Water Pollution Contents: Water contaminants Water supply Water treatment Wastewater collection...

Water Pollution

Contents: Water contaminants Water supply Water treatment Wastewater collection Wastewater treatment Sludge treatment

Sources of Water Pollution

industrial pollution: chemicals municipal pollution: combined

sewage agriculture sediment erosion petroleum products mine leaching

Effect of Pollution on Rivers When high energy organic material

is discharged to a river, several changes can occur downstream: decomposition of organics causes

drop in dissolved oxygen (DO). If DO>0 can get oxygen sag curve. If DO=0; anaerobic. get changes in biodiversity.

Water Supply and Treatment

Water Supplyhydrological cyclegroundwater suppliessurface water supplieswater transmission

Treatment MethodsCoaggulation and flocculationSettlingFiltrationDisinfection

Source of Water - 1

Potential drinking water sources: groundwater surface water

Groundwater comes from underground aquifers, into which wells are bored to recover the water.

Water in some aquifers is non-renewable. There is therefore a risk of depleting the aquifer.

Also possibly for the city to sink as the water is withdrawn

Source of Water - 2

Surface water is drawn from a lake or a river. For major rivers (e.g. Mississippi River, Rhine,

Danube), the water is used by many communities along the river.

Groundwater tends to be less contaminated than surface water because organic matter in the water has time to be decomposed by soil bacteria.

The soil itself acts as a filter so that less suspended matter is present in groundwater than in surface water.

Contamination of Groundwater

Inorganic contaminants: The most important inorganic contaminant is

nitrate ion, NO3-.

Excess nitrate ion in drinking water is a potential health hazard: risk of methemoglobinemia (or “blue-baby” syndrome).

Sources of nitrate in groundwater:- nitrogen fertilizers- atmospheric deposition- human sewage deposited in septic systems

Purification of Drinking Water

Sedimentation

Drinking water

Filtration Removal of unsettled hardness precipitate and residual floc

Chemical additions

Water supply

Flocculation

DisinfectionChlorine, to establish residual 0.3 mg/L preventing bacterial growing

Removal of suspended materials and precipitation by gravity

Conventional water treatment

Flow diagram


Steps in a typical water treatment plan

- Coagulation (settling and

precipitation)

- Hardness removal

- Disinfection

Coagulation - 1 Coagulation (or settling and precipitation) The finest particles, such as colloidal

minerals, bacteria, and pollen do not settle in the raw water.

Removal of this colloidal particles is necessary: to give the finished water a clear

appearance because they contain viruses and bacteria

that are resistant to later disinfection.

Coagulation - 2 The capture of the fine particles is done by

adding to the water either iron(III) sulfate, Fe2(SO4)3, or aluminum sulfate, Al2(SO4)3.

In the case of aluminum sulfate, Al(OH)3 is formed (in the pH range 6-8):

At this pH values, Al(OH)3 is close to its minimum solubility and at equilibrium very little aluminum is left dissolved in the water.

(aq)CO 3 (s)Al(OH) (aq) HCO 3 (aq)Al 23-3

3

Coagulation - 3 Aluminum hydroxide forms a very

gelatinous precipitate, which settles very slowly and which incorporate the colloidal particles.

With iron(III) sulfate the chemistry is analogous: Fe3+ forms gelatinous iron hydroxide

Fe(OH)3. These reactions consume hydroxide

pH decreases (neutralize alkaline water)

Hardness Removal - 1 Hardness is characterized by the

concentration of Ca2+ and Mg2+. Major problem caused by hard water:

formation of mineral deposits. Calcium can be removed by addition

of phosphate (see later). A more common way is by

precipitation and filtering of insoluble CaCO3

Hardness Removal - 2 When the calcium is present primarily as

“bicarbonate hardness” (intermediate pH), it can be removed by direct addition of Ca(OH)2 alone:

When bicarbonate ion is not present at substantial levels, a source of CO3

- must be provided at a high pH to prevent conversion of most of the carbonate to bicarbonate.

OH 2 (s)CaCO 2 Ca(OH) 2HCO Ca 232-3

2

Hardness Removal - 3 Source of carbonate ion: sodium carbonate, Na2CO3.

