant (http:/ / www. water-technology. net/ projects/ hidd/ ). Water Technology. Retrieved on 2011-05-29.

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[56] Durrat Al Bahrain desalination plant (http:/ / www. water-technology. net/ projects/ durrat-desalination/ ). Water Technology. Retrieved on2011-05-29.

[57] Construction starts on Durrat Al Bahrain desalination plant (http:/ / www. desalination. biz/ news/ news_story. asp?id=4775).Desalination.biz. Retrieved on 2011-05-29.

[58] Watts, Jonathan (2011-01-24). "Can the sea solve China's water crisis?" (http:/ / www. guardian. co. uk/ environment/ 2011/ jan/ 24/china-water-crisis?INTCMP=ILCNETTXT3487). The Guardian. . Retrieved 2011-04-19.

[59] Larnaca SWRO Water Desalination Plant (http:/ / www. water-technology. net/ projects/ larnaca/ ). Water Technology. Retrieved on2011-03-20.

[60] Marangou, V; Savvides, K (2001). "First desalination plant in Cyprus — product water aggresivity and corrosion control1" (http:/ / www.cyprus. gov. cy/ moa/ wdd/ wdd. nsf/ All/ E59112ED2B3034B2C22571C4001BAFE6/ $file/ Page1_8. pdf?OpenElement). Desalination 138:251. doi:10.1016/S0011-9164(01)00271-5. .

[61] AquaGib: Gibraltar – Present Plant (http:/ / www. aquagib. gi/ present_plant. html). Aquagib.gi. Retrieved on 2011-03-20.[62] "GIBRALTAR PROVING PLANT EXCEEDING EXPECTATIONS" (http:/ / www. modernwater. co. uk/ files/ files/ 2009-06-29. pdf). .

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Beyond (http:/ / www. israel21c. org/ technology/ archive) Retrieved 2009-12-21[64] Ashkelon Desalination Plant Seawater Reverse Osmosis (SWRO) Plant (http:/ / www. water-technology. net/ projects/ israel/ ).

Water-technology.net. Retrieved on 2011-05-29.[65] Sauvetgoichon, B (2007). "Ashkelon desalination plant — A successful challenge". Desalination 203: 75–81.

doi:10.1016/j.desal.2006.03.525.[66] Public-Private Partnership Projects (http:/ / ppp. mof. gov. il/ Mof/ PPP/ MofPPPTopNavEnglish/ MofPPPProjectsEnglish/ ), Accountant

General, Ministry of Finance[67] water-technology.net: "Ashkelon Desalination Plant Seawater Reverse Osmosis (SWRO) Plant, Israel" (http:/ / www. water-technology. net/

projects/ israel/ )[68] Globes Business and Technology News: "Palmachim desalination plant inaugurates expansion" (http:/ / www. globes. co. il/ serveen/

globes/ docview. asp?did=1000601526), November 17, 2010[69] Globes Business and Technology News: "Funding agreed for expanding Hadera desalination plant" (http:/ / archive. globes. co. il/ searchgl/

Production at the plants in Hadera, Palmachim and_s_hd_2L34nD3aqCbmnC30mD3KtE3GsBcXqRMm0. html), November 6, 2009[70] Desalination & Water Reuse (http:/ / www. desalination. biz/ about. asp?channel=0): "Spanish/Israeli JV awarded Ashdod desalination

contract" (http:/ / www. desalination. biz/ news/ news_story. asp?id=5133), 24 November 2009[71] Globes Business and Technology News: "Mekorot wins battle to build Ashdod desalination plant" (http:/ / www. globes. co. il/ serveen/

globes/ docview. asp?did=1000625564& fid=1725), February 22, 2011[72] Desalination & Water Reuse (http:/ / www. desalination. biz/ about. asp?channel=0): "IDE reported winner of Soreq desalination contract"

(http:/ / www. desalination. biz/ news/ news_story. asp?id=5163), 15 December 2009[73] Sasakura, Samsung $1.89bn bid lowest for Saudi plant (http:/ / www. reuters. com/ article/ idUSLDE64A0WL20100511). Reuters.com.

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article984409. ece). Tampabay.com. Retrieved on 2011-03-20.[79] "Yuma Desalting Plant" (http:/ / www. usbr. gov/ lc/ yuma/ facilities/ ydp/ yao_ydp. html) U.S. Bureau of Reclamation, retrieved May 1,

2010[80] "A fresh start for Yuma desalting plant" (http:/ / www. latimes. com/ news/ custom/ topofthetimes/ topstories/

la-me-water-20100501-15,0,1233621. story) Los Angeles Times, May 1, 2010[81] Ionics to build $120M desalination plant in Trinidad|Boston Business Journal (http:/ / www. bizjournals. com/ boston/ stories/ 1999/ 10/ 04/

story7. html). Bizjournals.com. Retrieved on 2011-03-20.[82] Trinidad Desalination Plant (http:/ / www. waterindustry. org/ New Projects/ ionics-2. htm). Waterindustry.org (2000-10-26). Retrieved on

2011-03-20.

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Further reading• Committee on Advancing Desalination Technology, National Research Council. (2008). Desalination: A National

Perspective (http:/ / www. nap. edu/ catalog. php?record_id=12184). National Academies Press.

Articles• Desalination: The next wave in global water consumption (http:/ / www. tlvinsider. com/ tlvinsider/ nl3/

interviews?name=nl3_interview2) from TLVInsider (http:/ / www. tlvinsider. com)

External links• International Desalination Association (http:/ / www. idadesal. org)• Examples of sea water desalination plants by the WWWS AG (http:/ / wwws-ag. com/ Sea-water-treatment. 732.

0. html)• GeoNoria Solar Desalination Process (http:/ / geonoria. org)• National Academies Press|Desalination: A National Perspective (http:/ / books. nap. edu/ openbook.

php?record_id=12184& page=R1)• World Wildlife Fund|Desalination: option or distraction? (http:/ / assets. panda. org/ downloads/

desalinationreportjune2007. pdf)• European Desalination Society (http:/ / www. edsoc. com)• IAEA – Nuclear Desalination (http:/ / www. iaea. org/ nucleardesalination/ )• DME – German Desalination Society (http:/ / www. dme-ev. de)• Large scale desalination of sea water using solar energy (http:/ / citeseerx. ist. psu. edu/ viewdoc/

summary?doi=10. 1. 1. 142. 5296)• Desalination by humidification and dehumidification of air: state of the art (http:/ / www. desline. com/ articoli/

4107. pdf)• Zonnewater – optimized solar thermal desalination (distillation) (http:/ / www. zonnewater. net)• SOLAR TOWER Project – Clean Electricity Generation for Desalination. (http:/ / www. enviromission. com. au)• Desalination bibliography Library of Congress (http:/ / www. loc. gov/ rr/ scitech/ tracer-bullets/ desalinationtb.

html)• Water-Technology (http:/ / www. water-technology. net/ projects/ )• Cheap Drinking Water from the Ocean (http:/ / www. technologyreview. com/ read_article. aspx?ch=nanotech&

sc=& id=16977& pg=1) – Carbon nanotube-based membranes will dramatically cut the cost of desalination• Solar thermal-driven desalination plants based on membrane distillation (http:/ / www. desline. com/ articoli/

5140. pdf)• Encyclopedia of Water Sciences, Engineering and Technology Resources (http:/ / www. eolss. net/ )• wind-powered desalinization plant in Perth, Australia, is an example of how technology is insulating rich

countries from impacts of climate change, while poor countries remain particularly vulnerable. (http:/ / www.nytimes. com/ 2007/ 04/ 03/ science/ earth/ 03clim. html/ The)

• The Desal Response Group (http:/ / www. desalresponsegroup. org)• Encyclopedia of Desalination and water and Water Resources (http:/ / www. desware. net/ )• Desalination & Water Reuse – Desalination news (http:/ / www. desalination. biz/ )• Desalination: The Cyprus Experience (http:/ / www. ewra. net/ ew/ pdf/ EW_2004_7-8_04. pdf)

Electrocoagulation 129

ElectrocoagulationElectrocoagulation, also known as Radio Frequency Diathermy or Short Wave Electrolysis, is a technique usedfor medical treatment and wastewater treatment.

Medical treatment

ElectrocoagulationIntervention

MeSH D004564 [1]

A fine wire probe or other delivery mechanism is used to transmit radio waves to tissues near the probe. Moleculeswithin the tissue are caused to vibrate which lead to a rapid increase of the temperature, causing coagulation of theproteins within the tissue, effectively killing the tissue. At higher powered applications, full desiccation of tissue ispossible.

Electrocoagulation in water treatmentAlthough electrocoagulation (EC) is an evolving technology that has for the past 100 years been effectively appliedin industrial wastewater treatment, the paucity of scientific understanding of the complex chemical and physicalprocesses involved as well as the limitations (in terms of size and cost) of the needed power supplies in the past,have curbed large scale applications and hindered progress.[2] In addition, the powerful manufacturers of chemicalshave been able to restrict the market penetration of this effective, environmentally friendly non-chemical procedure.With the latest technologies, reduction of electricity requirements, and miniaturization of the needed power supplies,EC systems have now become within reach of water treatment plants and industrial processes worldwide.

BackgroundThe need for clean water is particularly critical in developing countries. Rivers, canals, estuaries and other waterbodies are being constantly polluted due to indiscriminate discharge of industrial effluents as well as otheranthropogenic activities and natural processes. In the latter, unknown geochemical processes have contaminatedgroundwater with arsenic in many counties. Highly developed countries are also experiencing a critical need forwastewater cleaning because of an ever-increasing population, urbanization and climatic changes.Both the treatment of wastewater prior to discharge and the reuse of wastewater have become absolute necessities.There is, therefore, an urgent need to develop innovative, more effective and inexpensive techniques for treatment ofwastewater.A wide range of wastewater treatment techniques are known, which includes biological processes for nitrification,denitrification and phosphorus removal, as well as a range of physico-chemical processes that require chemicaladdition. The commonly used physico-chemical treatment processes are filtration, air stripping, ion exchange,chemical precipitation, chemical oxidation, carbon adsorption, ultrafiltration, reverse osmosis, electrodialysis,volatilization, and gas stripping.

Electrocoagulation 130

TechnologyTreatment of wastewater by EC has been practiced for most of the 20th century with limited success and popularity.In the last decade, this technology has been increasingly used in South America and Europe for treatment ofindustrial wastewater containing metals.[3] It has also been noted that in North America EC has been used primarilyto treat wastewater from pulp and paper industries, mining and metal-processing industries. A large one-thousandgallon per minute cooling tower application in El Paso Texas illustrates electrocoagulations growing recognition andacceptance to the industrial community. In addition, EC has been applied to treat water containing foodstuff waste,oil wastes, dyes, suspended particles, chemical and mechanical polishing waste, organic matter from landfillleachates, defluorination of water, synthetic detergent effluents, and solutions containing heavy metals.[4]

Coagulation process

Coagulation is one of the most important physio-chemical reactions used in water treatment. The precipitation ofions (heavy metals) and colloids (organic and inorganic) are mostly held in solution by electrical charges. By theaddition of ions with opposite charges, these colloids can be destabilized; coagulation can be achieved by chemicalor electrical methods. The coagulant is added in the form of suitable chemical substances. Alum [Al2(SO4)3

.18H2O]is such a chemical substance, which has been widely used for ages for wastewater treatment.The mechanism of coagulation has been the subject of continual review. It is generally accepted that coagulation isbrought about primarily by the reduction of the net surface charge to a point where the colloidal particles, previouslystabilized by electrostatic repulsion, can approach closely enough for van der Waals forces to hold them together andallow aggregation. The reduction of the surface charge is a consequence of the decrease of the repulsive potential ofthe electrical double layer by the presence of an electrolyte having opposite charge. In the EC process, the coagulantis generated in situ by electrolytic oxidation of an appropriate anode material. In this process, charged ionic species -metals or otherwise - are removed from wastewater by allowing it to react with an ion having an opposite charge, orwith floc of metallic hydroxides generated within the effluent.Electrocoagulation offers an alternative to the use of metal salts or polymers and polyelectrolyte addition forbreaking stable emulsions and suspensions. The technology removes metals, colloidal solids and particles, andsoluble inorganic pollutants from aqueous media by introducing highly charged polymeric metal hydroxide species.These species neutralize the electrostatic charges on suspended solids and oil droplets to facilitate agglomeration orcoagulation and resultant separation from the aqueous phase. The treatment prompts the precipitation of certainmetals and salts.

"Chemical coagulation has been used for decades to destabilize suspensions and to effect precipitationof soluble metals species, as well as other inorganic species from aqueous streams, thereby permittingtheir removal through sedimentation or filtration. Alum, lime and/or polymers have been the chemicalcoagulants used. These processes, however, tend to generate large volumes of sludge with high boundwater content that can be slow to filter and difficult to dewater. These treatment processes also tend toincrease the total dissolved solids (TDS) content of the effluent, making it unacceptable for reuse withinindustrial applications."[5]

"Although the electrocoagulation mechanism resembles chemical coagulation in that the cationic speciesare responsible for the neutralization of surface charges, the characteristics of the electrocoagulatedflock differ dramatically from those generated by chemical coagulation. An electrocogulated flock tendsto contain less bound water, is more shear resistant and is more readily filterable" [6]

Electrocoagulation 131

Description of the technology

In its simplest form, an electrocoagulation reactor is made up of an electrolytic cell with one anode and one cathode.When connected to an external power source, the anode material will electrochemically corrode due to oxidation,while the cathode will be subjected to passivation.An EC system essentially consists of pairs of conductive metal plates in parallel, which act as monopolar electrodes.It furthermore requires a direct current power source, a resistance box to regulate the current density and amultimeter to read the current values. The conductive metal plates are commonly known as "sacrificial electrodes."The sacrificial anode lowers the dissolution potential of the anode and minimizes the passivation of the cathode. Thesacrificial anodes and cathodes can be of the same or of different materials.The arrangement of monopolar electrodes with cells in series is electrically similar to a single cell with manyelectrodes and interconnections. In series cell arrangement, a higher potential difference is required for a givencurrent to flow because the cells connected in series have higher resistance. The same current would, however, flowthrough all the electrodes. On the other hand, in parallel or bipolar arrangement the electric current is dividedbetween all the electrodes in relation to the resistance of the individual cells, and each face on the electrode has adifferent polarity.During electrolysis, the positive side undergoes anodic reactions, while on the negative side, cathodic reactions areencountered. Consumable metal plates, such as iron or aluminum, are usually used as sacrificial electrodes tocontinuously produce ions in the water. The released ions neutralize the charges of the particles and thereby initiatecoagulation. The released ions remove undesirable contaminants either by chemical reaction and precipitation, or bycausing the colloidal materials to coalesce, which can then be removed by flotation. In addition, as water containingcolloidal particulates, oils, or other contaminants move through the applied electric field, there may be ionization,electrolysis, hydrolysis, and free-radical formation which can alter the physical and chemical properties of water andcontaminants. As a result, the reactive and excited state causes contaminants to be released from the water anddestroyed or made less soluble.It is important to note that electrocoagulation technology cannot remove infinitely soluble matter. Therefore ionswith molecular weights smaller than Ca+2 or Mg+2 cannot be dissociated from the aqueous medium.

Reactions within the electrocoagulation reactorWithin the electrocoagulation reactor, several distinct electrochemical reactions are produced independently. Theseare:• Seeding, resulting from the anode reduction of metal ions that become new centers for larger, stable, insoluble

complexes that precipitate as complex metal ions.• Emulsion Breaking, resulting from the oxygen and hydrogen ions that bond into the water receptor sites of oil

molecules creating a water-insoluble complex separating water from oil, driller's mud, dyes, inks, etc.• Halogen Complexing, as the metal ions bind themselves to chlorines in a chlorinated hydrocarbon molecule

resulting in a large insoluble complex separating water from pesticides, herbicides, chlorinated PCBs, etc.• Bleaching by the oxygen ions produced in the reaction chamber oxidizes dyes, cyanides, bacteria, viruses,

biohazards, etc. Electron Flooding of the water eliminates the polar effect of the water complex, allowingcolloidal materials to precipitate and the increase of electrons creates an osmotic pressure that ruptures bacteria,cysts, and viruses.

• Oxidation Reduction reactions are forced to their natural end point within the reaction tank which speeds up thenatural process of nature that occurs in wet chemistry.

• Electrocoagulation Induced pH swings toward neutral.

Electrocoagulation 132

Optimizing EC reactionsCareful selection of the reaction tank material is essential along with control of the current, flow rate and pH.Electrodes can be made of iron, aluminum, titanium, graphite or other materials, depending upon the wastewater tobe treated and the contaminants to be removed. Temperature and pressure have little effect on the process.In the EC process the water-contaminant mixture separates into a floating layer, a mineral-rich sediment, and clearwater. The floating layer is removed by means of a patented overflow/removal method, and moved to a sludgecollection tank. The aggregated mass settles down due to gravitational force, and is subsequently removed through adrainage valve at the bottom of the EC reaction tank, and moved to a sludge collection tank. The clear, treated wateris pumped to a buffer tank for later disposal and/or reuse in the plant’s designated process.

Advantages of EC• EC requires simple equipment and is easy to operate with sufficient operational latitude to handle most problems

encountered on running.• Wastewater treated by EC gives palatable, clear, colorless and odorless water.• Sludge formed by EC tends to be readily settable and easy to de-water, because it is composed of mainly metallic

oxides/hydroxides.• Flocs formed by EC are similar to chemical floc, except that EC floc tends to be much larger, contains less bound

water, is acid-resistant and more stable, and therefore, can be separated faster by filtration.• EC produces effluent with less TDS content as compared with chemical treatments. If this water is reused, the low

TDS level contributes to a lower water recovery cost.• The EC process has the advantage of removing the smallest colloidal particles, because the applied electric field

sets them in faster motion, thereby facilitating the coagulation.• The EC process avoids uses of chemicals and so there is no problem of neutralizing excess chemicals and no

possibility of secondary pollution caused by chemical substances added at high concentration as when chemicalcoagulation of wastewater is used.

• The gas bubbles produced during electrolysis can carry the pollutant to the top of the solution where it can bemore easily concentrated, collected and removed by a motorised skimmer.

• The electrolytic processes in the EC cell are controlled electrically and with no moving parts, thus requiring lessmaintenance.

• Dosing the incoming sewage waste water with sodium hypochlorite helps in tremendous reduction of biochemicaloxygen demand (BOD) and consequent chemical oxygen demand (COD). Sodium hypochlorite can be generatedusing electrochlorinators.[7]

• Due to the excellent EC removal of suspended solids and the simplicity of the EC operation ... tests conducted forthe Office of Naval research concluded that ... the most promising application of EC in a membrane system wasfound to be as pretreatment to a multi-membrane system of UF / RO or MF / RO. In this function the EC providesprotection of the low-pressure membrane that is more general than that provided by chemical coagulation andmore effective. EC is more effective at removing species that chemical coagulation and other alternatives canremove and it removes many species that chemical coagulation cannot remove. This has since been adopted in theindustrial arena with the use of a 1,000-gpm Powell Water / Quantum-ionics EC / UF / RO system at El PasoElectric, in El Paso Texas.>

Electrocoagulation 133

References[1] http:/ / www. nlm. nih. gov/ cgi/ mesh/ 2011/ MB_cgi?field=uid& term=D004564[2] Holt, Peter K.; Barton, Geoffrey W.; Mitchell, Cynthia A. (2004-12-08). "The future for electrocoagulation as a localised water treatment

technology". Chemoshpere (Elsevier) 59 (3): 355–67. doi:10.1016/j.chemosphere.2004.10.023. ISSN 0045-6535.[3] Rodriguez J, Stopić S, Krause G, Friedrich B (2007). "Feasibility Assessment of Electrocoagulation Towards a New Sustainable Wastewater

Treatment." (http:/ / www. springerlink. com/ content/ 28801600066879m8/ fulltext. pdf) Environmental Science and Pollution Research 14(7), pp. 477–482.

[4] Lai, C. L., Lin, S. H. 2003. "Treatment of chemical mechanical polishing wastewater by electrocoagulation: system performances and sludgesettling characteristics." (http:/ / dx. doi. org/ 10. 1016/ j. chemosphere. 2003. 08. 014) Chemosphere (http:/ / www. elsevier. com/ wps/product/ cws_home/ 362) 54 (3), January 2004, pp. 235-242.

[5] Benefield, Larry D.; Judkins, Joseph F.; Weand, Barron L. (1982). Process Chemistry for Water and Wastewater Treatment. EnglewoodCliffs, NJ: Prentice-Hall. p. 212. ISBN 0137229755.

[6] Woytowich, David L.; Dalrymple, C.W.; Britton, M.G. (Spring 1993). "Electrocoagulation (CURE) Treatment of Ship Bilge Water for theUS Coast Guard in Alaska" (http:/ / www. mtsociety. org). Marine Technology Society Journal (Columbia, MD: Marine Technology Society,Inc.) 27 (1): 92. ISSN 0025-3324. .

[7] United States Bureau of Reclamation. Yuma, AZ. "Research Facilities and Test Equipment - Chemistry Research Units." (http:/ / www. usbr.gov/ lc/ yuma/ facilities/ wqic/ yao_wqic_research_chemical. html) Updated August 2009.

Expanded granular sludge bed digestionAn expanded granular sludge bed (EGSB) reactor is a variant of the UASB concept.[1] The distinguishing featureis that a faster rate of upward-flow velocity is designed for the wastewater passing through the sludge bed. Theincreased flux permits partial expansion (fluidisation) of the granular sludge bed, improving wastewater-sludgecontact as well as enhancing segregation of small inactive suspended particle from the sludge bed. The increasedflow velocity is either accomplished by utilizing tall reactors, or by incorporating an effluent recycle (or both). Ascheme depicting the EGSB design concept is shown in this EGSB diagram [2].The EGSB design is appropriate for low strength soluble wastewaters (less than 1 to 2 g soluble COD/l) or forwastewaters that contain inert or poorly biodegradable suspended particles which should not be allowed toaccumulate in the sludge bed.

External links• UASB & EGSB Website [3]

References[1] UASB and EGSB (http:/ / www. uasb. org/ discover/ agsb. htm#egsb) Field, J. (2002) Anaerobic granular sludge bed technology pages,

anaerobic granular sludge bed reactor technology[2] http:/ / www. uasb. org/ discover/ agsb. htm#egsb[3] http:/ / www. uasb. org

Fine bubble diffusers 134

Fine bubble diffusersFine bubble diffusers are a pollution control technology used to aerate wastewater for sewage treatment. Theyproduce a plethora of very small air bubbles which rise slowly from the floor of a wastewater treatment plant orsewage treatment plant aeration tank and provide substantial and efficient mass transfer of oxygen to the water. Theoxygen, combined with the food source, sewage, allows the bacteria to produce enzymes which help break down thewaste so that it can settle in the secondary clarifiers or be filtered by membranes. A fine bubble diffuser is commonlymanufactured in various forms: tube, disc, plate, and dome.[1]

Bubble SizeThe subject of bubble size is important because the aeration system in a wastewater or sewage treatment plantconsumes an average of 50 to 70% of the energy of the entire plant.[2] Increasing the oxygen transfer efficiencydecreases the power the plant requires to provide the same quality of effluent water. Furthermore, fine bubblediffusers evenly spread out (often referred to as a 'grid arrangement') on the floor of a tank, provide the operator ofthe plant a great deal of operational flexibility. This can be used to create zones with high oxygen concentrations(oxic or aerobic), zones with minimal oxygen concentration (anaerobic) and zones with no oxygen (anoxic). Thisallows for more precise targeting and removal of specific contaminants.The importance of achieving ever smaller bubble sizes has been a hotly debated subject in the industry as ultra finebubbles (micrometre size) are generally perceived to rise too slowly and provide too little "pumpage" to provideadequate mixing of sewage in an aeration tank. On the other hand, the industry standard "fine bubble" with a typicaldischarge diameter of 2 mm is probably larger than it needs to be for many plants. Average bubble diameters of0.9 mm are possible nowadays, using special polyurethane (PUR) or special recently developed EPDM membranes.Fine bubble diffusers have largely replaced coarse bubble diffusers and mechanical aerators in most of the developedworld and in much of the developing world. The exception would be in secondary treatment phases, such asactivated sludge processing tanks, where 85%-90% of any remaining solid materials (floating on the surface) areremoved through settling or biological processes. The biological process uses air to encourage bacterial growth thatwould consume many of these waste materials, such as phosphorus and nitrogen that are dissolved in the wastewater.The larger air release openings of a coarse bubble diffuser helps to facilitate a higher oxygen transfer rate andbacterial growth. One disadvantage of using fine bubble diffusers in activated sludge tanks is the tendency of floc(particle) clogging the small air release holes.

A Fine Bubble Diffuser in a Tank, courtesy ofSSI Aeration, Inc..

Aerating water by means of fine poremembrane diffuser. Compliments of

Environmental Dynamics Inc.

Aerating water by means offine pore 9" Disc membrane

diffuser. Compliments ofEnvironmental Dynamics Inc

Fine bubble diffusers 135

References[1] http:/ / www. epa. gov/ owm/ mtb/ fine. pdf[2] http:/ / www. scipub. org/ fulltext/ ajeas/ ajeas22260-267. pdf

SedimentationSedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, andcome to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them:these forces can be due to gravity, centrifugal acceleration or electromagnetism. In geology sedimentation is oftenused as the polar opposite of erosion, i.e., the terminal end of sediment transport. In that sense it includes thetermination of transport by saltation or true bedload transport. Settling is the falling of suspended particles throughthe liquid, whereas sedimentation is the termination of the settling process.Sedimentation may pertain to objects of various sizes, ranging from large rocks in flowing water to suspensions ofdust and pollen particles to cellular suspensions to solutions of single molecules such as proteins and peptides. Evensmall molecules such as aspirin can be sedimented, although it can be difficult to apply a sufficiently strong force toproduce significant sedimentation.The term is typically used in geology, to describe the deposition of sediment which results in the formation ofsedimentary rock, and in various chemical and environmental fields to describe the motions of often-smallerparticles and molecules. Process is also used in biotech industry to separate out cells from the culture media.

ExperimentsIn a sedimentation experiment called tripothsis, the applied force accelerates the particles to a terminal velocity

at which the applied force is exactly canceled by an opposing drag force. For small enough particles (lowReynolds number), the drag force varies linearly with the terminal velocity, i.e., (Stokes flow)where f depends only on the properties of the particle and the surrounding fluid. Similarly, the applied forcegenerally varies linearly with some coupling constant (denoted here as q) that depends only on the properties of theparticle, . Hence, it is generally possible to define a sedimentation coefficient that

depends only on the properties of the particle and the surrounding fluid. Thus, measuring s can reveal underlyingproperties of the particle.In many cases, the motion of the particles is blocked by a hard boundary; the resulting accumulation of particles atthe boundary is called a sediment. The concentration of particles at the boundary is opposed by the diffusion of theparticles.The sedimentation of particles under gravity is described by the Mason–Weaver equation, which has a simple exactsolution. The sedimentation coefficient s in this case equals , where is the buoyant mass.The sedimentation of particles under the centrifugal force is described by the Lamm equation, which likewise has anexact solution. The sedimentation coefficient s also equals , where is the buoyant mass. However, theLamm equation differs from the Mason–Weaver equation because the centrifugal force depends on radius from theorigin of rotation, whereas gravity is presumed constant. The Lamm equation also has extra terms, since it pertains tosector-shaped cells, whereas the Mason–Weaver equation pertains to box-shaped cells (i.e., cells whose walls arealigned with the three Cartesian axes).Particles with a charge or dipole moment can be sedimented by an electric field or electric field gradient, respectively. These processes are called electrophoresis and dielectrophoresis, respectively. For electrophoresis, the sedimentation coefficient corresponds to the particle charge divided by its drag (the electrophoretic mobility). Similarly, for dielectrophoresis, the sedimentation coefficient equals the particle's electric dipole moment divided by

Sedimentation 136

its drag.Classification of sedimentation:• Type 1 sedimentation is characterized by particles that settle discretely at a constant settling velocity. They settle

as individual particles and do not flocculate or stick to other during settling. Example: sand and grit material• Type 2 sedimentation is characterized by particles that flocculate during sedimentation and because of this their

size is constantly changing and therefore their settling velocity is changing. Example: alum or iron coagulation• Type 3 sedimentation is also known as zone sedimentation. In this process the particles are at a high concentration

(greater than 1000 mg/L) such that the particles tend to settle as a mass and a distinct clear zone and sludge zoneare present. Zone settling occurs in lime-softening, sedimentation, active sludge sedimentation and sludgethickeners.

Geology

Siltation

In geology, sedimentation is the deposition of particles carried by afluid flow. For suspended load, this can be expressed mathematicallyby the Exner equation, and results in the formation of depositionallandforms and the rocks that constitute sedimentary record. Anundesired increased transport and sedimentation of suspended materialis called siltation, and it is a major source of pollution in waterways insome parts of the world.[1] [2] Climate change also affect siltationrates.[3]

ChemistryIn chemistry, sedimentation has been used to measure the size of large molecules (macromolecule), where the forceof gravity is augmented with centrifugal force in a centrifuge.

BiologyIn biology, the sedimentation of organisms is a critical issue for planktonic organisms, as sinking under gravitymoves them away from the surface, where sunlight provides energy.[4]

Notes[1] "Siltation & Sedimentation" (http:/ / blackwarriorriver. org/ siltation-sedimentation. html). blackwarriorriver.org. . Retrieved 2009-11-16.[2] "Siltation killed fish at Batang Rajang - Digest on Malaysian News" (http:/ / malaysiadigest. blogspot. com/ 2009/ 02/

siltation-killed-fish-at-batang-rajang. html). malaysiadigest.blogspot.com. . Retrieved 2009-11-16.[3] U.D. Kulkarni, et al. "The International Journal of Climate Change: Impacts and Responses » Rate of Siltation in Wular Lake, (Jammu and

Kashmir, India) with Special Emphasis on its Climate & Tectonics" (http:/ / ijc. cgpublisher. com/ product/ pub. 185/ prod. 38). TheInternational Journal of Climate Change: Impacts and Responses. . Retrieved 2009-11-16.

[4] Dusenbery, David B. (2009). Living at Micro Scale, Chapter 12. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.

Membrane bioreactor 137

Membrane bioreactorMembrane bioreactor (MBR) is the combination of a membrane process like microfiltration or ultrafiltration with asuspended growth bioreactor, and is now widely used for municipal and industrial wastewater treatment with plantsizes up to 80,000 population equivalent (i.e. 48 MLD).[1]

Overview

Simple schematic describing the MBR process

When used with domestic wastewater, MBR processes could produce effluent of high quality enough to bedischarged to coastal, surface or brackish waterways or to be reclaimed for urban irrigation. Other advantages ofMBRs over conventional processes include small footprint, easy retrofit and upgrade of old wastewater treatmentplants.It is possible to operate MBR processes at higher mixed liquor suspended solids (MLSS) concentrations compared toconventional settlement separation systems, thus reducing the reactor volume to achieve the same loading rate.Two MBR configurations exist: internal/submerged, where the membranes are immersed in and integral to thebiological reactor; and external/sidestream, where membranes are a separate unit process requiring an intermediatepumping step.

Membrane bioreactor 138

Schematic of conventional activated sludge process (top) and membrane bioreactor (bottom)

Recent technical innovation and significant membrane cost reduction have pushed MBRs to become an establishedprocess option to treat wastewaters.[1] As a result, the MBR process has now become an attractive option for thetreatment and reuse of industrial and municipal wastewaters, as evidenced by their constantly rising numbers andcapacity. The current MBR market has been estimated to value around US$216 million in 2006 and to rise toUS$363 million by 2010.[2]

Schematic of a submerged MBR

MBR history and basic operating parametersThe MBR process was introduced by the late 1960s, as soon as commercial scale ultrafiltration (UF) andmicrofiltration (MF) membranes were available. The original process was introduced by Dorr-Olivier Inc. andcombined the use of an activated sludge bioreactor with a crossflow membrane filtration loop. The flat sheetmembranes used in this process were polymeric and featured pore sizes ranging from 0.003 to 0.01 μm. Although theidea of replacing the settling tank of the conventional activated sludge process was attractive, it was difficult tojustify the use of such a process because of the high cost of membranes, low economic value of the product (tertiaryeffluent) and the potential rapid loss of performance due to membrane fouling. As a result, the focus was on theattainment of high fluxes, and it was therefore necessary to pump the mixed liquor suspended solids (MLSS) at highcrossflow velocity at significant energy penalty (of the order 10 kWh/m3 product) to reduce fouling. Due to the pooreconomics of the first generation MBRs, they only found applications in niche areas with special needs like isolatedtrailer parks or ski resorts for example.

Membrane bioreactor 139

The breakthrough for the MBR came in 1989 with the idea of Yamamoto and co-workers to submerge themembranes in the bioreactor. Until then, MBRs were designed with the separation device located external to thereactor (sidestream MBR) and relied on high transmembrane pressure (TMP) to maintain filtration. With themembrane directly immersed into the bioreactor, submerged MBR systems are usually preferred to sidestreamconfiguration, especially for domestic wastewater treatment. The submerged configuration relies on coarse bubbleaeration to produce mixing and limit fouling. The energy demand of the submerged system can be up to 2 orders ofmagnitude lower than that of the sidestream systems and submerged systems operate at a lower flux, demandingmore membrane area. In submerged configurations, aeration is considered as one of the major parameter on processperformances both hydraulic and biological. Aeration maintains solids in suspension, scours the membrane surfaceand provides oxygen to the biomass, leading to a better biodegradability and cell synthesis.The other key steps in the recent MBR development were the acceptance of modest fluxes (25% or less of those inthe first generation), and the idea to use two-phase bubbly flow to control fouling. The lower operating cost obtainedwith the submerged configuration along with the steady decrease in the membrane cost encouraged an exponentialincrease in MBR plant installations from the mid 90s. Since then, further improvements in the MBR design andoperation have been introduced and incorporated into larger plants. While early MBRs were operated at solidretention times (SRT) as high as 100 days with mixed liquor suspended solids up to 30 g/L, the recent trend is toapply lower solid retention times (around 10–20 days), resulting in more manageable mixed liquor suspended solids(MLSS) levels (10-15 g/L). Thanks to these new operating conditions, the oxygen transfer and the pumping cost inthe MBR have tended to decrease and overall maintenance has been simplified. There is now a range of MBRsystems commercially available, most of which use submerged membranes although some external modules areavailable; these external systems also use two-phase flow for fouling control. Typical hydraulic retention times(HRT) range between 3 and 10 hours. In terms of membrane configurations, mainly hollow fibre and flat sheetmembranes are applied for MBR applications.[3]

Despite the more favourable energy usage of submerged membranes, there continued to be a market for the sidestream configuration, particularly in industrial applications. For ease of maintenance the side stream configurationcan be installed at low level in a plant building. Membrane replacement can be undertaken without specialistequipment, and intensive cleaning of individual banks can be undertaken during normal operation of the other banksand without removing the membranes modules from the installation.As a result research continued with the side stream configuration, during which time it was found that full scaleplants could be operated with higher fluxes. This has culminated in recent years with the development of low energysystems which incorporate more sophisticated control of the operating parameters coupled with periodic backwashes, which enable sustainable operation at energy usage as low as 0.3 kWh/m3 product.

MBR configurations

Internal/submergedThe filtration element is installed in either the main bioreactor vessel or in a separate tank. The membranes can beflat sheet or tubular or combination of both, and can incorporate an online backwash system which reducesmembrane surface fouling by pumping membrane permeate back through the membrane. In systems where themembranes are in a separate tank to the bioreactor individual trains of membranes can be isolated to undertakecleaning regimes incorporating membrane soaks, however the biomass must be continuously pumped back to themain reactor to limit MLSS concentration increase. Additional aeration is also required to provide air scour to reducefouling. Where the membranes are installed in the main reactor, membrane modules are removed from the vessel andtransferred to an offline cleaning tank.

Membrane bioreactor 140

External/sidestreamThe filtration elements are installed externally to the reactor, often in a plant room. The biomass is either pumpeddirectly through a number of membrane modules in series and back to the bioreactor, or the biomass is pumped to abank of modules, from which a second pump circulates the biomass through the modules in series. Cleaning andsoaking of the membranes can be undertaken in place with use of an installed cleaning tank, pump and pipework.

Major considerations in MBR

Fouling and fouling controlThe MBR filtration performance inevitably decreases with filtration time. This is due to the deposition of soluble andparticulate materials onto and into the membrane, attributed to the interactions between activated sludge componentsand the membrane. This major drawback and process limitation has been under investigation since the early MBRs,and remains one of the most challenging issues facing further MBR development,.[4] [5]

Illustration of membrane fouling

In recent reviews covering membrane applications to bioreactors, it has been shown that, as with other membraneseparation processes, membrane fouling is the most serious problem affecting system performance. Fouling leads toa significant increase in hydraulic resistance, manifested as permeate flux decline or transmembrane pressure (TMP)increase when the process is operated under constant-TMP or constant-flux conditions respectively. In systemswhere flux is maintained by increasing TMP, the energy required to achieve filtration increases. Alternativelyfrequent membrane cleaning is therefore required, increasing significantly the operating costs as a result of cleaningagents and production downtime. More frequent membrane replacement is also expected.Membrane fouling results from interaction between the membrane material and the components of the activatedsludge liquor, which include biological flocs formed by a large range of living or dead microorganisms along withsoluble and colloidal compounds. The suspended biomass has no fixed composition and varies both with feed watercomposition and MBR operating conditions employed. Thus though many investigations of membrane fouling havebeen published, the diverse range of operating conditions and feedwater matrices employed, the different analyticalmethods used and the limited information reported in most studies on the suspended biomass composition, has madeit difficult to establish any generic behaviour pertaining to membrane fouling in MBRs specifically.

Membrane bioreactor 141

Factors influencing fouling (interactions in red)

The air-induced cross flow obtained in submerged MBR can efficiently remove or at least reduce the fouling layeron the membrane surface. A recent review reports the latest findings on applications of aeration in submergedmembrane configuration and describes the enhancement of performances offered by gas bubbling.[5] As an optimalair flow-rate has been identified behind which further increases in aeration have no effect on fouling removal, thechoice of aeration rate is a key parameter in MBR design.Many other anti-fouling strategies can be applied to MBR applications. They comprise, for example:• Intermittent permeation, where the filtration is stopped at regular time interval for a couple of minutes before

being resumed. Particles deposited on the membrane surface tend to diffuse back to the reactor; this phenomenabeing increased by the continuous aeration applied during this resting period.

• Membrane backwashing, where permeate water is pumped back to the membrane, and flow through the pores tothe feed channel, dislodging internal and external foulants.

• Air backwashing, where pressurized air in the permeate side of the membrane build up and release a significantpressure within a very short period of time. Membrane modules therefore need to be in a pressurised vesselcoupled to a vent system. Air usually does not go through the membrane. If it did, the air would dry themembrane and a rewet step would be necessary, by pressurizing the feed side of the membrane.

• Proprietary anti-fouling products, such as Nalco's Membrane Performance Enhancer Technology [6].In addition, different types/intensities of chemical cleaning may also be recommended:• Chemically enhanced backwash (daily);• Maintenance cleaning with higher chemical concentration (weekly);• Intensive chemical cleaning (once or twice a year).Intensive cleaning is also carried out when further filtration cannot be sustained because of an elevatedtransmembrane pressure (TMP). Each of the four main MBR suppliers (Kubota, Memcor, Mitsubishi and Zenon)have their own chemical cleaning recipes, which differ mainly in terms of concentration and methods (see Table 1).Under normal conditions, the prevalent cleaning agents remain NaOCl (Sodium Hypochlorite) and citric acid. It iscommon for MBR suppliers to adapt specific protocols for chemical cleanings (i.e. chemical concentrations andcleaning frequencies) for individual facilities.[3]

Membrane bioreactor 142

Intensive chemical cleaning protocols for four MBR suppliers (the exact protocol for chemical cleaning can vary from a plant to another)

Biological performances/kinetics

COD removal and sludge yield

Simply due to the high number of microorganism in MBRs, the pollutants uptake rate can be increased. This leads tobetter degradation in a given time span or to smaller required reactor volumes. In comparison to the conventionalactivated sludge process (ASP) which typically achieves 95%, COD removal can be increased to 96-99% in MBRs(see table,[7] ). COD and BOD5 removal are found to increase with MLSS concentration. Above 15g/L CODremoval becomes almost independent of biomass concentration at >96%.[8] Arbitrary high MLSS concentrations arenot employed, however, as oxygen transfer is impeded due to higher and Non-Newtonian fluid viscosity. Kineticsmay also differ due to easier substrate access. In ASP, flocs may reach several 100 μm in size. This means that thesubstrate can reach the active sites only by diffusion which causes an additional resistance and limits the overallreaction rate (diffusion controlled). Hydrodynamic stress in MBRs reduces floc size (to 3.5 μm in sidestream MBRs)and thereby increases the apparent reaction rate. Like in the conventional ASP, sludge yield is decreased at higherSRT or biomass concentration. Little or no sludge is produced at sludge loading rates of 0.01 kgCOD/(kgMLSSd).[9] Due to the biomass concentration limit imposed, such low loading rates would result in enormous tank sizes orlong HRTs in conventional ASP.

Nutrient removal

Nutrient removal is one of the main concerns in modern wastewater treatment especially in areas that are sensitive toeutrophication. Like in the conventional ASP, currently, the most widely applied technology for N-removal frommunicipal wastewater is nitrification combined with denitrification. Besides phosphorus precipitation, enhancedbiological phosphorus removal (EBPR) can be implemented which requires an additional anaerobic process step.Some characteristics of MBR technology render EBPR in combination with post-denitrification an attractivealternative that achieves very low nutrient effluent concentrations.[8]

Membrane bioreactor 143

Nutrients Removal in MBRs for Municipal Wastewater Treatment[7]

Anaerobic MBRs

Anaerobic MBRs were introduced in the 1980s in South Africa and currently see a renaissance in research. However,anaerobic processes are normally used when a low cost treatment is required that enables energy recovery but doesnot achieve advanced treatment (low carbon removal, no nutrients removal). In contrast, membrane-basedtechnologies enable advanced treatment (disinfection), but at high energy cost. Therefore, the combination of bothcan only be economically viable if a compact process for energy recovery is desired, or when disinfection is requiredafter anaerobic treatment (cases of water reuse with nutrients). If maximal energy recovery is desired, a singleanaerobic process will be always superior to a combination with a membrane process.

Mixing/HydrodynamicsLike in any other reactors, the hydrodynamics (or mixing) within an MBR plays an important role in determining thepollutant removal and fouling control within an MBR. It has a substantial effect on the energy usage and sizerequirements of an MBR, therefore the whole life cost of an MBR.The removal of pollutants is greatly influenced by the length of time fluid elements spend in the MBR (i.e. theresidence time distribution or RTD). The residence time distribution is a description of the hydrodynamics/mixing inthe system and is determined by the design of the MBR (e.g. MBR size, inlet/recycle flowrates,wall/baffle/mixer/aerator positioning, mixing energy input). An example of the effect of mixing is that a continuousstirred-tank reactor will not have as high pollutant conversion per unit volume of reactor as a plug flow reactor.The control of fouling, as previously mentioned, is primarily undertaken using coarse bubble aeration. The distribution of bubbles around the membranes, the shear at the membrane surface for cake removal and the size of the bubble are greatly influenced by the mixing/hydrodynamics of the system. The mixing within the system can also influence the production of possible foulants. For example, vessels not completely mixed (i.e. plug flow reactors) are

Membrane bioreactor 144

more susceptible to the effects of shock loads which may cause cell lysis and release of soluble microbial products.

Example of computational fluid dynamic (CFD) modelling results (streamlines) for a fullscale MBR (Adapted from the Project AMEDEUS – Australian Node Newsletter August

2007[10] ).