The precipitation of magnesium as the hydroxide requires a higher pH than the precipitation of calcium as the carbonate

The high pH required may be provided by the basic carbonate ion from soda ash (Na2CO3)

(s)CaCO CO Ca 3-2

32

(s)Mg(OH) OH 2 Mg 2-2

Hardness Removal - 4

There are two main problems: Supersaturation effect: Some CaCO3 and Mg(OH)2

usually remain in solution. They need to be removed. Use of highly basic sodium carbonate, which gives the

product water an excessively high pH, up to pH 11. Solution: water is recarbonated by bubbling CO2

into it. The carbon dioxide converts the slightly soluble

calcium carbonate and magnesium hydroxide to their soluble bicarbonate forms

Disinfection Disinfection is the most essential part of

water treatment Disinfectant used in water treatment:

chlorine

chlorine dioxide

ozone

ultraviolet radiation

Disinfection/Chlorine - 1 Chlorine dissolves in water:

Aqueous chlorine rapidly hydrolyzes to form hypochlorous acid:

Hypochlorous acid is a weak acid that dissociates according to the reaction: Ka = 3×108 mol/ L

(aq)Cl (g)Cl 22

HOCl(aq) Cl H O H (aq)Cl -22

-OCl H HOCl(aq)

Disinfection/Chlorine - 2

Speciation of active chlorine as a function of pH

Disinfection/Chlorine - 3 Sometimes (e.g. swimming pools)

hypochlorite salts, Ca(OCl)2, are substituted for chlorine gas as a disinfectant.

The hypochlorite salts are safer to handle than gaseous chlorine.

Sodium hypochlorite, NaOCl, can also be used as a substitute for chlorine.

The hypochlorite ions is then converted to hypochlorous acid:

-2

- OH HOCl O H OCl


The two chemical species formed by chlorine in water, HOCl and OCl-, are known as free available chlorine.

Free available chlorine is very effective in killing bacteria (in particular HOCl).

HOCl(aq) about 10× more effective than ClO-(aq) –result of the more lipophilic HOCl crossing bacterial membranes more easily

water with pH > 7.5 requires more chlorine – or longer disinfection time – than water with pH < 7.5

In the presence of ammonia chloroamines are formed. Alkaline pH will prevent the formation of these chloroamines. The chloroamines are called combined available chlorine. Breakpoint (Cl:N (wt/wt) =8:1)

• important terms: chlorine dose = concentration originally used chlorine residual = concentration in the finished water chlorine demand = concentration consumed by oxidizable

substances present in the water free available chlorine = sum of concentrations of HOCl(aq) and

ClO-(aq) combined available chlorine = the concentration of chloroamines


Chlorine Demand Curve

Chlorine dose (mg/L)

Ch

lori

ne

re

sid

ua

l (m

g/L

)

chlorine demandappl

ied

chlo

rine

free residual

breakpoint

combined residual


Problems with the use of chlorine as a disinfectant: Simultaneous production of some toxic chlorinated

organic compounds (e.g. chlorinated phenol). Production of trihalomethanes (THMs), CHX3. Of

particular concern is the formation of chloroform, CHCl3 (carcinogen, suspected to affect reproductive system)

chlorination byproducts, notably trihalomethanes. Example: chloroform CHCl3 which is often present at

10ppb or more. Source is natural substances (humic acids)

>C(=O)CH3 + 3HOCl —> –CO2- + CHCl3 + 2H2O

Disinfection/Chlorine Dioxide - 1

Chlorine dioxide, ClO2, is an effective water disinfectant

In the absence of impurity Cl2, it does not produce THMs in water treatment.

Chlorine dioxide oxidize organic molecules by extracting electrons from them.

Chlorine dioxide is a gas that is violently reactive with organic matter and explosive when exposed to light.


Chlorine dioxide is generated on-site, for example by the reaction of chlorine gas with solid sodium hypochlorite:

unstable, must be made in situ

an oxidizing agent, not a chlorinating agent — no taste and odour problems

no residual effect – rapidly decomposes – must add Cl2 afterward

Some concern has been raised over possible health effects of its main degradation byproducts, ClO2

- and ClO3- (chlorate) ions.

NaCl(s) 2 (g)ClO 2 (g)Cl (s)NaClO 2 222

O4H 2HCl SO5Na 8ClO SO5H NaClO 10 2422422

Disinfection/Ozone - 1

Ozone (stronger oxidizer than O2) is sometimes used as a disinfectant instead of chlorine, particularly in Europe.

unstable, must be made in situ – by electric discharge on dry O2 (air)

Process: air is filtered, cooled, dried, and pressurized, then subjected to an electrical discharge of approximately 20,000 volts.

3O2 —> 2O3 formed as a dilute mixture in air The ozone produced is then pumped into a

contact chamber where water contacts the ozone for 10-15 minutes.


• ozonation equipment is expensive, only economic on a large scale

• an oxidizing agent, not a chlorinating agent – no taste and odour problems, but cannot be used like ClO2 as a temporary replacement for chlorine

• no residual in the water, decomposition is pH dependent (also faster at higher water temperature)

• rate [OH-]0.55•[O3]2

A typical ozone water-treatment system

air

water

purified water

oxygen

contactchamber

ozonegenerator

coronadischarge

desiccator

refrigeration

cooledair

driedair

com

pres

sed

air

air

filter

pump20,000 volts

Disinfection/Ozone - 3 Interest in ozonation arises from the

possible production of toxic organochlorine compounds by water chlorination.

ozone is more destructive to viruses than is chlorine.

Unfortunately, the solubility of ozone in water is relatively low, which limits its disinfective power.

Disinfection/Ultraviolet light - 1

Ultraviolet radiation having wavelenghts below 300 nm is very damaging to life, including microorganisms by photochemical cross-linking of DNA, which absorbs strongly at this wavelength

Mercury lamps (germicidal lamps) are available, having their output radiation principally at 254 nm (UV-C at 254 nm – major output of a low pressure)

Advantage of UV method:- Short contact time: 1-10 s. Ozone and chlorine both

require contact of 10-50 minutes, therefore construction of a large reaction tank. UV disinfection can be run on a simple “flow-through” system (no holding tank)


Advantage of UV method (cont’d):- Low installation costs. Ozone generators are complex

and expensive to install; chlorine equipment is less so. - Not influenced by pH or temperature. Chlorination and

ozonation work best at lower pH, chlorine because more of it is in the HOCl rather than the OCl- form, ozone because it decomposes more rapidly at high pH.

- applicable to large and small scale installations, even domestic use

- No toxic residues. water must be clear and free of absorbing solutes


Cost comparison between the various disinfectants:

Disinfectant Plant capacitya Costb Cl2 0.05 26.0

0.5 3.0 1.0 1.7

ClO2 0.05 52.5 0.5 11.1 1.0 8.6

O3 0.05 72.5 0.5 9.6 1.0 6.6

UV 0.05 21.9 0.5 7.2 1.0 5.3

• small installations: UV is cheapest, then chlorine

• large installations: chlorine is cheapest by a wide margin

The Treatment of Wastewater and Sewage

Pollutants in Sewage - 1 Typical municipal sewage contains

oxygen-demanding materials, sediments, grease, oil, scum, pathogenic bacteria, viruses, salts, algal nutrients, pesticides, refractory organic compounds and heavy metals.

Major disposal problem with sewage: the sludge produced as a product of the sewage treatment process.

Pollutants in Sewage - 2Constituent Potential sources Effects in water

Oxygen-demanding substances

Mostly organic materials, particularly human feces

Consume dissolved oxygen

Refractory organics Industrial wastes, household products

Toxic to aquatic life

Viruses Human wastes Cause disease (possibly cancer); major deterrent to sewage recycle through water systems

Detergents Household detergents Esthetics, prevent grease and oil removal, toxic to aquatic life

Phosphates Detergents Algal nutrients Grease and oil Cooking, food processing,

industrial wastes Esthetics, harmful to some aquatic life

Salts Human wastes, water softeners, industrial wastes

Increase water salinity

Heavy metals Industrial wastes, chemical laboratories

Toxicity

Chelating agents Some detergents, industrial wastes

Heavy metal ion solubilization and transport

Solids All sources Esthetics, harmful to aquatic life

Sewage Treatment - 1 Three main categories:- primary treatment: primary settling,

mechanical treatment - secondary treatment: biological

treatment, include the related problem of disposal of sewage sludge

- tertiary treatment: include advanced treatment.

Sewage Treatment - 2

Primary Treatment - 1 Primary treatment of waste water consists of

the removal of insoluble matter such as grit, grease, and scum from water.- First step: screening to remove or reduce

the size of trash and large solids that get into the sewage system. The solids are collected on screens and scraped off for subsequent disposal.

- Second step: Grit removal.

Primary Treatment - 2 In the second step the sewage enters a

large lagoon and moves through slowly enough that any solid particles settle out.

Some materials float at the surface of the sewage (Those materials are called grease). They are removed by a skimming device.

The effluent form the primary settler is almost clear, but has a high BOD (several hundred milligrams per liter).