Many factors affect the hydrodynamicsof wastewater processes and henceMBRs. These range from physicalproperties (e.g. mixture rheology andgas/liquid/solid density etc.) to thefluid boundary conditions (e.g.inlet/outlet/recycle flowrates,baffle/mixer position etc.). However,many factors are peculiar to MBRs,these cover the filtration tank design(e.g. membrane type, multiple outletsattributed to membranes, membranepacking density, membrane orientationetc.) and it’s operation (e.g. membranerelaxation, membrane back flush etc.).

The mixing modelling and designtechniques applied to MBRs are verysimilar to those used for conventionalactivated sludge systems. They include the relatively quick and easy compartmental modelling technique which willonly derive the RTD of a process (e.g. the MBR) or the process unit (e.g. membrane filtration vessel) and relies onbroad assumptions of the mixing properties of each sub unit. Computational fluid dynamics modelling (CFD) on theother hand does not rely on broad assumptions of the mixing characteristics and attempts to predict thehydrodynamics from a fundamental level. It is applicable to all scales of fluid flow and can reveal much informationabout the mixing in a process, ranging from the RTD to the shear profile on a membrane surface. Visualisation ofMBR CFD modelling results is shown below.

Investigations of MBR hydrodynamics have occurred at many different scales, ranging from examination of shearstress at the membrane surface to RTD analysis of the whole MBR. Cui et al. (2003) [5] investigated the movementof Taylor bubbles through tubular membranes, Khosravi, M. (2007) [11] examined the entire membrane filtrationvessel using CFD and velocity measurements, while Brannock et al. (2007) [12] examined the entire MBR usingtracer study experiments and RTD analysis.

References[1] S. Judd, The MBR book (2006) Principles and applications of membrane bioreactors in water and wastewater treatment, Elsevier, Oxford[2] S. Atkinson, research studies predict strong growth for MBR markets. Membrane Technology (2006) 8-10[3] P. Le-Clech, V. Chen, A.G. Fane, Fouling in membrane bioreactors used for wastewater treatment – A review. J. Memb. Sci. 284 (2006)

17-53[4] Membrane Bioreactors (http:/ / www. membrane. unsw. edu. au/ research/ mbr. htm)[5] Z.F. Cui, S. Chang, A.G. Fane, The use of gas bubbling to enhance membrane process, J. Memb. Sci. 2211 (2003) 1-35[6] http:/ / www. nalco. com/ ASP/ applications/ membrane_tech/ products/ mpe. asp[7] M. Kraume, U. Bracklow, M. Vocks, A. Drews, Nutrients Removal in MBRs for Municipal Wastewater Treatment. Wat. Sci. Tech. 51

(2005), 391-402[8] A. Drews, H. Evenblij, S. Rosenberger, Potential and drawbacks of microbiology-membrane interaction in membrane bioreactors,

Environmental Progress 24 (4) (2005) 426-433[9] T. Stephenson, S. Judd, B. Jefferson, K. Brindle, Membrane bioreactors for wastewater treatment, IWA Publishing (2000)[10] MBR-Network (http:/ / www. mbr-network. eu/ mbr-forum/ forum_entry. php?id=194)[11] ., Khosravi, M. and Kraume, M. (2007) Prediction of the circulation velocity in a membrane bioreactor, IWA Harrogate, UK[12] Brannock, M.W.D., Kuechle, B., Wang, Y. and Leslie, G. (2007) Evaluation of membrane bioreactor performance via residence time

distribution analysis: effects of membrane configuration in full-scale MBRs, IWA Berlin, Germany

Retention basin 145

Retention basin

Trounce Pond, a retention basin landscaped withnatural grassland plants, in Saskatoon,

Saskatchewan, Canada

The Corporate Park retention basin in Stafford,Texas

Retention basin in Pinnau, Germany

A retention basin is a type of best management practice (BMP) that isused to manage stormwater runoff to prevent flooding and downstreamerosion, and improve water quality in an adjacent river, stream, lake orbay. Sometimes called a wet pond or wet detention basin, it is anartificial lake with vegetation around the perimeter, and includes apermanent pool of water in its design.[1] [2]

It is distinguished from a detention basin, sometimes called a dry pond,which temporarily stores water after a storm, but eventually emptiesout at a controlled rate to a downstream water body. It also differs froman infiltration basin which is designed to direct stormwater togroundwater through permeable soils.

Wet ponds are frequently used for water quality improvement,groundwater recharge, flood protection, aesthetic improvement or anycombination of these. Sometimes they act as a replacement for thenatural absorption of a forest or other natural process that was lostwhen an area is developed. As such, these structures are designed toblend into neighborhoods and viewed as an amenity.[3]

Design features

Storm water is typically channeled to a retention basin through asystem of street and/or parking lot storm drains, and a network of drainchannels or underground pipes. The basins are designed to allowrelatively large flows of water to enter, but discharges to receivingwaters are limited by outlet structures that function only during verylarge storm events.

Retention ponds are often landscaped with a variety of grasses, shrubsand/or wetland plants to provide bank stability and aesthetic benefits.Vegetation also provides water quality benefits by removing solublenutrients through uptake.[4] In some areas the ponds can attractnuisance types of wildlife like ducks or Canada Geese, particularlywhere there is minimal landscaping and grasses are mowed. Thisreduces the ability of foxes, coyotes and other predators to approachtheir prey unseen. Such predators tend to hide in the cattails and othertall, thick grass surrounding natural water features.

Other meanings

A retention basin can also be a part of a nuclear reactor used to contain a core meltdown.

References[1] Water Environment Federation (http:/ / wef. org), Alexandria, VA; and American Society of Civil Engineers (http:/ / www. asce. org),

Reston, VA. "Urban Runoff Quality Management." (http:/ / books. google. com/ books?id=AdU-VXXV_H0C) WEF Manual of Practice No.23; ASCE Manual and Report on Engineering Practice No. 87. 1998. ISBN 1-57278-039-8. Chapter 5.

Retention basin 146

[2] U.S. Environmental Protection Agency. Washington, D.C. "Preliminary Data Summary of Urban Storm Water Best Management Practices."(http:/ / epa. gov/ guide/ stormwater/ ) Chapter 5. August 1999. Document No. EPA-821-R-99-012.

[3] Mississippi State University. College of Engineering. Stormwater Retention Basins. Chapter 4, Best Management Practices. (http:/ / www.abe. msstate. edu/ csd/ p-dm/ all-chapters/ chapter4/ chapter4/ srb1. pdf)

[4] Urban Drainage & Flood Control District (http:/ / www. udfcd. org), Denver, CO. Urban Storm Drainage Criteria Manual. Volume 3,Structural BMPs. (http:/ / www. udfcd. org/ downloads/ pdf/ critmanual/ Volume 3 PDFs/ 04 Structural BMP 2008-04 rev. pdf)

External links• Virginia retention basin standards (http:/ / www. dcr. virginia. gov/ sw/ docs/ swm/ Chapter_3-06. pdf)• Detention vs. retention (http:/ / www. projectbrays. org/ detention. html) – Harris County, Texas Flood Control

District• Stormwater Ecological Enhancement Project (http:/ / natl. ifas. ufl. edu/ seep. htm) – University of Florida• The use of retention ponds in residential settings (http:/ / www. southalabama. edu/ geography/ fearn/ 480page/

02Jordan/ Jordan. htm)• International Stormwater BMP Database (http:/ / bmpdatabase. org) – Performance Data on Urban Stormwater

BMPs

Reverse osmosis

Schematics of a reverse osmosis system (desalination) using a pressure exchanger.1:Sea water inflow,

2: Fresh water flow (40%),3:Concentrate Flow (60%),

4:Sea water flow (60%),5: Concentrate (drain),

A: High pressure pump flow (40%),B: Circulation pump,

C:Osmosis unit with membrane,D: Pressure exchanger

Reverse osmosis (RO) is a filtration methodthat removes many types of large moleculesand ions from solutions by applyingpressure to the solution when it is on oneside of a selective membrane. The result isthat the solute is retained on the pressurizedside of the membrane and the pure solvent isallowed to pass to the other side. To be"selective," this membrane should not allowlarge molecules or ions through the pores(holes), but should allow smallercomponents of the solution (such as thesolvent) to pass freely.

In the normal osmosis process the solventnaturally moves from an area of low soluteconcentration, through a membrane, to anarea of high solute concentration. Themovement of a pure solvent to equalizesolute concentrations on each side of amembrane generates a pressure and this isthe "osmotic pressure." Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverseosmosis. The process is similar to membrane filtration. However, there are key differences between reverse osmosisand filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so theprocess can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influentpressure and concentration. Reverse osmosis, however, involves a diffusive mechanism so that separation efficiency

is dependent on solute concentration, pressure, and water flux rate.[1] . Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other substances from the water

Reverse osmosis 147

molecules.

HistoryThe process of osmosis through semipermeable membranes was first observed in 1748 by Jean Antoine Nollet. Forthe following 200 years, osmosis was only a phenomenon observed in the laboratory. In 1949, the University ofCalifornia at Los Angeles (UCLA) first investigated desalination of seawater using semipermeable membranes.Researchers from both UCLA and the University of Florida successfully produced fresh water from seawater in themid-1950s, but the flux was too low to be commercially viable[2] . By the end of 2001, about 15,200 desalinationplants were in operation or in the planning stages worldwide.[1]

Process

A semipermeable membrane coil used indesalinization.

Osmosis is a natural process. When two liquids of differentconcentration are separated by a semi permeable membrane, the fluidhas a tendency to move from low to high concentrations for chemicalpotential equilibrium.Formally, reverse osmosis is the process of forcing a solvent from aregion of high solute concentration through a semipermeablemembrane to a region of low solute concentration by applying apressure in excess of the osmotic pressure.

The membranes used for reverse osmosis have a dense barrier layer inthe polymer matrix where most separation occurs. In most cases, themembrane is designed to allow only water to pass through this denselayer, while preventing the passage of solutes (such as salt ions). Thisprocess requires that a high pressure be exerted on the highconcentration side of the membrane, usually 2–17 bar (30–250 psi) forfresh and brackish water, and 40–82 bar (600–1200 psi) for seawater,which has around 27 bar (390 psi)[3] natural osmotic pressure that mustbe overcome. This process is best known for its use in desalination (removing the salt and other minerals from seawater to get fresh water), but since the early 1970s it has also been used to purify fresh water for medical, industrial,and domestic applications.

Osmosis describes how solvent moves between two solutions separated by a permeable membrane to reduceconcentration differences between the solutions. When two solutions with different concentrations of a solute aremixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent fromthe two solutions. Instead of mixing the two solutions together, they can be put in two compartments where they areseparated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutesto move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achievedby the movement of solutes from the compartment with high solute concentration to the one with low soluteconcentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areasof high solute concentration. When the solvent moves away from low concentration areas, it causes these areas tobecome more concentrated. On the other side, when the solvent moves into areas of high concentration, soluteconcentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membranecan be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential. Osmosis is anexample of diffusion.In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with highconcentration. In this case, there are two forces influencing the movement of water: the pressure caused by the

Reverse osmosis 148

difference in solute concentration between the two compartments (the osmotic pressure) and the externally appliedpressure.

Applications

Drinking water purification

Marines from Combat Logistics Battalion 31operate ROWPUs for relief efforts after the 2006

Southern Leyte mudslide

Around the world, household drinking water purification systems,including a reverse osmosis step, are commonly used for improvingwater for drinking and cooking.

Such systems typically include a number of steps:• a sediment filter to trap particles, including rust and calcium

carbonate• optionally, a second sediment filter with smaller pores• an activated carbon filter to trap organic chemicals and chlorine,

which will attack and degrade TFC reverse osmosis membranes• a reverse osmosis (RO) filter, which is a thin film composite

membrane (TFM or TFC)• optionally, a second carbon filter to capture those chemicals not removed by the RO membrane• optionally an ultra-violet lamp for disinfecting any microbes that may escape filtering by the reverse osmosis

membrane

In some systems, the carbon prefilter is omitted, and cellulose triacetate membrane (CTA) is used. The CTAmembrane is prone to rotting unless protected by chlorinated water, while the TFC membrane is prone to breakingdown under the influence of chlorine. In CTA systems, a carbon postfilter is needed to remove chlorine from thefinal product, water.

Portable reverse osmosis (RO) water processors are sold for personal water purification in various locations. Towork effectively, the water feeding to these units should be under some pressure (40 psi or greater is the norm).Portable RO water processors can be used by people who live in rural areas without clean water, far away from thecity's water pipes. Rural people filter river or ocean water themselves, as the device is easy to use (saline water mayneed special membranes). Some travelers on long boating, fishing, or island camping trips, or in countries where thelocal water supply is polluted or substandard, use RO water processors coupled with one or more UV sterilizers. ROsystems are also now extensively used by marine aquarium enthusiasts. In the production of bottled mineral water,the water passes through an RO water processor to remove pollutants and microorganisms. In European countries,though, such processing of Natural Mineral Water (as defined by a European Directive[4] ) is not allowed underEuropean law. In practice, a fraction of the living bacteria can and do pass through RO membranes through minorimperfections, or bypass the membrane entirely through tiny leaks in surrounding seals. Thus, complete RO systemsmay include additional water treatment stages that use ultraviolet light or ozone to prevent microbiologicalcontamination.Membrane pore sizes can vary from 0.1 nanometres (3.9×10−9 in) to 5000 nanometres (0.00020 in) depending onfilter type. "Particle filtration" removes particles of 1 micrometre (3.9×10−5 in) or larger. Microfiltration removesparticles of 50 nm or larger. "Ultrafiltration" removes particles of roughly 3 nm or larger. "Nanofiltration" removesparticles of 1 nm or larger. Reverse osmosis is in the final category of membrane filtration, "hyperfiltration", andremoves particles larger than 0.1 nm.In the United States military, Reverse Osmosis Water Purification Units are used on the battlefield and in training. Capacities range from 1500 to 150000 imperial gallons (6800 to l) per day, depending on the need. The most common of these are the 600 and 3,000 gallons per hour units; both are able to purify salt water and water

Reverse osmosis 149

contaminated with chemical, biological, radiological, and nuclear agents from the water. During 24-hour period, atnormal operating parameters, one unit can produce 12000 to 60000 imperial gallons (55000 to l) of water, with arequired 4-hour maintenance window to check systems, pumps, RO elements and the engine generator. A singleROWPU can sustain a force the size of a battalion, or roughly 1,000 to 6,000 servicemembers.

Water and wastewater purificationRain water collected from storm drains is purified with reverse osmosis water processors and used for landscapeirrigation and industrial cooling in Los Angeles and other cities, as a solution to the problem of water shortages.In industry, reverse osmosis removes minerals from boiler water at power plants. The water is boiled and condensedrepeatedly. It must be as pure as possible so that it does not leave deposits on the machinery or cause corrosion. Thedeposits inside or outside the boiler tubes may result in under-performance of the boiler, bringing down its efficiencyand resulting in poor steam production, hence poor power production at turbine.It is also used to clean effluent and brackish groundwater. The effluent in larger volumes (more than 500 cu. meterper day) should be treated in an effluent treatment plant first, and then the clear effluent is subjected to reverseosmosis system. Treatment cost is reduced significantly and membrane life of the RO system is increased.The process of reverse osmosis can be used for the production of deionized water.RO process for water purification does not require thermal energy. Flow through RO system can be regulated byhigh pressure pump. The recovery of purified water depend upon various factor including - membrane sizes,membrane pore size, temperature, operating pressure and membrane surface area.In 2002, Singapore announced that a process named NEWater would be a significant part of its future water plans. Itinvolves using reverse osmosis to treat domestic wastewater before discharging the NEWater back into thereservoirs.

Food IndustryIn addition to desalination, reverse osmosis is a more economical operation for concentrating food liquids (such asfruit juices) than conventional heat-treatment processes. Research has been done on concentration of orange juiceand tomato juice. Its advantages include a lower operating cost and the ability to avoid heat-treatment processes,which makes it suitable for heat-sensitive substances like the protein and enzymes found in most food products.Reverse osmosis is extensively used in the dairy industry for the production of whey protein powders and for theconcentration of milk to reduce shipping costs. In whey applications, the whey (liquid remaining after cheesemanufacture) is concentrated with RO from 6% total solids to 10–20% total solids before UF (ultrafiltration)processing. The UF retentate can then be used to make various whey powders, including whey protein isolate used inbodybuilding formulations. Additionally, the UF permeate, which contains lactose, is concentrated by RO from 5%total solids to 18–22% total solids to reduce crystallization and drying costs of the lactose powder.Although use of the process was once avoided in the wine industry, it is now widely understood and used. Anestimated 60 reverse osmosis machines were in use in Bordeaux, France in 2002. Known users include many of theelite classed growths (Kramer) such as Château Léoville-Las Cases in Bordeaux.

Reverse osmosis 150

Car WashingBecause of its lower mineral content, reverse osmosis water is often used in car washes during the final vehicle rinseto prevent water spotting on the vehicle. Reverse osmosis is often used to conserve and recycle water within thewash/pre-rinse cycles, especially in drought stricken areas where water conservation is important. Reverse osmosiswater also enables the car wash operators to reduce the demands on the vehicle drying equipment, such as airblowers.

Maple Syrup ProductionIn 1946, some maple syrup producers started using reverse osmosis to remove water from sap before being furtherboiled down to syrup. The use of reverse osmosis allows approximately 54-42% of the water to be removed from thesap, reducing energy consumption and exposure of the syrup to high temperatures. Microbial contamination anddegradation of the membranes has to be monitored.

Hydrogen productionFor small scale production of hydrogen, reverse osmosis is sometimes used to prevent formation of minerals on thesurface of electrodes.

Reef aquariumsMany reef aquarium keepers use reverse osmosis systems for their artificial mixture of seawater. Ordinary tap watercan often contain excessive chlorine, chloramines, copper, nitrogen, phosphates, silicates, or many other chemicalsdetrimental to the sensitive organisms in a reef environment. Contaminants such as nitrogen compounds andphosphates can lead to excessive, and unwanted, algae growth. An effective combination of both reverse osmosisand deionization (RO/DI) is the most popular among reef aquarium keepers, and is preferred above other waterpurification processes due to the low cost of ownership and minimal operating costs. Where chlorine andchloramines are found in the water, carbon filtration is needed before the membrane, as the common residentialmembrane used by reef keepers does not cope with these compounds.

DesalinationAreas that have either no or limited surface water or groundwater may choose to desalinate seawater or brackishwater to obtain drinking water. Reverse osmosis is the most common method of desalination, although 85 percent ofdesalinated water is produced in multistage flash plants.[5]

Large reverse osmosis and multistage flash desalination plants are used in the Middle East, especially Saudi Arabia.The energy requirements of the plants are large, but electricity can be produced relatively cheaply with the abundantoil reserves in the region. The desalination plants are often located adjacent to the power plants, which reducesenergy losses in transmission and allows waste heat to be used in the desalination process of multistage flash plants,reducing the amount of energy needed to desalinate the water and providing cooling for the power plant.Sea Water Reverse Osmosis (SWRO) is a reverse osmosis desalination membrane process that has beencommercially used since the early 1970s. Its first practical use was demonstrated by Sidney Loeb and SrinivasaSourirajan from UCLA in Coalinga, California. Because no heating or phase changes are needed, energyrequirements are low in comparison to other processes of desalination, but are still much higher than those requiredfor other forms of water supply (including reverse osmosis treatment of wastewater).The Ashkelon seawater reverse osmosis (SWRO) desalination plant in Israel is the largest in the world.[6] [7] Theproject was developed as a BOT (Build-Operate-Transfer) by a consortium of three international companies: Veoliawater, IDE Technologies and Elran.[8]

The typical single-pass SWRO system consists of the following components:

Reverse osmosis 151

• Intake• Pretreatment• High pressure pump• Membrane assembly• Remineralisation and pH adjustment• Disinfection• Alarm/control panel

PretreatmentPretreatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiralwound design. The material is engineered in such a fashion as to allow only one-way flow through the system. Assuch, the spiral wound design does not allow for backpulsing with water or air agitation to scour its surface andremove solids. Since accumulated material cannot be removed from the membrane surface systems, they are highlysusceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF system.Pretreatment in SWRO systems has four major components:• Screening of solids: Solids within the water must be removed and the water treated to prevent fouling of the

membranes by fine particle or biological growth, and reduce the risk of damage to high-pressure pumpcomponents.

• Cartridge filtration: Generally, string-wound polypropylene filters are used to remove particles between 1 – 5micrometres.

• Dosing: Oxidizing biocides, such as chlorine, are added to kill bacteria, followed by bisulfite dosing to deactivatethe chlorine, which can destroy a thin-film composite membrane. There are also biofouling inhibitors, which donot kill bacteria, but simply prevent them from growing slime on the membrane surface and plant walls.

• Prefiltration pH adjustment: If the pH, hardness and the alkalinity in the feedwater result in a scaling tendencywhen they are concentrated in the reject stream, acid is dosed to maintain carbonates in their soluble carbonic acidform.

CO3−2 + H3O+ = HCO3

- + H2OHCO3

- + H3O+ = H2CO3 + H2O• Carbonic acid cannot combine with calcium to form calcium carbonate scale. Calcium carbonate scaling tendency

is estimated using the Langelier saturation index. Adding too much sulfuric acid to control carbonate scales mayresult in calcium sulfate, barium sulfate or strontium sulfate scale formation on the RO membrane.

• Prefiltration antiscalants: Scale inhibitors (also known as antiscalants) prevent formation of all scales compared toacid, which can only prevent formation of calcium carbonate and calcium phosphate scales. In addition toinhibiting carbonate and phosphate scales, antiscalants inhibit sulfate and fluoride scales, disperse colloids andmetal oxides. Despite claims that antiscalants can inhibit silica formation, there is no concrete evidence to provethat silica polymerization can be inhibited by antiscalants. Antiscalants can control acid soluble scales at afraction of the dosage required to control the same scale using sulfuric acid [9] .

Reverse osmosis 152

High pressure pumpThe pump supplies the pressure needed to push water through the membrane, even as the membrane rejects thepassage of salt through it. Typical pressures for brackish water range from 225 to 375 psi (15.5 to 26 bar, or 1.6 to2.6 MPa). In the case of seawater, they range from 800 to 1,180 psi (55 to 81.5 bar or 6 to 8 MPa). This requires alarge amount of energy.

Membrane assembly

The layers of a membrane.

The membrane assembly consists of a pressure vessel with amembrane that allows feedwater to be pressed against it. Themembrane must be strong enough to withstand whatever pressure isapplied against it. RO membranes are made in a variety ofconfigurations, with the two most common configurations beingspiral-wound and hollow-fiber.

Remineralisation and pH adjustment

The desalinated water is very corrosive and is "stabilized" to protectdownstream pipelines and storages, usually by adding lime or causticto prevent corrosion of concrete lined surfaces. Liming material is usedto adjust pH between 6.8 and 8.1 to meet the potable water specifications, primarily for effective disinfection and forcorrosion control.

DisinfectionPost-treatment consists of preparing the water for distribution after filtration. Reverse osmosis is an effective barrierto pathogens, however post-treatment provides secondary protection against compromised membranes anddownstream problems. Disinfection by means of UV lamps (sometimes called germicidal or bactericidal) may beemployed to sterilize pathogens which bypassed the reverse osmosis process. Chlorination or chloramination(chlorine and ammonia) protects against pathogens which may have lodged in the distribution system downstream,such as from new construction, backwash, compromised pipes, etc.

DisadvantagesHousehold reverse osmosis units use a lot of water because they have low back pressure. As a result, they recoveronly 5 to 15 percent of the water entering the system. The remainder is discharged as waste water. Because wastewater carries with it the rejected contaminants, methods to recover this water are not practical for household systems.Wastewater is typically connected to the house drains and will add to the load on the household septic system. AnRO unit delivering 5 gallons of treated water per day may discharge 40 to 90 gallons of wastewater per day.[10]

Large-scale industrial/municipal systems have a production efficiency closer to 48%, because they can generate thehigh pressure needed for more efficient RO filtration.

Reverse osmosis 153

New developmentsPrefiltration of high fouling waters with another, larger-pore membrane with less hydraulic energy requirement, hasbeen evaluated and sometimes used, since the 1970s. However, this means the water passes through two membranesand is often repressurized, requiring more energy input in the system, increasing the cost.Other recent development work has focused on integrating RO with electrodialysis to improve recovery of valuabledeionized products or minimize concentrate volume requiring discharge or disposal.

Notes[1] [Crittenden, John; Trussell, Rhodes; Hand, David; Howe, Kerry and Tchobanoglous, George. Water Treatment Principles and Design, Edition

2. John Wiley and Sons. New Jersey. 2005.][2] Glater, J. (1998). "The early history of reverse osmosis membrane development". Desalination 117: 297–309.[3] Lachish, Uri. "Optimizing the Efficiency of Reverse Osmosis Seawater Desalination" (http:/ / urila. tripod. com/ Seawater. htm). .[4] http:/ / eur-lex. europa. eu/ LexUriServ/ LexUriServ. do?uri=CONSLEG:1980L0777:19961213:EN:PDF[5] Water Technology – Shuaiba Desalination Plant (http:/ / www. water-technology. net/ projects/ shuaiba)[6] Israel is No. 5 on Top 10 Cleantech List (http:/ / www. israel21c. org/ briefs/ israel-is-no-5-on-top-10-cleantech-list) in Israel 21c A Focus

Beyond (http:/ / www. israel21c. org/ technology/ archive) Retrieved 2009-12-21[7] Desalination Plant Seawater Reverse Osmosis (SWRO) Plant (http:/ / www. water-technology. net/ projects/ israel/ Ashkelon)[8] Ashkelon desalination plant — A successful challenge (http:/ / www. sciencedirect. com/ science?_ob=ArticleURL&

_udi=B6TFX-4MWRDDH-B& _user=10& _rdoc=1& _fmt=& _orig=search& _sort=d& _docanchor=& view=c&_searchStrId=1143068268& _rerunOrigin=google& _acct=C000050221& _version=1& _urlVersion=0& _userid=10&md5=50ff032e4f44023c49838d77d7febfc5)

[9] Malki, M.,Optimizing scale inhibition costs in reverse osmosis desalination plants,INTERNATIONAL DESALINATION AND WATERREUSE QUARTERLY,2008, VOL 17; NUMB 4, pages 28-29 http:/ / www. membranechemicals. com/ english/Optimizing%20Operational%20Costs%20in%20Reverse%20Osmosis%20Desalination%20Plants%20-%202008. pdf

[10] Treatment Systems for Household Water Supplies (http:/ / www. ag. ndsu. edu/ pubs/ h2oqual/ watsys/ ae1047w. htm#disadvantage)

References• Kramer, Matt. Making Sense of Wine. Philadelphia: Running Press, 2003.

External links• Sidney Loeb – Co-Inventor of Practical Reverse Osmosis (http:/ / www. weizmann. ac. il/ ICS/ booklet/ 8/ pdf/

sidney. pdf)

Rotating biological contactor 154

Rotating biological contactor

Schematic diagram of a typical rotating biological contactor (RBC). The treated effluentclarifier/settler is not included in the diagram.

A rotating biological contactor orRBC is a biological treatment processused in the treatment of wastewaterfollowing primary treatment.[1] [2] [3]

[4] [5] The primary treatment processremoves the grit and other solidsthrough a screening process followedby a period of settlement. The RBCprocess involves allowing thewastewater to come in contact with abiological medium in order to removepollutants in the wastewater beforedischarge of the treated wastewater tothe environment, usually a body ofwater (river, lake or ocean). A rotating biological contactor is a type of secondary treatment process. It consists of aseries of closely spaced, parallel discs mounted on a rotating shaft which is supported just above the surface of thewaste water. Microorganisms grow on the surface of the discs where biological degradation of the wastewaterpollutants takes place.

Biotechnology for wastewaterEnvironmental consciousness and concern sustainable society have driven the society to the direction of re-organization of the infrastructures and the urban systems. To build an environmental management system that satisfies various social needs simultaneously in the water environment, it is essential to optimize environmental control technologies by comprehensive and systematic approaches. In this course, we critically discuss several key issues that are important in achieving desirable environmental technology systems. Biochemistry to understand the technology of wastewater treatment technologies using microorganisms is the main topic. The characteristics of complex microbial community and mathematical design modeling for Rotating Biological Contactors are discussed in this project. Biotechnology for wastewater treatment is needed so that we can use our rivers and stream for fishing, swimming and drinking water. For the first half of the 20th century, population in the Nation’s urban waterways resulted in frequent occurrences of low dissolved oxygen, fish kills, algal blooms and bacterial contamination. Early efforts in water pollution control prevented human waste from reaching water supplies or reduced floating debris that obstructed shipping. Pollution problems and their control were primarily local, not national, concerns. Since then, population and industrial growth have increased demand on our natural resources, altering the situation dramatically. Progress in abating pollution has barely kept ahead of population growth, changes in industrial processes, technological developments, and changes in land use, business innovations, and many other factors. Increases in both the quantity and variety of goods produced can greatly alter the amount and complexity of industrial wastes and challenge traditional treatment technology. The application of commercial fertilizers and pesticides, combined with sediment from growing development activities, continue to be source of significant pollution as runoff washes off the land. Water pollution issues now dominate public concerns about national water quality and maintaining healthy ecosystems. Although a large investment in water pollution control has helped to reduce the problems, many miles of streams are still impacted by variety of different pollutants. This, in turn, affects the ability of people to use the water for beneficial purpose. Past approaches used to control must be modified to accommodate current and emerging issues. Hence the appropriate biotechnology should be used for wastewater

Rotating biological contactor 155

treatment plant.

OperationThe rotating packs of disks (known as the media) are contained in a tank or trough and rotate at between 2 and 5revolutions per minute. Commonly used plastics for the media are polythene, PVC and expanded polystyrene. Theshaft is aligned with the flow of wastewater so that the discs rotate at right angles to the flow with several packsusually combined to make up a treatment train. About 40% of the disc area is immersed in the wastewater. RBC’s areclosely packed circular discs submerged in wastewater and rotated slowly. Biological growth is attached to thesurface of the disc and forms a slime layer. The disc contact wastewater and air for oxidation as it rotates. Helps toslough off excess solids. About one third of the disc is submerged. The disc system can be staged in series to obtainnearly any detention time or degree of removal required. Since the systems are staged, the culture of the later stagescan be acclimated to the slowly degraded materials. RBC media in the form of large, flat disc mounted on commonshaft are rotated through specially contoured tanks in which waste water flow on a continuous basis. The mediumconsists of plastic sheets ranging from 2 to 4 m in dia and up to 10 mm thick. Several modules may be arranged inparallel and / or in series to meet the flow and treatment requirements. The discs are submerged in waste water toabout 40% of there diameter and are rotated by power supplied to the shaft. Approximately 95% of the surface areais thus alternately immerged in waste water in then exposed to the atmosphere above the liquid under normaloperating conditions; carbonaceous substrate is removed in the initial stage of RBC. Carbon conversion may becompleted in the first stage of a series of modules, with nitrification being completed after the 5th stage. Most designof RBC systems will include a minimum of 4 or 5 modules in series to obtain nitrification of waste water.

A schematic cross-section of the contact face of the bed media in arotating biological contactor (RBC)

Biofilms, which are biological growths that becomeattached to the discs, assimilate the organic materials inthe wastewater. Aeration is provided by the rotatingaction, which exposes the media to the air aftercontacting them with the wastewater, facilitating thedegradation of the pollutants being removed. Thedegree of wastewater treatment is related to the amountof media surface area and the quality and volume of theinflowing wastewater. RBC’s were first installed inWest Germany in 1960 and were later introduce in U.Sand Canada, 70% of the RBC systems installed areused for carbonaceous. BOD removal only,25 % forcombine carbonaceous BOD removal and nitrification,and 5% for the nitrification of secondary effluent.

Construction

Rotating Biological contactor is the attached growth process. Rotating biological consist of 3-4m diameter plasticsheet of thickness 10mm attached to a shaft which is connected to a motor power 40kW, rotate at 1-2 rpm. 1 modulecontains 4-6 discs. And 5-6 module in series to assure complete nitrification Process-in this process the disc rotate inthe tank at 1-2 rpm to assure proper growth of bio logical film on the disc. The disc is submerged in the waste waterabout 45% to 90% of it dia according to the characteristic of waste water. When the disc rotates outside the tank theair enters the voids of the disc and water inside the disc trickles out the surface of the disc on the biological growth.During the submergence period the microbes present in the waste water get attached to the disc and from abio-logical film. The film is around 3-4mm thick. This film when enter in to the waste water it consumes the organic

Rotating biological contactor 156

waste by breaking the complex organic matter into the compound organic matter. Again when the disc surface facesthe open atmosphere to receive enough oxygen to sustain and carry out their metabolic activities. Since the bio filmis oxygenated externally from the wastewater, aerobic condition may develop in the liquid. Under normal operatingcondition the carbonaceous sustain in the initial stage of RBC. The carbon conversion may be completed in the firststage of a series of modules with nitrification being completed after the fifth stage. Nitrification proceeds only aftercarbon concentration is substantially reduced. Most design of RBC system will include minimum of four to fivemodule in series to obtain nitrification of wastewater. The sloughed bio mass is relatively dense and settles well insecondary clarifier. Since it is continuous process it has no detention time.

Details

HistoryThe first RBC was installed in Germany in 1960, later it was introduced in U.S.A. In U.S.A Rotating biologicalcontactors are used for industries producing high B.O.D. i.e. for petroleum industry, dairy industries etc.

Detail:- size of disc- <1m up to 5m

Length of shaft- up to 8m

Disc material: PVC is commonly used.

Plastic media is typically 1-2mm thick and corrugated for strength.

Submergence of disc in wastewater: 40 - 100%.

Speed of disc -: 1-2 rpm (revolutions per minute)

Hydraulic loading -: 30 – 160 lit/day/m²Organic loading -: 5 to 20 gBOD/day/m²

Film of micro organism on disc -: 1-2mm as per characteristics of wastewater.

Detention time -: 40 mins - 4 hours.

The rotating biological contactor reactor is a unique adaptation of the attached-growth process. Media in the form oflarge, flat disks mounted on a common shaft are rotated through specially contoured tanks in which wastewaterflows on a continuous basis. The medium consist of many plastic sheets of up to 4 m in diameter, arranged on acentral horizontal shaft. Spacing between flat disks is approximately 15 to 40 mm.The discs are submerged in wastewater to about 40 percent of their diameter and are rotated by electric motor via agearbox, or by air or water drive. The discs are alternately immersed in the waste water and then exposed to theatmosphere above the liquid. Rotational speed of the unit ranges from 1 to 2 r/min. Micro-organisms growing on themedium surface remove nutrients from the wastewater and consume oxygen from the air to sustain their metabolicprocesses. As the biofilm grows and thickens, anaerobic conditions may develop in the lower layers and parts of thebiofilm will slough from the media, to be replaced by new growth.Under normal operating conditions, carbonaceous substrate (BOD) is removed in the initial stages of the R.B.C. andnitrification occurs in later stages. The stages may be configured as multiple RBCs in series, or by modules of mediaplates within a single R.B.C..Most designs of R.B.C include minimum of 4 to 5 module in series to obtain nitrification of the wastewater. Onemodule of 3.7 m in diameter by 7.6 m long contains approximately 10,000 m² of surface area for bio-film growth. A40-kw motor is sufficient to turn the 3.7 by 7.6 m unit.

Rotating biological contactor 157

Secondary clarificationSecondary clarifiers following R.B.C.s are identical in design to conventional humus tanks, as used downstream oftrickling filters. Sludge is generally removed daily, or pumped automatically to the primary settlement tank forco-settlement. Regular sludge removal reduces the risk of anaerobic conditions from developing within the sludge,with subsequent sludge flotation due to the release of gases.

References[1] C.P. Leslie Grady, Glenn T. Daigger and Henry C. Lim (1998). Biological wastewater Treatment (2nd Edition ed.). CRC Press. ISBN

0-8247-8919-9.[2] C.C. Lee and Shun Dar Lin (2000). Handbook of Environmental Engineering Calculations (1st Edition ed.). McGraw Hill. ISBN

0-07-038183-6.[3] Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc. (4th

Edition ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0.[4] Frank R. Spellman (2000). Spellman's Standard Handbook for Wastewater Operators. CRC Press. ISBN 1-56676-835-7.[5] Mechanical Evolution of the Rotating Biological Contactor Into the 21st Century (https:/ / aerade. cranfield. ac. uk/ bitstream/ 1826/ 1086/ 1/

Mechanical+ evolution+ of+ the+ rotating. pdf) by D. Mba, School of Engineering, Cranfield University

External links• Design Criteria for Rotating Biological Contactors (http:/ / www. state. sd. us/ denr/ des/ P& s/ designcriteria/

design-10. html)• Implementing Rotating Biological Contactor Solutions (http:/ / www. coe. org. mt/ html/ Ryan Falzon FM Env

COE 140405up. ppt)• Applying the Rotating Biological Contactor Process (http:/ / www. waow. net/ Brochures/ RBC. pdf)

API oil-water separator 158

API oil-water separatorAn API oil-water separator is a device designed to separate gross amounts of oil and suspended solids from thewastewater effluents of oil refineries, petrochemical plants, chemical plants, natural gas processing plants and otherindustrial sources. The name is derived from the fact that such separators are designed according to standardspublished by the American Petroleum Institute (API). [1] [2]

Description of the design and operation

A typical gravimetric API separator

The API separator is a gravityseparation device designed by usingStokes Law to define the rise velocityof oil droplets based on their densityand size. The design of the separator isbased on the specific gravity differencebetween the oil and the wastewaterbecause that difference is much smallerthan the specific gravity differencebetween the suspended solids andwater. Based on that design criterion,most of the suspended solids will settleto the bottom of the separator as asediment layer, the oil will rise to topof the separator, and the wastewaterwill be the middle layer between theoil on top and the solids on thebottom.[2]

Typically, the oil layer is skimmed offand subsequently re-processed ordisposed of, and the bottom sedimentlayer is removed by a chain and flightscraper (or similar device) and a sludgepump. The water layer is sent to further treatment consisting usually of a dissolved air flotation (DAF) unit forfurther removal of any residual oil and then to some type of biological treatment unit for removal of undesirabledissolved chemical compounds.

API oil-water separator 159

A typical parallel plate separator

Parallel plate separators are similar toAPI separators but they include tiltedparallel plate assemblies (also knownas parallel packs).[2] The underside ofeach parallel plate provides moresurface for suspended oil droplets tocoalesce into larger globules. Anysediment slides down the topside ofeach parallel plate. Such separatorsstill depend upon the specific gravitybetween the suspended oil and thewater. However, the parallel platesenhance the degree of oil-waterseparation. The result is that a parallelplate separator requires significantlyless space than a conventional APIseparator to achieve the same degree of separation.

HistoryThe API separator was developed by the API and the Rex Chain Belt Company (now USFilter Envirex Products).The first API separator was installed in 1933 at the Atlantic Refining Company (ARCO) refinery in Philadelphia.Since that time, virtually all of the refineries worldwide have installed API separators in their wastewater treatmentplants. The majority of those refineries installed the API separators using the original design based on the specificgravity difference between oil and water. However, many refineries now use plastic parallel plate packing to enhancethe gravity separation.[1] [2]

Other oil-water separation applicationsThere are other applications requiring oil-water separation. For example:• Oily water separators (OWS) for separating oil from the bilge water accumulated in ships as required by the

international MARPOL Convention.[3] [4]

• Oil and water separators are commonly used in electrical substations. The transformers found in substations use alarge amount of oil for cooling purposes. Moats are constructed surrounding unenclosed substations to catch anyleaked oil, but these will also catch rainwater. Oil and water separators therefore provide a quicker and easiercleanup of an oil leak.[5]

API oil-water separator 160

References[1] American Petroleum Institute (API) (February 1990). Management of Water Discharges: Design and Operations of Oil-Water Separators

(1st Edition ed.). American Petroleum Institute.[2] Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st Edition ed.). John Wiley & Sons. LCCN

67019834.[3] International Convention for the Prevention of Pollution from Ships, 1973 (and later amendments) (http:/ / www. imo. org/ Conventions/

mainframe. asp?topic_id=258& doc_id=678)[4] Oily water separator (http:/ / www. free-marine. com/ i3oilywsep. htm)[5] Leonard L.Grigsby (2001). The Electrical Power Engineering Handbook. CRC Press. ISBN 0-8493-8578-4.

• Photographs, drawings and design discussion of gravimetric API Separators (http:/ / www. water. siemens. com/en/ Industries/ Hydrocarbon_Processing/ Solutions_Newsletter/ )

• Oil/Water Separators (http:/ / www. brentw. com/ water/ pdfs/ owda. pdf) Diagrams and description of separatorsusing plastic parallel plate packing.

• Oil-in-water Separation (http:/ / www. etna-usa. com/ zertech. pdf) Good discussion and explanation ofwastewater treatment processes.

• Monroe Environmental Clarifiers (http:/ / www. mon-env. com/ water_wastewater_treatment. htm) Manufacturer,drawings, photographs, diagrams and descriptions.

• Oil Water Separators (http:/ / www. washbaysolutions. com/ oil-water-separators. php) Features, Case Studies,Technology, Photos

Septic tank

A septic tank before installation

The same tank partially installed in the ground

A septic tank is a key component of the septic system, a small-scalesewage treatment system common in areas with no connection to mainsewage pipes provided by local governments or private corporations.(Other components, typically mandated and/or restricted by localgovernments, optionally include pumps, alarms, sand filters, andclarified liquid effluent disposal means such as a septic drain field,ponds, natural stone fiber filter plants or peat moss beds.) Septicsystems are a type of On-Site Sewage Facility (OSSF). In NorthAmerica, approximately 25% of the population relies on septic tanks;this can include suburbs and small towns as well as rural areas(Indianapolis is an example of a large city where many of the city'sneighborhoods are still on separate septic systems). In Europe, they aregenerally limited to rural areas only.

The term "septic" refers to the anaerobic bacterial environment thatdevelops in the tank and which decomposes or mineralizes the wastedischarged into the tank. Septic tanks can be coupled with other on-sitewastewater treatment units such as biofilters or aerobic systemsinvolving artificial forced aeration.[1]

Periodic preventive maintenance is required to remove the irreduciblesolids which settle and gradually fill the tank, reducing its efficiency.In most jurisdictions this maintenance is required by law, yet often notenforced. Those who ignore the requirement will eventually be faced

Septic tank 161

Septic tank scheme

Septic tank and septic drain field

with extremely costly repairs when solids escape the tank and destroythe clarified liquid effluent disposal means. A properly maintainedsystem, on the other hand, can last for decades or possibly even alifetime.

Description

A septic tank generally consists of a tank (or sometimes more than onetank) of between 4000 and 7500 litres (1,000 and 2,000 gallons) in sizeconnected to an inlet wastewater pipe at one end and a septic drainfield at the other. These pipe connections are generally made via a Tpipe which allows liquid entry and exit without disturbing any crust onthe surface. Today, the design of the tank usually incorporates twochambers (each of which is equipped with a manhole cover) which areseparated by means of a dividing wall which has openings locatedabout midway between the floor and roof of the tank.

Wastewater enters the first chamber of the tank, allowing solids tosettle and scum to float. The settled solids are anaerobically digested,reducing the volume of solids. The liquid component flows through thedividing wall into the second chamber where further settlement takesplace, with the excess liquid then draining in a relatively clearcondition from the outlet into the leach field, also referred to as a drainfield or seepage field, depending upon locality.

The remaining impurities are trapped and eliminated in the soil, withthe excess water eliminated through percolation into the soil(eventually returning to the groundwater), through evaporation, and byuptake through the root system of plants and eventual transpiration. A piping network, often laid in a stone filledtrench (see weeping tile), distributes the wastewater throughout the field with multiple drainage holes in the network.The size of the leach field is proportional to the volume of wastewater and inversely proportional to the porosity ofthe drainage field. The entire septic system can operate by gravity alone, or where topographic considerationsrequire, with inclusion of a lift pump. Certain septic tank designs include siphons or other methods of increasing thevolume and velocity of outflow to the drainage field. This helps to load all portions of the drainage pipe more evenlyand extends the drainage field life by preventing premature clogging.