Secondary Treatment Objective of the secondary treatment:

to reduce the BOD to acceptable level (below 100 mg/L).

The basic principle consists of the action of microorganisms provided with added oxygen degrading organic material in solution or in suspension.

Two main systems: Trickling filter Activated sludge

Trickling Filter The simplest biological waste treatment. The trickling filter is a large round bed of

sand and gravel. A rotating boom sprinkle sprays the

wastewater over rocks or other solid support material covered with microorganisms.

Main threat: presence of toxic substances, which would kill the microorganisms.

Disadvantage: require large space Advantage: low energy consumption

Activated Sludge Reactor - 1

Very effective wastewater treatment. Require less land and, being enclosed, can

be maintained at the optimum temperatures for biological activity.

The reactor is a large tank in which the wastewater is agitated and aerated to provide the oxygen required by the microorganisms.

A portion of the sludge of microorganisms is removed from the exit stream of the reactor and recycled into the influent stream.


Schematic of activated sludge reactor:

Activate Sludge Reactor - 3

BOD may be removed by: oxidation of organic matter to provide

energy for the metabolic process. Synthesis, incorporation of the organic

matter into cell mass.


The water content in the sludge may be removed by some drying process and the resulting dewatered sludge may be incinerated or used as landfill.

To a certain extent, sewage sludge may be digested in the absence of oxygen by methane-producing anaerobic bacteria to produce methane and carbon dioxide

A well-designed plant may produce enough methane to provide for all of its power needs.

Tertiary Treatment - 1 In some cases a portion of the drinking

water is actually water that has been discharged from a municipal sewage treatment.

Tertiary waste treatment (also called advanced waste treatment): term used to describe a variety of processes performed on the effluent from the secondary waste treatment.

Tertiary Treatment - 2 The contaminants removed by tertiary

treatment fall into three general categories: suspended solids: responsible for residual

biological oxygen demand in secondary sewage effluent waters.

dissolved organic compounds: they are potentially the most toxic

dissolved inorganic materials: the major problem: nitrates and phosphates (nutrient for algae). Also, potentially hazardous toxic metals may be found among the dissolved inorganics.

Removal of Solids

Some of the colloidal particles are removed using aluminum salt which forms Al(OH)3.

Similar to the process described during the purification of the drinking water.

Removal of Dissolved Organics

The standard method for removal of dissolved organic material is by adsorption on activated carbon.

Activated carbon is characterized by a very large surface area.

The carbon is regenerated by heating it to 950C in a steam-air atmosphere.

Adsorbent synthetic polymers can also be used instead of activated carbon. They are regenerated by using solvent such as isopropanol and acetone.

Removal of Dissolved Inorganics

The effluent of secondary waste treatment generally contains 300-400 mg/L more dissolved inorganic material than does the municipal water supply.

The most cost-effective methods of removing inorganic material from water is currently membrane processes.

Methods considered: reverse osmosis, electrodialysis, and ion exchange.

Reverse Osmosis Basic principle: force water through a

semipermeable membrane that allows the passage of water but not other material.

Electrodialysis - 1

Basic principle: apply a direct current across a body of water separated into vertical layers by membranes alternately permeable to cations and anions.

Cations migrate toward the cathode and anions toward the anode.

Layers of water enriched in salts alternate with those from which salts have been removed.

Electrodialysis - 2

Ion Exchange Basic principle: passing the water

successively over a solid cation exchanger and a solid anion exchanger, which replace cations and anions by hydrogen ion and hydroxide ion, respectively.

The cation exchanger is regenerated with strong acid and the anion exchanger with strong base.

---- X H {Cat(s)} M X M {Cat(s)}H

O H {An(s)}X X H {An(s)}OH 2-

Phosphorus Removal - 1 Even when water is not destined for

immediate reuse, the removal of the inorganic nutrients phosphorus and nitrogen is highly desirable to reduce eutrophication downstream.

Organic phosphorus is converted to orthophosphate (H3PO4, H2PO4

-,HPO42-,PO4

3-). The main sources of phosphate are

polyphosphates in detergents, raw sewage, and the runoff from farms that used phosphate fertilizers.

Phosphorus Removal - 2

Algae may grow at PO43- (phosphate ions)

levels as low as 0.05 mg/L. Growth inhibition requires levels well below 0.5 mg/L.