An Imhoff tank is a two-stage septic system where the sludge is digested in a separate tank. This avoids mixingdigested sludge with incoming sewage. Also, some septic tank designs have a second stage where the effluent fromthe anaerobic first stage is aerated before it drains into the seepage field.Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank, or elsethe septic tank fills up and undecomposed wastewater discharges directly to the drainage field. Not only is this badfor the environment, but if the sludge overflows the septic tank into the leach field, it may clog the leach field pipingor decrease the soil porosity itself, requiring expensive repairs.How often the septic tank has to be emptied depends on the volume of the tank relative to the input of solids, the amount of indigestible solids and the ambient temperature (as anaerobic digestion occurs more efficiently at higher temperatures). The required frequency varies greatly depending on jurisdiction, usage, and system characteristics. Some health authorities require tanks to be emptied at prescribed intervals, while others leave it up to the determination of the inspector. Some systems require pumping every few years or sooner, while others may be able to go 10–20 years between pumpings. Contrary to what many believe, there is no "rule of thumb" for how often

Septic tank 162

tanks should be emptied. An older system with an undersized tank that is being used by a large family will requiremuch more frequent pumping than a new system used by only a few people. Anaerobic decomposition is rapidlyre-started when the tank re-fills.A properly designed and normally operating septic system is odor free and, besides periodic inspection and pumpingof the septic tank, should last for decades with no maintenance.A well designed and maintained concrete, fibreglass or plastic tank should last about 50 years.[2]

Potential problems• Excessive dumping of cooking oils and grease can cause the inlet drains to block. Oils and grease are often

difficult to degrade and can cause odour problems and difficulties with the periodic emptying.• Flushing non-biodegradable items such as cigarette butts and hygiene products such as sanitary napkins, tampons

and cotton buds/swabs will rapidly fill or clog a septic tank; these materials should not be disposed of in this way.• The use of garbage disposers for disposal of waste food can cause a rapid overload of the system and early failure.• Certain chemicals may damage the working of a septic tank, especially pesticides, herbicides, materials with high

concentrations of bleach or caustic soda (lye) or any other inorganic materials such as paints or solvents.• Roots from trees and shrubbery growing above the tank or the drainfield may clog and/or rupture them.• Playgrounds and storage buildings may cause damage to a tank and the drainage field. In addition, covering the

drainage field with an impervious surface, such as a driveway or parking area, will seriously affect its efficiencyand possibly damage the tank and absorption system.

• Unsupervised septic tanks may cause serious injury or death to children playing nearby.• Excessive water entering the system will overload it and cause it to fail. Checking for plumbing leaks and

practicing water conservation will help the system's operation.• Very high rainfall, rapid snow-melt, and flooding from rivers or the sea can all prevent a drain field from

operating and can cause flow to back up and stop the normal operation of the tank.• Over time, biofilms develop on the pipes of the drainage field which can lead to blockage. Such a failure can be

referred to as "biomat failure".• Septic tanks by themselves are ineffective at removing nitrogen compounds that can potentially cause algal

blooms in receiving waters; this can be remedied by using a nitrogen-reducing technology,[3] or by simplyensuring that the leach field is properly sited to prevent direct entry of effluent into bodies of water.

• Historically at least, not all varieties of toilet paper were suitable for disposal in a septic tank as they did notdeteriorate sufficiently (or, at least at some points in history, some toilet paper was specifically marked as suitablefor use in septic systems and some was not).

Helpful tips• As mentioned above, many chemicals such as household cleaners and detergents can damage the septic system

and kill the "good" bacteria that is needed to properly operate. A way to minimize the negative impact of theseitems is to invest in bacterial additives, which help to regulate the colonies of bacteria which are useful innaturally breaking down solids. This helps to avoid backups and ensures easy flow from the outlet T pipe into thedistribution box and out to the leach field. Bacterial additives can be found at certain hardware or home stores oryour local septic service company.[4]

• Oftentimes, septic systems backup due to non-biodegradable items — either flushed objects or hair from showerdrains — that make their way through the inlet T pipe and into the septic tank which then can clog the outlet Tpipe and cause the liquid to overflow. To try to avoid such a problem, installation of a filter is recommended. Afilter acts as a security guard and stops unwanted items from making their way into the tank. It can be installed ineither the inlet or outlet T and protects the septic system.

Septic tank 163

• Septic system cover safety is an extremely important topic and ongoing issue in the U.S. Older septic systemcovers were concrete, which over time can crack and corrode and should be regularly checked for guaranteedsecurity. Metal or cast iron covers also have the ability to be unsafe. Metal and cast iron are such heavy materialsthat if not properly installed and/or with an incorrect bottom structure, can flip themselves like a revolving doorwhich can be especially unsafe for children and animals. It is so important that covers should be regularlychecked, preferably by a professional, who can determine whether the cover is up to code standards. It isrecommended that covers be checked twice per season (such as once in the beginning of spring and once at theend of spring, once at the beginning of summer and once at the end of summer, etc.). It is also highlyrecommended that covers are inspected after winter in colder regions as heavy snow and ice can damage even anewer cover.

Environmental issuesSome pollutants, especially sulfates, under the anaerobic conditions of septic tanks, are reduced to hydrogen sulfide,a pungent and toxic gas. Likewise, methane, a potent greenhouse gas is another by-product. Nitrates and organicnitrogen compounds are reduced to ammonia. Because of the anaerobic conditions, fermentation processes takeplace, which ultimately generate carbon dioxide and methane.The fermentation processes cause the contents of a septic tank to be anaerobic with a low redox potential, whichkeeps phosphate in a soluble and thus mobilized form. Because phosphate can be the limiting nutrient for plantgrowth in many ecosystems, the discharge from a septic tank into the environment can trigger prolific plant growthincluding algal blooms which can also include blooms of potentially toxic cyanobacteria.Soil capacity to retain phosphorus is large compared with the load through a normal residential septic tank. Anexception occurs when septic drain fields are located in sandy or coarser soils on property adjoining a water body.Because of limited particle surface area, these soils can become saturated with phosphate. Phosphate will progressbeyond the treatment area, posing a threat of eutrophication to surface waters.[5]

In areas with high population density, groundwater pollution levels often exceed acceptable limits. Some smalltowns are facing the costs of building very expensive centralized wastewater treatment systems because of thisproblem, owing to the high cost of extended collection systems.To slow development, building moratoriums and limits on the subdivision of property are often imposed. Ensuringexisting septic tanks are functioning properly can also be helpful for a limited time, but becomes less effective as aprimary remediation strategy as population density increases.Trees in the vicinity of a concrete septic tank have the potential to penetrate the tank as the system ages and theconcrete begins to develop cracks and small leaks. Tree roots can cause serious flow problems due to plugging andblockage of drain pipes, but the trees themselves tend to grow extremely vigorously due to the continuous influx ofnutrients into the septic system.

References[1] "Septic Systems for Waste Water Disposal" (http:/ / www. agwt. org/ info/ septicsystems. htm). American Ground Water Trust. . Retrieved

2008-05-20.[2] "Septic Tanks: The Real Poop" (http:/ / cecalaveras. ucdavis. edu/ realp. htm). University of California Extension. . Retrieved 2006-07-11.[3] Residential nutrient reduction (http:/ / www. epa. gov/ etv/ pubs/ 600s07004. pdf)[4] Septic System FAQ's. "Septic System FAQ's" (http:/ / www. wrenvironmental. com). Wind River Environmental. . Retrieved 4 April 2011.[5] Craig G. Cogger. "eb1475 Septic System Waste Treatment in Soil" (http:/ / cru. cahe. wsu. edu/ CEPublications/ eb1475/ eb1475. html).

College of Agriculture and Home Economics, Pullman, Washington. . Retrieved 2006-07-11.

Septic tank 164

External links• The Septic Systems Information Website - Inspecting, Testing, Designing, & Maintaining Residential Septic

Systems (http:/ / www. inspect-ny. com/ septbook. htm)• Site and Soil Evaluation for Onsite Waste Water Systems (http:/ / www. oznet. ksu. edu/ library/ h20ql2/ mf2645.

pdf)• Homeowner's Guide to Septic Systems (http:/ / www. epa. gov/ npdespub/ pubs/

homeowner_guide_long_customize. pdf)• U.S. Environmental Protection Agency (http:/ / www. epa. gov)• Massachusetts Department of Environmental Protection (http:/ / www. mass. gov/ dep/ water/ )• Rhode Island Department of Environmental Management (http:/ / www. dem. ri. gov/ )

Stabilization pondStabilization pond technology - sometimes also called facultative pond technology - is a natural method forwastewater treatment.

TechnologyStabilization ponds [1] consist of shallow man-made basins comprising a single or several series of anaerobic,facultative or maturation ponds. The primary treatment takes place in the anaerobic pond, which is mainly designedfor removing suspended solids, and some of the soluble element of organic matter (BOD). During the secondarystage in the facultative pond most of the remaining BOD is removed through the coordinated activity of algae andheterotrophic bacteria. The main function of the tertiary treatment in the maturation pond is the removal ofpathogens and nutrients (especially nitrogen).

Application and suitabilityStabilization ponds are particularly well suited for tropical and subtropical countries because the intensity of thesunlight and temperature are key factors for the efficiency of the removal processes. [2] It is also recommended bythe WHO for the treatment of wastewater for reuse in agriculture and aquaculture, especially because of itseffectiveness in removing nematodes (worms) and helminth eggs.[3]

Stabilization ponds, as described here, use no aerators. High-performance lagoon technology with aerators has muchmore in common with that of activated sludge. Such aerated lagoons are common in small towns in the UnitedStates, among other places.[4]

Cost considerationsAccording to the IRC International Water and Sanitation Centre, stabilization pond technology is the mostcost-effective wastewater treatment technology for the removal of pathogenic micro-organisms.[5] A World Bankstudy carried out in Sana’a, Yemen, in 1983 makes a detailed economic comparison of waste stabilization ponds,aerated lagoons, oxidation ditches and trickling filters. According to this study, stabilization pond technology is acheaper option up to a land cost of US$ 7.8/m2. Above this cost, oxidation ditches become the cheapest option. [6]

However, often the main constraint against selecting this technology is not land cost but land availability. If land isavailable, stabilization ponds have the advantage of very low operating costs since they use no energy compared toother wastewater treatment technologies. This makes them particularly suitable to developing countries where many“conventional” wastewater treatment plants (usually using activated sludge technology) have had to be shut downbecause water and sewer utilities did not generate sufficient revenue to pay the electricity bill for the plant.

Stabilization pond 165

UseStabilization ponds are used for municipal waste water treatment in many countries with ample sunshine, includingColombia, El Salvador, Guatemala, Honduras, Israel, Jordan, Morocco, Nicaragua, Tunisia and Uganda.[7] They aretypically used in smaller towns where land availability and cost is less of a constraint. In some cities largerstabilization ponds have been replaced in the early 2000s by activated sludge waste water treatment plants, such as inAmman (Jordan) and in Adelaide (Australia) in 2004.[8]

Removal of pathogensA study of ten stabilization ponds in Honduras has shown that they were effective in removing helminth eggs, apathogen, from the effluent and to satisfy the World Health Organization microbiological guidelines for Category Birrigation with wastewater effluent. However, sludge from all ponds was heavily contaminated with helminth eggs.[9]

External links• EPA Wastewater Technology Fact Sheet: Facultative Lagoons [10]

References[1] Design and Performance of Waste Stabilization Ponds, Hamzeh H. Ramadan & Victor M. Ponce (http:/ / stabilizationponds. sdsu. edu)[2] IRC Waste stabilization ponds for wastewater treatment, May 2004, prepared by Cinara, Colombia (http:/ / www. irc. nl/ page/ 8237)[3] WHO: Guidelines for the safe use of wastewater, excreta and greywater (http:/ / www. who. int/ water_sanitation_health/ wastewater/ gsuww/

en/ index. html)[4] Lagoons online (http:/ / www. lagoonsonline. com/ technotes. htm)[5] IRC Waste stabilization ponds for wastewater treatment, May 2004, prepared by Cinara, Colombia (http:/ / www. irc. nl/ page/ 8237)[6] Arthur, J.P. (1983). Notes on the design and operation of waste stabilization ponds in warm climates of developing countries. Technical paper

No 7. Washington D.C[7] OAKLEY S. M.; POCASANGRE A.; FLORES C.; MONGE J.; ESTRADA M.: Waste stabilization pond use in Central America : The

experiences of El Salvador, Guatemala, Honduras and Nicaragua (http:/ / cat. inist. fr/ ?aModele=afficheN& cpsidt=783370), Water scienceand technology 2000, vol. 42, no 10-11, pp. 51-58

[8] South Australia Water (http:/ / www. sawater. com. au/ SAWater/ WhatsNew/ MajorProjects/ PtAdel_Plant_Disposal. htm)[9] Stewart M. Oakley, Department of Civil Engineering, California State University: The Need for Wastewater Treatment in Latin America: A

Case Study of the Use of Wastewater Stabilization Ponds in Honduras (http:/ / www. nesc. wvu. edu/ nsfc/ Articles/ SFQ/ SFQ_sp05/SFQ_Sp05_juried. pdf), Small Flows Quarterly Juried Article, Srping 2005, Volume 6, Number 2

[10] http:/ / www. epa. gov/ OWM/ mtb/ faclagon. pdf

Ultrafiltration (industrial) 166

Ultrafiltration (industrial)

A selectively permeable membrane can be mounted ina centrifuge tube. The buffer is forced through the

membrane by centrifugation, leaving the protein in theupper chamber.

Ultrafiltration is a type of filtration. Industries such as chemicaland pharmaceutical manufacturing, food and beverage processing,and waste water treatment, employ ultrafiltration in order torecycle flow or add value to later products. Ultrafiltration iscommonly abbreviated to "UF."

UF's main attraction is its ability to purify, separate, andconcentrate target macromolecules in continuous systems. UFdoes this by pressurizing the solution flow. The solvent and otherdissolved components that pass through the membrane are knownas permeate. The components that do not pass through are knownas retentate. Depending on the Molecular Weight Cut Off(MWCO) of the membrane used, macromolecules may bepurified, separated, or concentrated in either fraction.

Currently, the study of UF processing occurs mainly in laboratorysetups because it is very prone to membrane fouling caused byincreased solute concentration at the membrane surface (either bymacromolecular adsorption to internal pore structure ofmembrane, or aggregation of protein deposit on surface ofmembrane), which leads to concentration polarization (CP)). CP is the major culprit in decreasing permeate flux.Ultrafiltration is used as a pre-treatment step in reverse osmosis processes in many Middle Eastern countries topotable drinking water, as there is little fresh water available in those areas.

Drinking water treatment 300 m³/h usingultrafiltration in Grundmühle waterworks

(Germany)

Treatment pond 167

Treatment pondA treatment pond treats water fouled by anaerobic bacteria. It is used mainly by tree nurseries, dairy farms andother agricultural companies near horse or cattle sheds or barns. The pond treats polluted stormwater and animalwastewater so that it may be returned to the environment as fertilizer and irrigation water.

Uses of treatment ponds

An ecological swimming pond

A treatment pond may be used in combination with a rainwater reservoir toform an ecological, self-purifying irrigation reservoir or swimming pond.[1]

[2] A pond may dispose or treat industrial liquid wastes. An example is theMartinez, California treatment ponds developed by IT Corporation.Small-scale treatment can be done in small ponds if the effluent is given timeto break into harmless nutrients. However, smaller ponds may need to bedivided in much the same manner as septic tanks.

Set-up

Urban areas

Three reedbed flow configurations are commonly in use. Each is being usedin commercial systems (usually together with septic tanks).[3] They are:

• Surface flow (SF) reedbeds• Subsurface Flow (SSF) reedbeds• Vertical Flow (VF) reedbeds

Different flow configurations for reedbed treatment.

Treatment pond 168

All three are placed in a closed basin with a substrate. For most commercial purposes (e.g., agriculture) ponds arelined with rubber to ensure being watertight (essential in urban areas). The substrate can be gravel, sand or lavastone.

Design characteristics

A treatment pond next to an irrigation reservoir,forming a self-purifying reservoir

Surface flow reedbeds use a horizontal flow of waste water betweenplant roots. They are no longer much used, as they need considerablespace (A person requires 20 m2 to purify the water they use.) Theyhave increased smell and poorer purification in winter.[4]

With subsurface flow reedbeds, the flow of waste water is betweenplant roots, but not at the water surface. This is more efficient, lesssmelly and less sensitive to winter conditions. The soil to purify wateris 5–10 m2 per person. Intakes, which can clog easily, are a potentialproblem.[4]

Vertical flow reedbeds are similar to subsurface flow reedbeds(subsurface wastewater flow is present here as well). Therefore they

have similar efficiency and winter hardiness. Wastewater flow is somewhat different though, as it is vertical. Thewater is distributed through a set of perforated distribution pipes in the top layer of gravel. The wastewater thenpasses a layer of fine sand mixed with iron and chalk and there it is purified by bacteria that live on the sand grainsand in higher concentrations near the plant roots. The purified water is then collected in drainage pipes that areembedded in the bottom layer of gravel. Other than the two previous systems, this system almost makes exclusiveuse of fine sand to increase bacteria counts. Intake of oxygen into the water is better. Pumping is done in pulses toreduce obstructions with the intakes. Only 3 m2 is needed to purify the water for one person.[4]

OrganismsUsually, common reed or Phragmites australis are used in treatment ponds (e.g., in greywater treatment systems topurify wastewater). In self-purifying water reservoirs(used to purify rainwater), other plants are used as well. Thesereservoirs need to be filled with 1/4 lavastones and water-purifying plants.[5]

Treatment ponds use a wide variety of plants, depending on the local climate and other conditions. Plants aregenerally chosen which are indigenous, for environmental reasons and optimum performance. In addition to waterpurifying (de-nutrifying) plants, plants that supply oxygen, and shade are added in ecological water catchments,ponds. This allows a complete ecosystem. Local bacteria and non-predatory fish may be added to eliminate pests.The bacteria are usually grown by submerging straw in water and allowing bacteria from the surrounding air to formon it. Plants are divided in four water depth-zones:1. A water depth from 0–20 cm. Iris pseudacorus, Sparganium erectum may be placed here (temperate climates).2. A water depth from 40–60 cm. Stratiotes aloides, Hydrocharis morsus-ranae may be placed here (temperate

climates).3. A water depth from 60–120 cm. Nymphea alba, may be placed here (temperate climates).4. A submerged water depth. Myriophyllum spicatum may be placed here (temperate climates).Three non-predatory fish (surface, bottom and ground-swimmers) are chosen. This ensures the fish 'get along'.Examples for temperate climates are:• Surface swimming fish: Leuciscus leuciscus, Leuciscus idus, and Scardinius erythrophthalmus• Middle-swimmers: Rutilus rutilus• Bottom-swimming fish: Tinca tinca

Treatment pond 169

The plants are usually grown on coconut fibre.[6] At the time of implantation to water-purifying ponds, de-nutrifiedsoil is used to prevent growth of algae and other unwanted organisms

FinishingThese systems, for example, aerate the water after the final reedbed using cascades such as flowforms before holdingthe water in a shallow pond.[7] Primary treatments such as septic tanks, and pumps such as grinder pumps may beadded.[8]

Rooftop treatment pondsRooftop water purifying ponds are being used on rooftops. These green roofs can be built from a simple substrate (asis being done in Dongtan)[9] or with plant-based ponds (as is being done by WaterWorks UK Grow System).[10]

Waterzuiveren.be[11] Plants used include calamus, Menyanthes trifoliata, Mentha aquatica, ...[12]

External links• Pictures of a treatment pond [13]

• SuSaNa treatment pond schematics [14]

References[1] "Ecologic water basins used for agriculture/irrigation" (http:/ / www. oieau. fr/ ciedd/ contributions/ atriob/ contribution/ russian. htm).

Oieau.fr. . Retrieved 2010-10-05.[2] SwimPond Incorporated. "reservoirs made self-purifying through addition of treatment pond" (http:/ / www. swimpond. com/

pools_or_ponds. html). Swimpond.com. . Retrieved 2010-10-05.[3] "reedbed descriptions" (http:/ / www. certipro. be/ docs/ Certificering van plantenwaterzuiveringssystemen. pdf) (PDF). . Retrieved

2010-10-05.[4] "reedbed secriptions" (http:/ / www. certipro. be/ docs/ Certificering van plantenwaterzuiveringssystemen. pdf) (PDF). . Retrieved

2010-10-05.[5] "Overview of lavafilters" (http:/ / www. stowa-selectedtechnologies. nl/ Sheets/ Sheets/ Lava. Filters. html). Stowa-selectedtechnologies.nl. .

Retrieved 2010-10-05.[6] Coconut growing medium used for water purifying plants (http:/ / www. lukmertens. be/ kwekerij. html)[7] (http:/ / www. sheepdrove. com/ article. asp?art_id=115) reedbed and flowform cascade polishing, Sheepdrove Organic Farm, England[8] "Pictures of hybrid reedbed systems" (http:/ / www. pure-milieutechniek. be/ Page22. htm). Pure-milieutechniek.be. . Retrieved 2010-10-05.[9] Dongtan green roofs filter water (http:/ / www. eukn. org/ eukn/ themes/ Urban_Policy/ Urban_environment/ Environmental_sustainability/

dongtan-eco-city_1348. html)[10] "WWUK rooftop water purification with plants" (http:/ / www. wwuk. co. uk/ grow. htm). Wwuk.co.uk. 2008-02-22. . Retrieved

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id=684& Itemid=58). Toontoelen.be. . Retrieved 2010-10-05.[13] http:/ / www. facstaff. bucknell. edu/ kirby/ 4ponds. html[14] http:/ / www. susana. org/ index. php?option=com_content& view=article& id=152& Itemid=145& lang=en

Wet oxidation 170

Wet oxidationWet oxidation is a form of hydrothermal treatment. It is the oxidation of dissolved or suspended components inwater using oxygen as the oxidizer. It is referred to as "Wet Air Oxidation" (WAO) when air is used. The oxidationreactions occur in superheated water at a temperature above the normal boiling point of water (100° C), but belowthe critical point (374° C).The system must be maintained under pressure to avoid excessive evaporation of water. This is done to controlenergy consumption due to the latent heat of vaporization. It is also done because liquid water is necessary for mostof the oxidation reactions to occur. Compounds oxidize under wet oxidation conditions that would not oxidize underdry conditions at the same temperature and pressure.

Commercial applicationsWet oxidation has been used commercially for around 60 years. It is used predominantly for treating wastewater. Itis often referred to as the Zimpro process, after Fred T. Zimmermann who commercialized it in the mid 20thcentury.[1]

Commercial systems typically use a bubble column reactor, where air is bubbled through a vertical column that isliquid full of the hot and pressurized wastewater. Fresh wastewater enters the bottom of the column and oxidizedwastewater exits the top. The heat released during the oxidation is used to maintain the operating temperature.The majority of commercial wet oxidation systems are used to treat industrial wastewaters, such as sulfide ladenspent caustic streams. Almost as many systems are also used for treating biosolids, in order to pasteurize and todecrease volume of material to dispose of.A special type of process was the so-called "VerTech process". Here was the required pressure supplied by installingthe system in de below ground pressure vessel (Gravity Pressure Vessel GPV). The pressure was supplied by feedingthe material to a reactor with a depth of 1200 meters. The deep shaft reactor also served as a heat exchanger, so nopre heating was required. The operating temperature was about 270 degrees Celsius, with a pressure of about 100bar. The installation was operational in Apeldoorn (the Netherlands) between 1994-2004, but was eventually shutdown due to operational problems.

References• Zimmermann, F. Waste Disposal, US Patent 2665249, 1950.• Mishra, V.; Mahajani, V.; Joshi, J. "Wet Air Oxidation", Ind. Eng. Chem. Res.", 34, 2-48, 1995.• Maugans C.; Ellis, C. "Wet Air Oxidation: A Review of Commercial Sub-Critical Hydrothermal Treatment",

Twenty First Annual International Conference on Incineration and Thermal Treatment Technologies, NewOrleans, May 13-17, 2002. WAO History Paper [2]

• Patria, L.; Maugans, C.; Ellis, C.; Belkhodja, M.; Cretenot, D.; Luck, F.; Copa, B. "Wet Air Oxidation Processes",Advanced Oxidation Processes for Water and Wastewater Treatment, S. Parsons Editor, pp 247–274. 2004, IWAPublishing.

• Giudici, D.; Maugans, C. "Improvement of Industrial Synthesis of Methyl Methacrylate Application of a Wet AirOxidation Process (WAO)", MMA WAO Paper [3]

[1] "Zimpro History" (http:/ / www. water. siemens. com/ en/ about_us/ legacy_brands/ Pages/ zimpro. aspx). . Retrieved February 2010.[2] http:/ / www. water. siemens. com/ SiteCollectionDocuments/ Product_Lines/ Zimpro/ Brochures/

IT3%202002%20WAO%20history%20paper. pdf[3] http:/ / www. water. siemens. com/ SiteCollectionDocuments/ Product_Lines/ Zimpro/ Brochures/

Improvement%20of%20Industrial%20Sysnthesis%20by%20Methly%20Methacrylate. pdf

Article Sources and Contributors 171

Article Sources and ContributorsWastewater  Source: http://en.wikipedia.org/w/index.php?oldid=422903640  Contributors: -Majestic-, 14zip34b, ABF, Aboalbiss, Alan Liefting, Alansohn, Amokbel, Anlace, AnonymousDissident, Anthony Appleyard, Avalon, Ayanoa, BigFatBuddha, Bloodshedder, Bloosyboy, Bobo12345, Boilerup12, BozMo, Brandon, CALR, CDM2, Cactusjack1981, Caichris, Calvin 1998,CapitalR, Casmith 789, Catgut, ChVA, Charivari, CharlesC, Christdk, Chriswaterguy, Cm6051, Cohesion, Cphi, Cruccone, DARTH SIDIOUS 2, Daniel Collins, Davewild, DeadEyeArrow,DerHexer, Derekristow, Dieselbub, EJF, Element16, Elfino, ErrantX, Ewen, Exhummerdude, FayssalF, Fred Bauder, Gabriel Kielland, Gsaup, Gurch, Guthrie, HaeB, HexaChord, Ida Shaw,J04n, JTN, Jgrabbs, John, Johnsrw, Jupix, KJD2011, Kaarel, Kakoui, Kbh3rd, Kjkolb, KudzuVine, Lear's Fool, Little Mountain 5, MPN, Mac, Mausy5043, Maximus Rex, Mbeychok, Mongol,MrOllie, Mrholybrain, Mschiffler, Okoivisto, OlEnglish, PBarak, PMDrive1061, Pablowerk, Paleorthid, Paxse, Petrb, Philip Trueman, Pilgaard, Quercusrobur, RJFJR, Rd232, RedHillian, RichFarmbrough, Robyvecchio, Ronz, RuM, Saga City, Saint Midge, Salvador95, Sam Hocevar, Seaphoto, Sewer Me, Shaddack, Shoombooly, Silencedmajority, Simon12, SmileJohn, TakuyaMurata,Templationist, That Guy, From That Show!, The Thing That Should Not Be, Themfromspace, Tide rolls, Tim1357, Tiptoety, TomFrankel, Tommy2010, Tuckya, Useight, Utilitysupplies,Vegaswikian, Vegetator, Veinor, Velella, Vortexrealm, WCFrancis, Wavelength, Wayne Slam, Woodshed, Xbspiro, Yodelheck, 200 anonymous edits

Sewage treatment  Source: http://en.wikipedia.org/w/index.php?oldid=432013257  Contributors: 14zip34b, 2over0, 4twenty42o, A Stop at Willoughby, ABCD, Aaavinash, Abdallahdjabi,Abhishank.jajur, AdultSwim, Afluent Rider, Aitias, Alansohn, Aldie, Ale jrb, Alf ea, Ali@gwc.org.uk, Allstarecho, AlphaEta, Amalthea, Andrewpmk, Anlace, AnnaLore, Antandrus, Anthere,Anthony Appleyard, AntiVan, Anwar saadat, Arjun01, Arnavwik, AubreyEllenShomo, Aurista25, Awanta, Awickert, AxelBoldt, Aymatth2, Azaroonus, BWKA, Badgernet, Bantman, Bartledan,Basar, BaseballDetective, Basseysam, Beetstra, Belg4mit, Ben James Ben, Bendzh, Berwinc, Bettymnz4, Bhadani, Bkonrad, Bletch, Bloodshedder, Bobjgalindo, Bobo192, Bogelund, Boilerup12,Bongwarrior, Bookofjude, Boxidon, BozMo, Brian0918, Brownstone Mr, BruceDLimber, Brutaldeluxe, Bryan Derksen, Bucketsofg, Bugtrio, Bumm13, Burn, Burpelson AFB, Burschik,Bushytails, C.lettingaAV, CDM2, CDN99, CactusWriter, Camembert, Can't sleep, clown will eat me, CanisRufus, Capricorn42, Captain-n00dle, Carlog3, Cathydunham, Cessator, Cfailde,Cgeers, Chongkian, Chris 73, Chris the speller, Chriswaterguy, Clairekcarpenter, Clarknova714, Cocacola789123, Cohesion, Cointyro, ComputerGuy, Control.optimization, Cromwellt, Cureden,Curps, DMahalko, Da monster under your bed, DanMS, Daniel Collins, DanielCD, Davidlburton, Davnor, Deen rose, Deirdre, Dejitarob, DennyColt, DerHexer, Dfrg.msc, Digitalminddubai,Dirkbb, Discospinster, Dlohcierekim's sock, Dmanning, DocWatson42, DouglasHeld, Dralwik, Dsmithsmithy, Dtaylor1984, Dureo, EERichards, Echuck215, Eclecticos, EdBever, Eequor,EgbertW, Elite782, Elvirs, Eng.Kalaji, EnvironmentalDynamics, Espoo, EthanL, EuTuga, Evertype, Everyking, Ewen, Excirial, F. 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Biochemical oxygen demand  Source: http://en.wikipedia.org/w/index.php?oldid=432176985  Contributors: Alan Liefting, Alansohn, Alqadri86, Amitauti, Anandsince, Andrea105, Aperea,Arthena, Azwaldo, Biker Biker, Biscuittin, BradyDale, Christopherfair, Chriswaterguy, Cmdrjameson, ConCompS, Diberri, Dj Capricorn, Euchiasmus, Exir Kamalabadi, Fanghong, Floria L,FramingArmageddon, Fullerenedream, Funnyfarmofdoom, Gas Panic42, Gene Nygaard, GhaziKakany, Giggs for Temporary, GrDn, Gurch, Ike9898, Jeffq, Jfeucht82, Kenneth Charles,KudzuVine, LFaraone, Lights, Mandarax, Materialscientist, Mateus Hidalgo, Mbeychok, Metamagician3000, MightyWarrior, Mnhb2948, Moreau1, MrSomeone, Nono64, Pablowerk, Paleorthid,Panchhee, Parmesan, PeN, Phgao, Pinethicket, Quadell, Rui Silva, SSpiffy, Sampo Torgo, Samw, Sandip90, Savh, Sercan K., Smack, Stemonitis, Tbhotch, Thewellman, Thue, Tide rolls, Vanhelsing, Velella, Vortexrealm, Vuong Ngan Ha, WCFrancis, Yaluen, Yilloslime, Yohan Duminda, Zarathushtra1111, 187 anonymous edits

Effluent  Source: http://en.wikipedia.org/w/index.php?oldid=420589150  Contributors: Akradecki, Alan Liefting, Alpha Ralpha Boulevard, Andres, Anlace, BeGenderNeutral, Bobo12345, C i d,Chris the speller, Correogsk, Daniel, Eagles247, EldKatt, Erasmussen, Geologyguy, Huw Powell, IByte, Markozeta, Mbeychok, MickWest, Moreau1, Mrfebruary, Nafinch, Nposs, Qwfp, Riskone, Robofish, The Nut, Theanphibian, Thejadefalcon, Wavelength, Wouterhagens, Yumeteor, Yvh11a, 29 anonymous edits

Biofilter  Source: http://en.wikipedia.org/w/index.php?oldid=425127348  Contributors: ASK, Anlace, Basicdesign, Bdonlan, Chriswaterguy, Dietdrpep, Dj Capricorn, Duk, Fish Bass, Frajolex,Gobonobo, Harry, KVDP, KudzuVine, LLucas, Look2See1, Mbeychok, Mion, Moreau1, Nono64, Paleorthid, Radiant!, Red58bill, Rkincorporated, S, Senators, Sewer Me, Shoefly, Sortan, TheVindictive, Tide rolls, Velella, Vortexrealm, Waterboy12, Webcomsystems, 21 anonymous edits

Trickling filter  Source: http://en.wikipedia.org/w/index.php?oldid=424242392  Contributors: Alchimista, Anticipation of a New Lover's Arrival, The, Aushulz, Bendel boy, Biofilter tech, C777,Copywriter74, DMahalko, DeadEyeArrow, Gregbard, Hmains, LilHelpa, Longhair, Mbeychok, Michael Hardy, Mschiffler, R'n'B, Reyk, Rickproser, Ronz, Sewer Me, Twirligig, Velella, 21anonymous edits

Chemical oxygen demand  Source: http://en.wikipedia.org/w/index.php?oldid=416332490  Contributors: Aksi great, Annabel, Aperea, Biscuittin, Bry9000, Christopherfair, Diberri, DjCapricorn, Dr.Soft, Enviroboy, Erud, Gene Nygaard, Kaarel, Karthickbala, LOL, Langbein Rise, Margoz, Mbeychok, Michael Devore, Mintleaf, Moreau1, MorningRazor, Morwen, NickNumber, Nono64, Pengo, Peruvianllama, Quadell, RSido, Roboa1983, Schewek, Spellmi, Spiral5800, Srleffler, VIGNERON, Velella, Vina, Vortexrealm, Vuong Ngan Ha, WCFrancis, Welsh,Yadavbasti, Yonatan, 78 anonymous edits

Chlorination  Source: http://en.wikipedia.org/w/index.php?oldid=423149899  Contributors: Agentmoose, Anthony Appleyard, Antifumo, Are you ready for IPv6?, Armking5, Aussie Alchemist,Beland, Benbest, Bender235, Bigboehmboy, Black Science Nerd, BozoTheScary, Bryan Derksen, CJ, Caillan, Chriswaterguy, Cliffb, Common Man, DDKnaus, DabMachine,DagErlingSmørgrav, Darkxsun, DeviNex, Dj Capricorn, Doodle77, DrFod, Edgar181, Ffangs, Freefighter, George100, Gioto, Gpvos, GregorB, Haham hanuka, Ike9898, Japo, Jason thenerd,Jimjamjak, Justanyone, Kate, La goutte de pluie, Mako, Marispotens, Martin S Taylor, Mikespedia, Nmrtian, Northgrove, Oasisbob, Ottre, Pethead, Physchim62, Pinethicket, Postlewaight,Puppy8800, Qaz, Red duck41, Rich Farmbrough, RobertG, SimonP, Spiffy sperry, Stemonitis, Sterichinderance, Syrthiss, Tarquin, TheKMan, Ulric1313, Van Flamm, Vanjagenije, Wmahan,X1987x, 85 anonymous edits

Ozone  Source: http://en.wikipedia.org/w/index.php?oldid=432288312  Contributors: 09jhowarth, AZard, Aarchiba, Abce2, Adambro, AdjustShift, AgentPeppermint, AkiBlast16, Alansohn, Albertgenii12, AlexiusHoratius, Alphachimp, Amazinms90, Amit6, AnakngAraw, Andres, AndrewLeeson, Andrewpmk, Anehmet, Angelofdeath275, Angilbas, Antandrus, Apollo, ArcyQwerty, Ashley kennedy3, Avoided, Ayesoe, Azgoldcom, BD2412, BRUTE, Baa, Backpackadam, Bagginz, Bahar101, Bart133, Bbik, BeefWellington, Beetstra, Beland, BenBildstein, Benjah-bmm27, Benjaminong, Betterworld, Bevo, Billy03, BirgitteSB, Bkell, Bluemoose, Bnelson13, BobbyShred, Boeseben, Bongwarrior, Borgx, Brainblaster52, Brighterorange, Brolin Empey, Brumski, BullRangifer, C.A.T.S. CEO, C3o, Cacycle, Caffelice, Caltas, CanDy sPiceR, Candy lny, Capricorn42, Captain-n00dle, Carbonpoppy, Carlos heise, Cedders, Cflm001, CharlesC, Chavas, Chem-awb, ChemJohn, ChemNerd, Chicocvenancio, Chinmay153, CiTrusD, Civil Engineer III, Clemwang, Clip9, Closedmouth, Colbuckshot, Conversion script, Coolhandscot, Corruptedozone, Craig rd, Cryptus92, Cxl198, DMacks, DRHagen, DVdm, Dachannien, DanielCD, Danielleevandenbosch, Danno uk, Darkfight, Darkteal, Darrien, Dataphile, David H Braun (1964), Delirium, Der lektor, DerHexer, Discospinster, Dkangjam, DocWatson42, DonaldVR, Donarreiskoffer, Dougofborg, Download, Dragonjimmyy2k, Dragons flight, E. Fokker, Ed Poor, Edgar181, Eduardo Napalm, Eequor, Egh0st, Elassint, Elm-39, Elrodriguo, Elysianfields, EncycloPetey, Enviroboy, Eog1916, Epbr123, Eridani, Erik9, Erislover, Erudy, Escape Orbit, Escape80, Eshhsu, Euneirophrenia, Evan1005, Evand, Evil Monkey, Evoact, Falcor84, Feezo, Flewis, Foobar, Fred Bauder, Freddyd945, Fshafique, GPHemsley, Gaius Cornelius, Gaorui, Garkbit, Gershwinrb, Ghiles, Giftlite, Gjd001, Glenn, Glob.au, Gparker, Graeme Bartlett, Grandia01, Greg5601, Ground Zero, Gurch, Gwernol, Hagedis, HalfShadow, Han G Thesobs, Hans Dunkelberg, Haymaker, Hdt83, Heimstern, Hellbus, Heron, Hiddenfromview, Horselover Frost, Hottubprod, Howcheng, Hr.easen, Hu Gadarn, Hu12, Hue White, Hut 8.5, Hydrargyrum, Hydrogen Iodide, Ian Pitchford, Ideyal, IndulgentReader, Inhumandecency, Iodine Galaxy, Itub, Ivy Shoots, J.delanoy, JForget, JIMRULES08, JWeytze, Japlin06, Jauerback, Jbourj, Jedidan747, Jfdwolff, Jimcham17, Jj137, Jkc1jkc1, Jmundo, Joanjoc, Joerogel, John, Jon186, JonHarder, Jonnyhum, Joyous!, Jpk, Jrobinjapan, Judicatus, Jujubee21, Julesd, Juniorgregory, JustAGal, KGasso, KPWM Spotter, Kakofonous, Karuna8, Katita621, Katoa, Keilana, Kelly Martin, Kerina yin, Kevinmlv, Keyesc, Kieff, King of Hearts, Klomenattowa, Konstable, Kostisl, Krash, Krepta3000, Kumar.ashwini64,

Article Sources and Contributors 172

LAX, La Pianista, Lalguy, LeaveSleaves, Leonard G., Lfh, Lightmouse, Ligulem, Little Mountain 5, Lkosci3, Looxix, Louis De Pena, Lova Falk, Lradrama, Luna Santin, Lupoblanco, MHendriks, M.nelson, M3suka, MECU, MK8, MPerel, Malafaya, Malke 2010, Manop, Marek69, Master12345678910x, Masterjamie, Masz, Matdrodes, Materialscientist, Mathman27, Mattabat,Mav, MaxEnt, Maxis ftw, McSly, Meltonkt, Melzipp, Mentifisto, Menwith, Miaow Miaow, Michaelritchie200, Michał Sobkowski, Michur, Mike Winters, Mikeo, Mikepittas, Milosh1414,Ming-xuan-shao, Minghong, Mipadi, Mistercow, Modify, MoleculeUpload, Mongoosedogg, Mstrkrft96, Mxn, Myaca, NHSavage, NReitzel, NSH001, Nabetse, Nagy, NatureA16, Naudefj,Nbennardo, Ned4500000, NellieBly, NeonMerlin, Nergaal, NickelShoe, Nihiltres, Nirmos, Nishkid64, Nivix, Nono64, Nova77, Nsaa, Nyenyec, Obradovic Goran, Ocdncntx, Ocdnctx,Olaf.rodriguez, OldakQuill, Omegatron, Onco p53, Onopearls, Ooga131booga, Optiguy54, Oraipl, Orphic, Oscar9000, Outriggr, Ozonejoe, PDH, Pakaran, Particlebry, PatGallacher, Paul August,Pauljyoung, PeepP, Pelirojopajaro, Pengo, Peterlewis, Pevarnj, Ph0987, PhilHibbs, Phillip.calvin, Physchim62, Piotrgoldstein, Pizza Puzzle, Plasmic Physics, Plasticup, Plugwash, Plvekamp,Ponyo, Prashanthns, Princess Lirin, Prolog, Pusshead, Quackslikeaduck, Quarem, Quest for Truth, Qxz, R. fiend, Rakela, Random account 47, Random user 39849958, Rapty, Ratsbew, RedAndr,Reddi, Reedy, Rejectedman, Resourcesforlife, Rettetast, RexNL, Reyk, Reytan, Rich Farmbrough, RickOzone, Rickstarz, Rifleman 82, Rivertorch, Rjwilmsi, RobertAustin, Rodhullandemu,RolfSander, Ronebofh, Rparson, Rrburke, Rror, Rtdrury, Rwarburton, Ryandinubilo, SDC, SHIMONSHA, SYSS Mouse, Sadistic monkey, Sampenglase, Savant13, Sbharris, Sceptre, Scharks,SchuminWeb, Scott.medling, Scottman783, Sedunova, SemperBlotto, Shaddack, Shantanu.m28, ShaunMacPherson, Shniken1, Sidism, Sietse Snel, Sillybilly, Silverchemist, SimonP, Sionus,SlimVirgin, Sloth monkey, Slowking Man, SmartBagel, Smartse, Smihael, Smokefoot, Snowolf, Some jerk on the Internet, Sonett72, SparrowsWing, Spiel496, Squiddy, Srpostma11, SteinbDJ,Stemonitis, StephanieM, Steve Quinn, Stone, StpetersIPT, SummerPhD, SunCreator, Susvolans, Synchronism, Syplex, Tabletop, Tangent747, Taoster, Tdonoughue, Techauthor, Techman224,Technolust, Tetracube, The Anome, The High Fin Sock Whale, The High Fin Sperm Whale, The Thing That Should Not Be, TheOtherJesse, Thesmothete, Thingg, Thorongil CVI, Thorpe,Thricecube, Tide rolls, Tim Starling, Tim1357, Timl2k4, Tommy2010, Traroth, Trusilver, Tscole90, Tyco.skinner, USANightmare, Ultraviolet scissor flame, Uncle G, Unfree, Unyoyega,V.narsikar, V8rik, Vadmium, Verox, Vezzu, Vishizs, Vrenator, Vsmith, WaterRabbit, Wavelength, Wayne Slam, WazzaMan, WelshMatt, Whatcanbrowndo, WhisperToMe, Whiteandnerdy52,Wickey-nl, Wikianon, Wikichick7, Wikieat, Will Beback Auto, William M. Connolley, Willking1979, Wimt, Wimvandorst, Wishritenow, Wtcdown, Xpanzion, Yaronf, Yilloslime, Yonatan,Zandperl, Zaytran, Zikiodotte, Zpb52, ~K, Александър, 1035 anonymous edits