Municipal wastes typically contain approximately 25 mg/L of phosphate (as orthophosphates, polyphosphates, and insoluble phosphate)

The efficiency of phosphate removal must be quite high to prevent algal growth.

Phosphorus Removal - 3

This removal may occur in the sewage treatment process in the primary settler in the aeration chamber of the activated sludge unit.

Activated sludge treatment removes about 20% of the phosphorus from sewage.

after secondary waste treatment.

Phosphorus is most commonly removed by precipitation, which are capable of at least 90-95% phosphorus removal at reasonable cost.

Phosphorus Removal - 4 Some chemical precipitants and their products:

Lime, Ca(OH)2, is the most commonly chemical used for phosphorus removal

The products formed are insoluble calcium phosphate, such as Ca5OH(PO4)3 and Ca3(PO4)2.

Precipitant(s) Products Ca(OH)2 Ca5OH(PO4)3 (hydroxyapatite)

Ca(OH)2 + NaF Ca5F(PO4)3 (fluorapatite) Al2(SO4)3 AlPO4

FeCl3 FePO4 MgSO4 MgNH4PO4

Nitrogen Removal - 1 Organic nitrogen is converted to

ammonium ion and nitrate Ammonia is the primary nitrogen product

produced by most biological waste treatment processes.

One method is to strip ammonia in the form NH3 gas from the water by air.

Another method is nitrification followed by denitrification.

Nitrogen Removal - 2

Nitrification - denitrification: First step: conversion of ammonia and

organic nitrogen to nitrate under strongly aerobic conditions:

These reactions occur in the aeration tank of the activated sludge plant.

O2H NO 2 H4 O 3 NH 2 2-2

bacteria asNitrosomon24

-3

rNitrobacte2

-2 NO 2 O NO 2

Nitrogen Removal - 3

Nitrification-denitrification (cont’d) Second step: reduction of nitrate to nitrogen

gas.This reaction is also bacterially catalyzed and requires a carbon source and a reducing agent such as methanol, CH3OH:

In pilot plant operation, conversions of 95% of the ammonia to nitrate and 86% of the nitrate to nitrogen have been achieved.

O H13 CO 5 (g)N 3 bacteria) ing(denitrify H6 OH CH 5 NO 6 2223-3

Waste Water Treatment Methods

aeration to remove volatile solutes precipitation of divalent cations coagulation and flocculation settling filtration disinfection

Raw water

Clean water

lime coagulant

aeratorfirst basin: insolubleMg2+ Ca2+

precipitate

second basin:flocculation

filter

carbondioxide

chlorine

sludge lagoon

A typical municipal water treatment plant

Aeration

Reasons for aeration systems: to reduce [dissolved CO2] to reduce [dissolved H2S] to promote oxidation of Fe and Mg to remove volatile organic compoundsTypes of aeration systems: gravity aerators, eg. cascade of steps spray aerators - require large land area

Coagulation and Flocculation

Filtration

Chemical Feed Injections - alum coagulant followed by polymeric flocculent

Depth Clarifier - flocs adhere to the sand as water passes through filter

Depth Filter - consists of three different media

waste sludge

treated effluent

primary clarifiergrit chamber

waste grit

bar rack

influent

Primary Treatment

Secondary Treatment (remove the BOD)

Tertiary Treatment Wastewater receiving tertiary treatment

is unable to support microbial growth, and can be of such high quality that it can be pumped directly into the water supply.

The most popular means of removing BOD is with biological treatment.

Physiochemical processes are used to remove inorganic nutrients, especially phophate and nitrate.

Sources of Sludge

Two kinds of sludge are generated in a waste treatment plant: -

organic sludge from activated sludge, trickling filter, or rotating biological reactors.

inorganic sludge from the additional of chemicals such as those used for phosphorous removal.

Anaerobic Sludge Digestor

gas outlet

methane

supernatentsupernatent

activated digesting sludge

stabilizedsludge

sludge outlet

sludge inlet supernatentremoval

scum layer scum removal

Sludge Thickening

99%water

80% of waterremoved

100%

1% solid 5% solid

Ultimate Disposal

Some alternatives for ultimate disposal of sludge have included ocean dumping, land spreading, and incineration.

Since 1992, there has been legislation prohibiting ocean dumping of sewage sludge.

Accumulation of heavy metals is of concern when sewage sludge is used on cropland. Prior control of heavy metal contamination from industrial sources enables sludge to be used more extensively.

Incineration is an expensive alternative to sludge disposal, and there is no benefit from its potential use as a fertilizer.

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