Ultraviolet germicidal irradiation  Source: http://en.wikipedia.org/w/index.php?oldid=412627213  Contributors: Acdx, Ali@gwc.org.uk, Atlant, Beland, Billymac00, Brewsum, ChrisKurtz,CosineKitty, Debeo Morium, Deglr6328, Dforest, Global8000, Granpire Viking Man, Ground Zero, January, Jeepday, Kleinost, Mattbash, Melchoir, Otisjimmy1, Pdream, Petercoccusfuriosus,Radagast83, Radiojon, Rubin joseph 10, SSpiffy, Scombs, Stagyar Zil Doggo, Stephanienox, Szhaider, Templatehater, TestPilot, The enthusiast 889, Trahana22, Tri400, Useight, WaterRabbit,Wezelboy, Whitepaw, Worksafe, WriterHound, X96lee15, 90 anonymous edits

Water treatment  Source: http://en.wikipedia.org/w/index.php?oldid=427664976  Contributors: 21655, Accurizer, Ahmad halawani, Alan Liefting, Alansohn, Alexwatt, Amokbel,Amplitude101, Applejaxs, Arvindrkale, BD2412, Bantman, Beland, Belovedfreak, Blah55, Bobo192, Bwil, CDM2, Caracaskid, Carter, Casomerville, Chanbc, Chasedog, Chongkian, ChristinaSilverman, Chriswaterguy, Cielomobile, Cindychu, Clarince63, Closedmouth, D, D6, DMacks, Dalcanale, Dalillama, DancingPenguin, DanielCD, Dannyherbon, Dantadd, Davewild, Deen rose,DeltaQuad, Discospinster, Dj Capricorn, Dlohcierekim's sock, Doorvery far, Drmies, Dspanogle, Duckdive117, EarthPerson, Email4mobile, Encyo, Espoo, Esrever, FayssalF, Femto, Ffaarr,Fischer.sebastian, Frymaster, Gabriel Kielland, Grumpyyoungman01, H2g2bob, Harrison Daxter, Hmains, Humboldt, Iconprocesssystems, Ike9898, Immunize, Irineliul, ItascaSystems, J04n,Jimjamjak, Jrcrin001, Juan de Vojníkov, Julia Rossi, Keilana, Korealover-jisung, Kurieeto, Laurentleap, Le.crouton, LeadSongDog, Lifeline69, Liz-ldavis, LizardJr8, Loren.wilton, Lotje,MER-C, Marek69, Matumeru, Mazarin07, Mbeychok, Michael Hardy, Modulatum, Moreau1, MrOllie, Mschiffler, NawlinWiki, NewEnglandYankee, OhanaUnited, PAR, Perspicacite,PeteShanosky, Phmoreno, Pilotguy, Plugwash, Possum, Proudmoose, Push desh, Radiojon, Rahim6464, Ralajer, Rayne117, Rbanzai, Register112, Rholton, Richard0612, Rjwilmsi, RockMFR,Ronz, Rossfi, Sal4u1982, Schzmo, Sean William, ShadowoftheL1ght, SmileJohn, SoAxVampyre, Speciate, Steven J. Anderson, Syrthiss, Taitoa, Thingg, Utcursch, VI, Velella, Vinmax,Vortexrealm, WDickens, Watertruth, Wavelength, Wiki Pushkar, WikiDan61, Xavexgoem, ZayZayEM, Zzuuzz, 279 anonymous edits

Settling  Source: http://en.wikipedia.org/w/index.php?oldid=422117384  Contributors: Aboalbiss, AcideAmine, Agne27, AndreiDukhin, Ansell, Attarparn, Awickert, Bluap, Bryan Derksen,Darkwind, Dwjones1, Dysprosia, Eastlaw, Fennec, Frap, H2O, Juan de Vojníkov, Ktr101, Li4kata, Lvzon, Marshman, Mattbrundage, N2e, Neelix, Ospalh, Piotrus, Radagast83, Rjwilmsi,SeventyThree, Symposiarch, TenPoundHammer, User A1, Vsmith, Woohookitty, X1987x, YixilTesiphon, 47 anonymous edits

Flocculation  Source: http://en.wikipedia.org/w/index.php?oldid=422250734  Contributors: Ali@gwc.org.uk, Allesddd, Arcadian, Atlant, Auntof6, Auric, Badagnani, Benbest, Bendel boy,Bogelund, Bozoid, Canberra User, Ccrrccrr, Chrislk02, Chriswaterguy, ClockworkSoul, Coagulant, Curps, DARTH SIDIOUS 2, DaGizza, Darkwraith, Dominic, EL STAVEO, Encyo, Eras-mus,Ezhiki, Fvw, Gabriel Kielland, Gaius Cornelius, Glane23, Golgofrinchian, Gunmetal, Hmains, Hollis01, Immunize, Itub, J04n, JIP, Jacksonic, Jcorgan, Jeff Dahl, John254, Jshadias, Jsolari,Karlhahn, Kesuari, Langbein Rise, Lifthrasir, Luna Santin, M dorothy, MarSch, Michael Hardy, MrBell, MrChupon, Mrzaius, Nukeless, Off2riorob, Onco p53, P199, Paleorthid, Peter Grey,Physchim62, Piano non troppo, Pizza1512, Procrastinatrix, Puppy8800, Rholtslander, Rifleman 82, Sam Hocevar, Sander123, Seansheep, Sgpsaros, Shadowfax0, Shinkolobwe, SmileJohn,Sonett72, Sophos II, Stan J Klimas, Stepa, Tb, Tempodivalse, The High Fin Sperm Whale, Toddst1, Tony May, Uq, UrsaFoot, Vortexrealm, Vuvar1, Weasel5i2, William Avery, Wkrocek,Zymatik, 107 anonymous edits

Activated sludge  Source: http://en.wikipedia.org/w/index.php?oldid=422004681  Contributors: Aomarks, Aushulz, BD2412, Bendel boy, Billymac00, Bogelund, Boilerup12, BozMo, BryanDerksen, Cimex, Cissi, Craig Pemberton, Cralize, D, Davidruben, Dj Capricorn, Eddyji999, Engineman, EnvironmentalDynamics, Exteray, Gaius Cornelius, GrDn, Ground Zero, Hmains, Kekel,Mbeychok, Mhdg, MrOllie, Ndenison, Opr8r, Pengo, Peter G Werner, Podesc, R'n'B, Rjwilmsi, Ryantuck, Sal4u1982, Salvador95, Sonic flashy101, SunCreator, Thewellman, Tom harrison,Trjumpet, TurboChan, Uncle G, Velella, Vojt V, Vortexrealm, Wavelength, Wieliczka, Wwengr, 77 anonymous edits

Slow sand filter  Source: http://en.wikipedia.org/w/index.php?oldid=416940046  Contributors: Ali@gwc.org.uk, Attilathefun, BenAveling, Bendel boy, Benjamindees, Bill Price, Bob Burkhardt,Brighterorange, Bulas, CAJ, CanisRufus, Chriswaterguy, Excirial, Hooperbloob, Immunize, Ingolfson, Jaxl, JdH, John254, Juan de Vojníkov, KVDP, LilHelpa, Moreau1, Muffinon, Paleorthid,Rich Farmbrough, SilentC, Stemonitis, Thumperward, Timo Honkasalo, Velella, Wikipelli, 49 anonymous edits

Aerated lagoon  Source: http://en.wikipedia.org/w/index.php?oldid=425848528  Contributors: Boilerup12, Brian0918, Ground Zero, Hdt83, Jaeger5432, Mattryanfry, Mbeychok, Mejor LosIndios, Moreau1, Myscrnnm, Osnetwork, Rich Farmbrough, Rjwilmsi, Sheeana, V8rik, Vortexrealm, Walter Hartmann, Wavelength, 9 anonymous edits

Advanced oxidation process  Source: http://en.wikipedia.org/w/index.php?oldid=417202260  Contributors: Adkoon, Fuzzywallaby, Graeme Bartlett, Nono64, Pxma, Ronz, Snehakrishna,Squids and Chips, Steven Zhang, Tweek, V8rik, Velella, Wikielwikingo, Ymalaika, 15 anonymous edits

Aerobic treatment system  Source: http://en.wikipedia.org/w/index.php?oldid=430778841  Contributors: AndrewKepert, Colonies Chris, Dugwiki, Everyking, Fish Bass, Fluzwup, Iridescent,Katalaveno, Keeper76, Legotech, Mbeychok, Parutakupiu, Preppernau, R2bomber, Ronz, Saimhe, Sewer Me, Spellmaster, Uyanga, Walter Hartmann, Wikiacctname, Woohookitty, 8 anonymousedits

Anaerobic digestion  Source: http://en.wikipedia.org/w/index.php?oldid=432045087  Contributors: A. B., AbhisheksinghWIKI, Aconescs, Ag2003, Agre22, Alan Liefting, Amberroom, AnnaLincoln, Ascidian, AstroHurricane001, Atasin11, Auntof6, Awis, AxelBoldt, Behun, Bogelund, Brandon, Brastein, Brighterorange, Brinerustle, Brusegadi, CStack3, Chowbok, Chriswaterguy,Cimbalom, Crusaderstar, Cult hero, Curtbeckmann, Cyclonenim, DARTH SIDIOUS 2, Dancter, Darkwind, David Woodward, Doodle77, Dorkapeter, Drphilharmonic, Ed lewis, Edward,Effenberger, Element16, EmmettLBrown, Energee5, Engineman, Epbr123, Euchiasmus, Eugene Kelly, Everyking, Fahadsadah, Farmideas, GHarrison14, GTZ-44-ecosan, Gaius Cornelius,Gpanzani, Granitethighs, Gregalton, Gregkaye, Ground Zero, Gullinbrusti, Hai398, Harperweb, Hbent, Hmains, Hu12, Hugo-cs, Iknowtrash, Incrediblehunk, Interiot, Iridescent, Iulian28ti,J.delanoy, Jaimemh, Jamesmorrison, Jcorgan, Jeff Dahl, Jezhotwells, Jmhollen, John of Reading, Johnfos, Jorfer, Joseph Solis in Australia, Juliancolton, Jxm211, Jyril, Katana0182, Kjkolb,KlickingKarl, Ksd5, Kweeket, L Kensington, Lawrencekhoo, Lcmoreno, Ldbio130, Ling.Nut, Ljohnson13, Lkinkade, Lotje, Malleus Fatuorum, Mark.murphy, Marxspiro, Matthewireland,Maurreen, Mbeychok, Mcchino64, Michael Devore, Mion, Mirgy, Mmeijeri, Moreno.laura, Mp3banker, Mr3641, NCurse, NormaMcDonald, OhanaUnited, Onetoremember, Parutakupiu, Paul P,Pinethicket, Quale, Quasirandom, Qwertyman, RAM, Resqbrett, Rich Farmbrough, RichardF, Rjwilmsi, Robert.carpenter, Roger Davies, Ronz, Salvador95, Satori Son, Shenme, Simesa, SimonP,Skier Dude, Slawomir D-K, Spanjers, Stillnotelf, Tabletop, Tim10000, Trafford09, User A1, Vanished user 47736712, Velella, Vinayakbhogan, Vortexrealm, Wavelength, Welsh, Wikibiogas,Withlyn, Woohookitty, Yeeak61, Yellowdesk, Yousaf465, Кузнецов, 223 anonymous edits

Bioreactor  Source: http://en.wikipedia.org/w/index.php?oldid=429537750  Contributors: 20Lukianto, Ahoerstemeier, AlGordo, Andy Dingley, Bogelund, Ceyockey, Chemical Engineer, Cquan,Cyclopia, Dieselbub, EagleFan, Elliott Drabek, Ithunn, Jaerik, Johan Lont, Jonkerz, Kanzie, Legend revolve, Loki 22890, Mmeijeri, N2e, Naddy, Nono64, Rivertorch, SCEhardt,ShakespeareFan00, Sonett72, Tarret, Taylorjc, Timberframe, Titoxd, Tobias Wolter, Velella, Yaser hassan 2006, YassineMrabet, Zenchu, 27 anonymous edits

Carbon filtering  Source: http://en.wikipedia.org/w/index.php?oldid=430199849  Contributors: Asianrewriter, Betacommand, Bobblewik, Chriswaterguy, D-Kuru, Domaingamer, FinalRapture,Giftlite, Haukurth, HenkvD, Hobosmacks, Hu12, Igiffin, Ipsomatic9, Ivan Akira, JMumford, Jscb, JustinBailey75, Kaesa, Keilana, Kgrr, Klosterdev, Lightmouse, Loren36, Mion, MusicTree3,Oatmeal batman, OfCourse13, Onorem, Pigman, Pinkpanther23, QuiteUnusual, Ravedave, Robofish, Steve Wise, V8rik, Vegaswikian, VirtualSteve, 58 anonymous edits

Constructed wetland  Source: http://en.wikipedia.org/w/index.php?oldid=429407937  Contributors: Alan Liefting, Anlace, Arch dude, Avalon, Basicdesign, Bb143143, Bperlman99, Brastein,CSWarren, Captain Selenium, Chriswaterguy, Conti, Covalent, Darklilac, Davidlburton, Deirdre, Duk, EagleFan, Edward, Epiphaross, GTZ-44-ecosan, Gaius Cornelius, Gate-way,GorillaWarfare, Griffin700, Ground Zero, Gveret Tered, Hmains, Hyronimus299, J04n, JaGa, KVDP, Laikalynx, Lamiot, Leolaursen, Lfstevens, LilHelpa, Lloyd roz, Look2See1,Materialscientist, Mcguirep2010, Michal Nebyla, Moreau1, MttJocy, Mushroom, Nicke L, Olawai, Paleorthid, Parutakupiu, Plrk, Quadell, Rich Farmbrough, Rjwilmsi, Rmrfstar, Rotational, Salixalba, Samw, Schaef, Shlomke, South atlantic ocean, Sqrlmstr, Squids and Chips, TDogg310, Twirligig, Uiew, Una Smith, Vegaswikian, Vortexrealm, Walrus heart, Waterbender kara,Wavelength, Whpq, Yingkeli, Ytrottier, 68 anonymous edits

Article Sources and Contributors 173

Dissolved air flotation  Source: http://en.wikipedia.org/w/index.php?oldid=408468011  Contributors: Amalas, BWKA, Beagel, Bharatdandekar, Chrishmt0423, Cleanwatertech, Dceron, Enysjg,Grafen, Hydroflo, ILike2BeAnonymous, IronGargoyle, James084, Karlhahn, Langbein Rise, Longhair, Mattisse, Maurreen, Mbeychok, My76Strat, Pearle, Shenme, Shinkolobwe, SmileJohn,Sophie VIS, Splodgeness, User A1, 17 anonymous edits

Desalination  Source: http://en.wikipedia.org/w/index.php?oldid=432308328  Contributors: 124eric, 12b3, 4I7.4I7, 5ko, 7, 7k7p1mnm, A. B., AJHalliwell, Acroterion, Aengus, Agyle, Aitias,Alansohn, Alex.muller, Alexius08, Allansiew, Andonic, Andrew wilson, AndrewLeeson, Anonymous Dissident, Antandrus, Anthere, Apacheneo, Aquaepulse, Ashvidia, Axeman, Axl, B4hand,BBird, BD2412, BW52, Badgernet, BastiaanNaber, Bayerischermann, Bdx, Beland, Benrmac129, Berserkerz Crit, Betterusername, Bevo, Bgpaulus, Bigtimepeace, Bioarchie1234, Bluewind,BobM, Bobbo, Bobo192, Bogey97, Bovineone, Bovlb, Bravo31, Bumm13, Bundy197, C45207, Calm, CalumH93, Camw, Can't sleep, clown will eat me, Capricorn42, Ceyockey, Charvest,Chefallen, Choi9999, Chriswaterguy, Chriswiki, ChungLW, Ckatz, Coemgenus, Common Man, Coro, Cremepuff222, Crystal sparkle96, Cureden, Curps, CurranH, CustardJack, Cxz111,DARTH SIDIOUS 2, DMacks, DVD R W, Dancxjo, Darth Mike, Davewho2, Dawnseeker2000, Dd1958, DeadEyeArrow, Deen rose, Dengua, Dgiroux, Dinopup, Dirtyharry2, Disavian,Discospinster, Doseiai2, Doug Coldwell, Doulos Christos, Drat, DrexRockman, Drmies, Dryguy, DynamoDegsy, Dysepsion, Eastlaw, EdBever, Edward321, El C, Engineman, Epbr123, Ereglibob, Everyking, FF2010, Fabrictramp, Figma, Finnrind, Fireice, Fivemack, Flumstead, Foobar, FrankTobia, Fratrep, Frosted14, Fui in terra aliena, Funnyhat, Furrykef, GHe, GVOLTT, GaiusCornelius, Gatemansgc, Geanixx, Ghunter3016, Gilliam, Gimboid13, Grafen, Ground Zero, Groyolo, Grundle2600, Guanaco, Gus 795, Gwernol, HHahn, Halesw, Halvorseno, Haoie,HarlandQPitt, HarryHenryGebel, Haukurth, Henare, Hmrox, Hobartimus, Hu12, Huangdi, Hungry homer, Hungry homer1, Huntster, ILOVEPONY, IRP, IReceivedDeathThreats, Iames,Ianisalifestyle, Ibnzintzo, ImperfectlyInformed, Inferno, Lord of Penguins, Inwind, Iorsh, Iridescent, Itschris, J.delanoy, JForget, JNW, JTN, Jackol, James xeno, James086, Jamott, Janko,Jasonwilson1972, Javawizard, Jcfdillon, Jclemens, Jeff Silvers, Jeremy Visser, Jessdogg, Jimjamjak, John Kjos, Johnfos, Joostvandeputte, JorisvS, Jpkotta, Jpp42, Jrtayloriv, Jsmith86, Julesd,Juliaa.italiana, Juliancolton, Jyril, KDS4444, KFP, KVDP, Kablammo, Karen Johnson, Katalaveno, Kevin Rector, Khoikhoi, King Zebu, Kingpin13, Kjkolb, Kneejerk, KnowledgeOfSelf,Krusty627, Ksyrie, Kuru, Kylefoley76, Kylemcinnes, Lcolson, Life=search, Lightmouse, Lihaas, LinguistAtLarge, Little Mountain 5, Lldenke, Loodog, Luckyherb, Luk, Mackdady8, Macpunc,Macy, Maharaja9933, Mahudhy, Marek69, MarkBolton, Materialscientist, MattieTK, Mav, Maxis ftw, Meg egg, Megan 189, Mennato, Mentality, Merzbow, Midgley, Midusunknown,Mikeycbaby, Mirffy, Mkweise, Mlpearc, Monterey Bay, MrOllie, Mucky Duck, Mueller42, Musides, NJR ZA, Nahteecirp, Narcberry, NathanHurst, NawlinWiki, NeilN, NerdyScienceDude,NescioNomen, Neum, NickBush24, Oliver202, Omega025, One Salient Oversight, Opelio, Pakaran, Paleorthid, Pandemias, Para, Pengo, Persian Poet Gal, Perx1, Peter Kaminski, PeterNetsu,Phantomsnake, Pharaoh of the Wizards, PhilKnight, Philip Trueman, Pinethicket, Plumbago, Plvekamp, Predator1087, PrinceRegentLuitpold, Professor Newcomb, Pstudier, Punkishlyevil,Pwt898, Quest for Truth, Quinn d, R'n'B, R.J.Oosterbaan, RJaguar3, RKloti, RUL3R, Rachaeldez, RadManCF, Rajivrkr, Ralpharama, Raymondwinn, Rbrandonmoore, Rd232, Rearden Metal,Revth, Rich Farmbrough, Rick Block, Rifleman 82, Rjwilmsi, Ronz, Rory096, SJP, Sam Korn, Sango123, Scarian, Shadowjams, Sillybilly, Sinus, Slakr, Slwnix, Smithbrenon, Spencer, Splette,Srbauer, Stanleycr1, Starsend, Stemonitis, Stepa, Stephen G. Brown, Steve G, Steven Zhang, Stormie, Struthious Bandersnatch, Suffusion of Yellow, Swallow2011, Taestell, Tamás Kádár,Tarheel95, Taxman, Tcncv, TeH nOmInAtOr, Teejay17, Telecineguy, TempestSA, Tempodivalse, Tewfik, Thatguyflint, The Thing That Should Not Be, TheKhakinator, TheMathinator,Themadchopper, Thingg, Thinking of England, Thorwald, Thumperward, Tide rolls, Tierneyjohn, Tiki God, TimProof, Timneu22, Titoxd, Tohd8BohaithuGh1, Tomcenc, Tommy2010, TonyFox, Tony14k, Tonythermo, True Pagan Warrior, U890, UV254, UbUb, Ultramarine, Uncle Dick, Unyoyega, Useight, Vague Rant, Vegaswikian, Verbum Veritas, Vianello, Viriditas, Vjockin,Vmenkov, Voyagerfan5761, Vsmith, WODUP, Wadems, Warut, Wavelength, Waynebeast, Wenli, Wiki12345678, WikiBrik, WikiDan61, WikiHaquinator, Wikieditor06, Wikip rhyre,Wikipeditor, Wikispaceman, Willking1979, Wisq, Wmas01, Wtshymanski, Xabian40409, Y2000, Yamamoto Ichiro, Yermi888, Yintan, Yobol, Yomangani, Youghurt, Yuckfoo, Yvh11a,ZooFari, Zsero, Žiedas, 949 anonymous edits

Electrocoagulation  Source: http://en.wikipedia.org/w/index.php?oldid=432095864  Contributors: Altenmann, Anaxial, Arcadian, Bduke, Captain Selenium, Ccdqi, Chaver4u, Chris 73,Dekimasu, DrAbeBeagles, Ebyabe, Ericelgressy, MenteMagica, Moreau1, Nono64, Pearle, Philwkpd, Quantum-ionics, RJFJR, Snafflekid, Spellmaster, Thatjenn, Thejadefalcon, 16 anonymousedits

Expanded granular sludge bed digestion  Source: http://en.wikipedia.org/w/index.php?oldid=430430040  Contributors: Ahoerstemeier, Johnfos, Vortexrealm, 3 anonymous edits

Fine bubble diffusers  Source: http://en.wikipedia.org/w/index.php?oldid=421794361  Contributors: Adgee, Boilerup12, Cathydunham, Editore99, Halogenated, Miss Madeline, MrOllie,OnePt618, Ovezea, Parutakupiu, Shoefly, TomFrankel, Trlabarge, Twirligig, 7 anonymous edits

Sedimentation  Source: http://en.wikipedia.org/w/index.php?oldid=425584542  Contributors: Alansohn, Amaltheus, Andres, Andrewpmk, Anonymous Dissident, Attarparn, Avjoska, Awickert,Can't sleep, clown will eat me, Ceyockey, Chriswaterguy, Dgp22 njitwill, Edward321, Glasnt, Gracenotes, GregorB, Hablu, HexaChord, IvanLanin, Jay Litman, Jeff G., KGasso, Karlhahn,Katherine, Ktr101, Lastdestination.89, LeaveSleaves, Leoiscool333333, Lost tourist, Lvzon, Martin451, Mattisse, Mbell, McSly, Meetikher, Michael Hardy, MickeyI04, Mikenorton, NrDg,PMDrive1061, PeHa, Pekaje, Persian Poet Gal, Pikiwyn, Pizza1512, Pornofreak, Radiojon, Redeagle688, Rijn, Salvio giuliano, SebastianHelm, Sheogorath, Stepa, The Random Editor, Theundertow, Vsmith, WillowW, Xavier J, Xezbeth, Zhou Yu, Zudduz, 89 anonymous edits

Membrane bioreactor  Source: http://en.wikipedia.org/w/index.php?oldid=423459792  Contributors: A930913, Aushulz, Beetstra, Bendel boy, Dbrothwell, Dthomsen8, Felsenbirne, Hmains,Hollaatchyoboy, M brannock, Mìthrandir, Ndaiju, Oleg Alexandrov, Poubelle29, Rich Farmbrough, Ronz, Sal4u1982, Skysmith, Soekandar, Stuartfost, WCFrancis, Withlyn, 30 anonymous edits

Retention basin  Source: http://en.wikipedia.org/w/index.php?oldid=414608905  Contributors: Anlace, Basar, Bobblewik, Cherrypj, Cobaltcigs, Emil76, Ewlyahoocom, Fbthye, Frog47, Hilpll,Hraefen, Ian Pitchford, John Maynard Friedman, Judahdavis, Moreau1, Namazu-tron, Paleorthid, Pit-yacker, ProfessorXY, Richard Arthur Norton (1958- ), Someguy1221, Vegaswikian,WereSpielChequers, WhisperToMe, Willow4, 17 anonymous edits

Reverse osmosis  Source: http://en.wikipedia.org/w/index.php?oldid=431637797  Contributors: 11Dunc11, Aarticles, AhmadH, Ahoerstemeier, Alex.tan, Ampac usa, Andrhyo, AndyBQ,Angstorm, Anonymi, Archanamiya, Arm, Armeria, Armymanmikey, ArthurDenture, Ashley.margery, Ashvidia, Astaroth5, Astronaut, AxelBoldt, Axl, BAMopedia, Bahamut0013, Beetstra,Behind The Wall Of Sleep, Beland, Bennythewoof, Berlo84, Bilbo1507, Blackangel25, Bob, Bob Kreisher, Bobblewik, Bobo192, Bogdangiusca, Borameer, BorgQueen, Bork, BrokenSegue,Buzzbo, CDM2, Cambrasa, Can't sleep, clown will eat me, Carroy, CharlesC, Chefallen, Chenzw, ChildofMidnight, Chowbok, Christian75, Chriswaterguy, Chrkl, Ckatz, Coasterlover1994,Cocoaguy, Col16, Coro, Corpx, Creavolution, Cristivivo, Ctsspruill13, Cymbalta, DMahalko, Da Joe, Dannable, David Shankbone, Davidpaulson, Deli nk, Delirium, Dennis Schmitz, Dlfelps,Dman727, DocWatson42, Doron, Dpl yho, Drahkrub, Ds13, Duckdive117, Duncan, Dureo, Długosz, Eike Welk, Emonkey, Epbr123, Espwater, Euryalus, Evalowyn, Fabiform, Falcon8765,Fieldday-sunday, Flatline, FreplySpang, From-cary, Froth, Fæ, Gail, Gary Cziko, Gem fr, George Kienzle, Gerwen, Getterstraight, Giftlite, Gigemag76, Gilderien, Gilliam, Give Peace A Chance,Glenn, Goldenrowley, Grundle2600, Gtk123, Gtstricky, Gwernol, Gwguffey, H2O, HEL, HYC, HarlandQPitt, Igodard, Immunize, Isaac, Isnow, ItascaSystems, Izakjacobuslouw, Jaganath,Jallotta, Jeremymobile, Jigibb, Jimscottuk, Jodarom, John Reaves, John Riemann Soong, Jonathan.s.kt, Juliancolton, Karuna8, Kateshortforbob, Kingfish101, Kjkolb, Knuckles, Kummi, Kuru,Landon1980, LeaveSleaves, Libertyblues, Ligar, Lightmouse, Lirnup, Llywelyn, Loewenstein, Lord Roem, Lpoulsen, Luk, LurkingInChicago, M7, MER-C, Maisy101, Maksdo, Mark.murphy,Markhurd, Markj99, MartinHarper, Materialscientist, Maxhugen, Mbell, Mbeychok, Mentifisto, Mgiganteus1, MikeChE, Mindak3, Mindmatrix, Mion, Mkweise, Mmeijeri, Momalki76, Mono,MrOllie, Mrosaclot, Msbchphdech, Mtensign, Mxn, Mysid, Nave.notnilc, NawlinWiki, Ndkl, NeetuBarmecha21, NewEnglandYankee, Newstrens, Nick UA, Nihonjoe, Nsaa, Oldmanwalyn,Omicronpersei8, Osmoflo, Palnatoke, Paloma Walker, Pcirrus, PeterSymonds, Philip Trueman, PierreAbbat, Pinethicket, Pinzo, Pjetter, Pmberry, Poeloq, Powderblue, R'n'B, R.J.Oosterbaan,Raghunathan.vs, Rahim6464, Randyoo, Raven in Orbit, Reaper Eternal, Redjar, Register112, Rhys jw, Rifleman 82, Robert Merkel, Rohan Jayasekera, Romanm, Ronz, Roscoe x, Rror,Rtfarrell5, Sagaciousuk, Sammy1462, Samtheboy, ScaldingHotSoup, ScientistInTraining, Scratch, Screen stalker, Sean Heron, Senator Palpatine, Shenme, Sicvolo, SidP, Sikkema, SirLamer,Sldenoble, Sleep pilot, Sleibler, Slimserver, Starsend, StaticGull, Stepa, Steve Wise, SyntaxError55, Syrthiss, TastyPoutine, Telso, ThatKid98, The Thing That Should Not Be, TheTranshumanist, Tickle me gusta, Tide rolls, Tiger304, Timo Honkasalo, Trilobitealive, TurboChan, Ubiquinaut, Unixcrab, Vald, Vatassery, Vegaswikian, Velella, Vinifera, Vinophilussylvestris,Vishnava, Vistro, Wade.david, WaterSuperhero, Wavelength, WeniWidiWiki, WhiteOakTree, Wmahan, Wnissen, Woudloper, Wvoutlaw2002, Xeno, Xeyback, Yaf, YordanGeorgiev, Zodon,Zundark, Zylox, 590 ,ناگداز یلق ,24.يدماغ.دمحأ anonymous edits

Rotating biological contactor  Source: http://en.wikipedia.org/w/index.php?oldid=418147137  Contributors: AlexGadsby, Ben.c.roberts, Biddleje, CapitalR, DivSundar, FironDraak,IndustrialChemist, KudzuVine, Kurieeto, Mbeychok, Pearle, Richhoncho, Rjwilmsi, Salad Days, Stemonitis, The Rambling Man, Vortexrealm, WikHead, Zvar, 17 anonymous edits

API oil-water separator  Source: http://en.wikipedia.org/w/index.php?oldid=415506872  Contributors: AndreasJS, Beagel, CZmarlin, Dceron, Hydroflo, Jimenvironmental, Lv58, Max736,Mbeychok, Micasta, Rjwilmsi, Snezzy, User A1, Wtmitchell, Zuluct, 11 anonymous edits

Septic tank  Source: http://en.wikipedia.org/w/index.php?oldid=428817043  Contributors: Aardark, Accountholder, Acha11, Aitias, Aleenf1, Alperen, Andromeda451, Ann Stouter, AnthonyAppleyard, Arch.satish, Armeria, Astanhope, Atlant, Badly Bradley, Barin69, Beenturns23, Bob111asdf, Bobo192, Bongwarrior, Boss911, Bozoid, BruceDLimber, Bryan Derksen,Caitlinfashion, Callumw 88, Caseyc, Chameleon, Cherkash, Chriswaterguy, CokeBear, Comrade009, Cpm990, Cybercobra, DMahalko, Dailynetworks, DavidFarmbrough, Deathawk,Dennyboy34, Dismas, Djwolfie, Doniago, Dougn, Duk, Duncharris, E. Fokker, El Cubano, Elkman, Engelbaet, Eric.weigle, Erud, Etimster, Evilandi, Faizhaider, Fanx, Fullobeans, Furrykef,GTZ-44-ecosan, Gadfium, GerardM, Graham87, Gregorydavid, Greybeard, Hbartlett, Hede2000, Hmains, HorsePunchKid, II MusLiM HyBRiD II, Ilamb94, Inwind, Isthisthingon, J04n, JTN,Janko, Jgrabbs, JohnWhitlock, Johntheattacker, Jooler, Kjkolb, Knuckles, Koenige, Koibeatu, KudzuVine, Kyle1081, Lankiveil, Lisasmall, Lot49a, Lowjay, Luna Santin, Mahkciwnad,MakeChooChooGoNow, Malcohol, Martarius, Mathew Scott Fitsgarrett, Mbeychok, Mboverload, Micromaster, Mike Rosoft, Monkeyblue, Mrs Trellis, Muad, Murderbike, N8hachi, Nblanton,Nkthen, Nuberger13, Numbersinstitute, Oatmeal batman, OlEnglish, Orem, Oxymoron83, Ozzykhan, Paleorthid, PaulJones, Pedant, Peter Horn, Philip Trueman, Poulson01, Pugwash, R2bomber,Rainkingcc, RanEagle, Rhkramer, Ringnick, Rjwilmsi, Rlandmann, Rockintovanhalen, Ronz, Ryanhanson, SaltyBoatr, Sam Hocevar, Samw, SchuminWeb, Septicsid, Sewer Me, Shipnerdbear,SimonP, Sjbcen, SoilMan2007, Somno, Sonett72, Stanley1976jesus, StaticGull, Sweetfilter, Tarquin, Teapotgeorge, The Red Hat of Pat Ferrick, Tiganusi, Tomwhite56, Tryptofish, Twyford,Uninvisible, Utilitysupplies, VMS Mosaic, Valfontis, Vanished 6551232, Velella, Versageek, Vortexrealm, Walter Hartmann, Ward3001, Wastetech, WhisperToMe, Will Beback, Will Decay,Wilochka, Wonkypixel, Xyzzyplugh, Yabbadab, Zedla, 216 anonymous edits

Article Sources and Contributors 174

Stabilization pond  Source: http://en.wikipedia.org/w/index.php?oldid=430415699  Contributors: Epipelagic, Hamzeh r, Mblumber, Mschiffler, Thewellman, 4 anonymous edits

Ultrafiltration (industrial)  Source: http://en.wikipedia.org/w/index.php?oldid=377282629  Contributors: Amokbel, Arcadian, Benreis, Dougher, Gabriel Kielland, Gary King, Geekdiva, Mion,Moreau1, Peter in s, R'n'B, Roger.rodin, T-borg, Woohookitty, Zereshk, 13 anonymous edits

Treatment pond  Source: http://en.wikipedia.org/w/index.php?oldid=431342157  Contributors: Alan Liefting, Alpha Quadrant, Alpha Ralpha Boulevard, Architectsf, Closedmouth, Conti,Darklilac, DaveTheRed, Epipelagic, Gaius Cornelius, Gloverepp, Hmains, Jkwchui, KVDP, Knoma Tsujmai, Koavf, MiltonT, Moreau1, No such user, Parutakupiu, Rjwilmsi, SchuminWeb,Sfahey, Signalhead, Slashme, Vortexrealm, WereSpielChequers, Woohookitty, WriterHound, Zoicon5, 13 anonymous edits

Wet oxidation  Source: http://en.wikipedia.org/w/index.php?oldid=413279510  Contributors: Alan Liefting, Desertsky85451, Gobonobo, Leftfoot69, MaugansC, Mindmatrix, OdVardara, PolKnops, Rjwilmsi, SeanWillard, Stainless316, UberScienceNerd, Vortexrealm, Woohookitty, 3 anonymous edits

Image Sources, Licenses and Contributors 175

Image Sources, Licenses and ContributorsFile:Wastewater effluent.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Wastewater_effluent.JPG  License: Public Domain  Contributors: Nigel WylieFile:Sewer Plant.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Sewer_Plant.jpg  License: Creative Commons Attribution 3.0  Contributors: RjgalindoFile:ESQUEMPEQUE-EN.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ESQUEMPEQUE-EN.jpg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: LeonardG.File:SchemConstructedWetlandSewage.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:SchemConstructedWetlandSewage.jpg  License: Public Domain  Contributors: YayasanIDEP Foundation and Wastewater GardensFile:Sedimentation tank.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Sedimentation_tank.jpg  License: Creative Commons Attribution 3.0  Contributors: MADeFile:Activated Sludge 1.png  Source: 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Commons Attribution 2.0  Contributors: User:Ikar.usFile:Discharge pipe.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Discharge_pipe.jpg  License: Public Domain  Contributors: Wouterhagens, 1 anonymous editsImage:biofilter.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Biofilter.jpg  License: GNU Free Documentation License  Contributors: LLucasImage:CVRD4chamber.air.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CVRD4chamber.air.jpg  License: Creative Commons Attribution 3.0  Contributors: Red58billImage:CVRD5biofilter.air.exhaust.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CVRD5biofilter.air.exhaust.jpg  License: Creative Commons Attribution 3.0  Contributors:Red58billImage:Trickle Filter.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Trickle_Filter.svg  License: Public Domain  Contributors: Trickle_Filter.png: Mbeychok derivative work:Malyszkz (talk)Image:Trickle Filter Cross-section.png  Source: http://en.wikipedia.org/w/index.php?title=File:Trickle_Filter_Cross-section.png  License: GNU Free Documentation License  Contributors:Milton Beychok - Mbeychok 02:03, 23 October 2007 (UTC)File:Yes check.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Yes_check.svg  License: Public Domain  Contributors: SVG by Gregory Maxwell (modified by WarX)Image:Hazard_O.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Hazard_O.svg  License: Public Domain  Contributors: AJenbo, BLueFiSH.as, Crazy-Chemist, MarianSigler,Matthias M., Phrood, Thuresson, W!B:, 5 anonymous editsFile:Ozone-resonance-Lewis-2D.png  Source: http://en.wikipedia.org/w/index.php?title=File:Ozone-resonance-Lewis-2D.png  License: Public Domain  Contributors: Ben MillsFile:Ozonolysis_scheme.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Ozonolysis_scheme.svg  License: Public Domain  Contributors: ~KFile:Atmospheric ozone.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Atmospheric_ozone.svg  License: Public Domain  Contributors: user:RedAndrFile:Nimbus ozone Brewer-Dobson circulation.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Nimbus_ozone_Brewer-Dobson_circulation.jpg  License: Public Domain Contributors: Bkell, Szdori, 2 anonymous editsFile:IM ozavg ept 200006.png  Source: http://en.wikipedia.org/w/index.php?title=File:IM_ozavg_ept_200006.png  License: Public Domain  Contributors: Monkeybait, NHSavage, 2 anonymouseditsFile:Ozone cracks in tube1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ozone_cracks_in_tube1.jpg  License: Public Domain  Contributors: PeterlewisFile:Alder showing ozone discolouration.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Alder_showing_ozone_discolouration.jpg  License: Public Domain  Contributors: PatTemple, U.S. Forest ServiceFile:SignboardAirQualityHouston.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:SignboardAirQualityHouston.JPG  License: Public Domain  Contributors: WhisperToMeImage:UV-ontsmetting laminaire-vloeikast.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:UV-ontsmetting_laminaire-vloeikast.JPG  License: Public Domain  Contributors:Elipongo, Karelj, Newbie, Pieter Kuiper, TeleComNasSprVen, 2 anonymous editsImage:Germicidal Lamp 1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Germicidal_Lamp_1.jpg  License: Creative Commons Attribution 2.5  Contributors: AtlantImage:Bragança43.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Bragança43.jpg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: User:DantaddImage:Water plant.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Water_plant.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Le.crouton (talk)File:Prázdná aerační místnost, ÚV Káraný.jpg  Source: 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Creative Commons Attribution 3.0  Contributors: Jakub BułasFile:Slow sand filter EPA.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Slow_sand_filter_EPA.jpg  License: Public Domain  Contributors: w:United States EnvironmentalProtection AgencyUS Environmental Protection AgencyFile:Vsakovací nádrže umělé infiltrace v ÚV Káraný.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Vsakovací_nádrže_umělé_infiltrace_v_ÚV_Káraný.jpg  License: CreativeCommons Attribution-Sharealike 3.0  Contributors: Che(Please credit as "Petr Novák, Wikipedia" in case you use this outside WMF projects.)Image:wikisource-logo.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Wikisource-logo.svg  License: logo  Contributors: Nicholas MoreauImage:Aerated Lagoon.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Aerated_Lagoon.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: C TharpImage:Haase Lubeck MBT.JPG  Source: 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Image Sources, Licenses and Contributors 176

Image:Anaerobic Lagoon at Cal Poly.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Anaerobic_Lagoon_at_Cal_Poly.jpg  License: Creative Commons Attribution-Sharealike 2.5 Contributors: Kjkolb, Zephynelsson VonImage:Biogasholder and flare.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Biogasholder_and_flare.JPG  License: Creative Commons Attribution 3.0  Contributors: Originaluploader was Vortexrealm at en.wikipediaImage:Biogas pipes.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Biogas_pipes.JPG  License: Creative Commons Attribution 3.0  Contributors: Original uploader wasVortexrealm at en.wikipediaImage:Anaerobic digestate.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Anaerobic_digestate.JPG  License: Creative Commons Attribution-Sharealike 2.5  Contributors:Original uploader was Vortexrealm at en.wikipediaImage:Final half coil vessel.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Final_half_coil_vessel.JPG  License: GNU 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planted constructed wetland.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Newly_planted_constructed_wetland.jpg  License: Public Domain  Contributors:en:User:Lloyd rozFile:Mature Constructed Wetland.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Mature_Constructed_Wetland.jpg  License: Public Domain  Contributors: en:User:Lloyd rozImage:Verschiedene Typen von Pflanzenkläranlagen.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Verschiedene_Typen_von_Pflanzenkläranlagen.jpg  License: Public Domain Contributors: bb143143Image:Lavafilter.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Lavafilter.jpg  License: Public Domain  Contributors: KVDP (talk)Image:Flowform.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Flowform.jpg  License: Creative Commons Attribution-Sharealike 3.0,2.5,2.0,1.0  Contributors: AalangImage:DAF Unit.png  Source: http://en.wikipedia.org/w/index.php?title=File:DAF_Unit.png  License: GNU Free Documentation License  Contributors: Original uploader was Mbeychok aten.wikipediaImage:REDOX DAF unit 225 m3-h-1000 GPM.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:REDOX_DAF_unit_225_m3-h-1000_GPM.jpg  License: GNU Free DocumentationLicense  Contributors: user:SmileJohn (enWP)File:Multiflash.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Multiflash.svg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: Multiflash.png: RubenCastelnuovo (myself) derivative work: NJR_ZA (talk)File:PlantaSchemaFiction.png  Source: http://en.wikipedia.org/w/index.php?title=File:PlantaSchemaFiction.png  License: GNU Free Documentation License  Contributors: Aushulz, Maksim,WstFile:Reverse osmosis desalination plant.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Reverse_osmosis_desalination_plant.JPG  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: James GrellierFile:Shevchenko BN350 desalinati.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Shevchenko_BN350_desalinati.jpg  License: Public Domain  Contributors: User:LcolsonImage:Image-DiscDiffuser.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Image-DiscDiffuser.JPG  License: GNU Free Documentation License  Contributors: TomFrankelImage:Fine Bubble Diffuser (Tube).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Fine_Bubble_Diffuser_(Tube).jpg  License: Creative Commons Attribution-Sharealike 3.0 Contributors: C TharpImage:Fine bubble diffuser (Disc).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Fine_bubble_diffuser_(Disc).jpg  License: Creative Commons Attribution-Sharealike 3.0 Contributors: C TharpFile:Siltation or Sedimentation.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Siltation_or_Sedimentation.jpg  License: Public Domain  Contributors: U.S. Fish and WildlifeService Original uploader was Mattisse at en.wikipediaImage:MBR Schematic.jpg  Source: 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Image:Landpeople s cc8.PNG  Source: http://en.wikipedia.org/w/index.php?title=File:Landpeople_s_cc8.PNG  License: Public Domain  Contributors: USGSImage:ultra filtration.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Ultra_filtration.JPG  License: Public Domain  Contributors: User:KjaergaardImage:Ultrafiltration Grundmühle.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ultrafiltration_Grundmühle.jpg  License: Unrestricted Use with attribution  Contributors: W.E.T.GmbH Original uploader was Benreis at de.wikipediaFile:Ecological swimming pond schematic.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ecological_swimming_pond_schematic.jpg  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: User:KVDPImage:Treatment-pond-raster.png  Source: http://en.wikipedia.org/w/index.php?title=File:Treatment-pond-raster.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors:Jkwchui

License 178

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