Wastewater- envi eng

8/13/2019 Wastewater- envi eng

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Gerald M. Lim

BSChE-4

Waste Water TreatmentWastewater treatment is closely related to the standards and/or expectations set

for the effluent quality. Wastewater treatment processes are designed to achieveimprovements in the quality of the wastewater. The various treatment processes mayreduce: Although there are many variations of wastewater treatment plants, most willhave the following steps: preliminary treatment, primary treatment, secondarytreatment, tertiary treatment, disinfection, and solids handling.

Process

1. Preliminary Treatment

During Preliminary Treatment, the incoming rawsewage, or influent, is strained to removed all large objectsthat make their way into the sewer system. These objectscan be anything from rags and sticks to toys, cans andeven snakes. Generally bar screens, which come in avariety of shapes and sizes, are used to remove the items.The influent flows across these screens, objects catch on
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the screens, are raised out of the water and are then raked (either mechanically ormanually) off the screens.

Another component of Preliminary Treatment is the grit channel where thevelocity of the incoming wastewater is carefully controlled to allow sand, grit, and

stones to settle to the bottom of the channel while keeping the majority of thesuspended organic material in the water column. The grit is removed from the channel,added to the larger objects removed by the bar screens, and taken to the landfill fordisposal.

Preliminary Treatment is vital for preventing damage to pumps and otherequipment in the remaining treatment stages.

2. Primary Treatment

Many plants have a sedimentation stage where the sewage is allowed to passslowly through large tanks, commonly called primaryclarifiers or primary sedimentation tanks . The tanksare large enough that sludge can settle and floatingmaterial such as grease and oils can rise to thesurface and be skimmed off. The main purpose ofprimary treatment is to produce both a generallyhomogeneous liquid capable of being treatedbiologically and a sludge that can be separatelytreated or processed. Primary clarifiers are usuallyequipped with mechanically driven scrapers thatcontinually drive the collected sludge towards ahopper in the base of the tank from where it can be pumped to further sludgetreatment stages. The clarified water flows on to the next step of treatment.

3. Secondary Treatment

Secondary treatment processes can remove up to 90% of the organic matter inwastewater by using biological treatment processes. The two most commonconventional methods used to achieve secondary treatment are attached growth

processes and suspended growth processes.

a. Attached Growth Processes . In attached growth (or fixed film) precesses,bacteria, algae, fungi and other microorganisms grow and multiply on thesurface of stone or plastic media, forming a microbial growth or slime layer(biomass) on the media. Wastewater passes over the media along with air toprovide oxygen, and the bacteria consume most of the organic matter in the


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wastewater as food. Attached growth process units include trickling filters,biotowers, and rotating biological contactors.

Trickling filters biotower biological contactors

b. Suspended Growth Processes. In suspended growth processes, the microbialgrowth is suspended in an aerated water mixturewhere the air is pumped in, or the water is agitatedsufficiently to allow oxygen transfer. Thesuspended growth process speeds up the work ofaerobic bacteria and other microorganisms thatbreak down the organic matter in the sewage byproviding a rich aerobic environment where themicroorganisms suspended in the wastewater canwork more efficiently. In the aeration tank ,wastewater is vigorously mixed with air andmicroorganisms acclimated to the wastewater in asuspension for several hours. This allows thebacteria and other microorganisms to break downthe organic matter in the wastewater. Suspendedgrowth process units include variations of activatedsludge, oxidation ditches and sequencing batchreactors.

After biological treatment, the water is

pumped to secondary clarifiers where any leftoversolids and the microorganisms sink to the bottom.These solids are handled separately from thesupernatant which continues on to disinfection.


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4. Tertiary Treatment

The purpose of tertiary treatment is to provide a final treatment stage to raise theeffluent quality to the desired level. This advanced treatment can be accomplished bya variety of methods such as coagulation sedimentation, filtration, reverse osmosis, and

extending secondary biological treatment to further stabilize oxygen-demandingsubstances or remove nutrients. In various combinations, these processes can achieveany degree of pollution control desired. As wastewater is purified to higher and higherdegrees by such advanced treatment processes, the treated effluent can then bereused for urban, landscape, and agricultural irrigaton, industrial cooling andprocessing, reacreational uses and water recharge, and even indirect and directaugmentation of drinking water supplies.

a. Coagulation sedimentation

Chemical coagulation sedimentation is used toincrease the removal of solids from effluent afterprimary and secondary treatment. Solids heavierthan water settle out of wastewater by gravity. Withthe addition of specific chemicals, solids canbecome heavier than water and will settle. Alum,lime, or iron salts are chemicals added to thewastewater to remove phosphorus. With thechemicals, the smaller particles clump or 'floc'together into large masses. The larger masses ofparticles will settle out in the sedimentation tank reducing the concentration of phosphorus by morethan 95%.

b. Filtration

A variety of filtration methods are available to ensure highquality water. Sand filtration, which consists of simplydirecting the flow of water through a sand bed, is used toremove residual suspended matter. Filtration overactivated carbon results in the removal of the followingtypes of contaminants: non-biodegradable organiccompounds, adsorbable organic halogens, toxins, colorcompounds and dyestuffs, aromatic compounds including

phenol and bis-phenol A (BPA), chlorinated/halogenatedorganic compounds, and pesticides.

Although there are a number of different methods ofmembrane filtration, the most mature is pressure drivenmembrane filtration. This relies on a liquid being forcedthrough a filter membrane with a high surface area.Membrane filtration is designed to remove bacteria,


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viruses, pathogens, metals, and suspended solids.

c. Reverse osmosis

In the reverse osmosis process, pressure is used to force effluent through a membranethat retains contaminants on one side and allows the clean water to pass to the otherside. Reverse osmosis is actually a type of membrane filtration called microfiltrationbecause it is capable of removing much smaller particles including dissolved solids suchas salt. This process is also effective at removing biological contaminants, metals,pharmaceuticals, pesticides, and endocrine disruptors.

d. Nutrient Removal

Nitrogen control. Ammonia in wastewater effluent can be toxic to aquatic life incertain instances. By providing additional biological treatment beyond the secondarystage, nitrifying bacteria present in wastewater can biologically convert ammonia tothe non-toxic nitrate through a process known as nitrification. The nitrification process isnormally sufficient to remove the toxicity associated with ammonia in the effluent.Since nitrate is a nutrient, excess amounts can contribute to eutrophication in thereceiving waters. In situations where nitrogen must be completely removed fromeffleunt, an additional biological process can be added to the system to convert thenitrate to nitrogen gas. The conversion of nitrate to nitrogen gas is accomplished bybacteria in a process known as denitrification. Effluent with nitrogen in the form ofnitrate is placed into a tank devoid of oxygen, where carbon-containing chemicals,such as methanol, are added. In this oxygen-free environment, bacteria use theoxygen attached to the nitrogen in the nitrate form releasing nitrogen gas. Becausenitrogen comprises almost 80% of the air in the earth's atmophere, the release ofnitrogen into the atmosphere does not cause any environmental harm.

Phosphorus control Like nitrogen, phosphorus is a necessary nutrient for the growth ofalgae. Phosphorus reduction is often needed to prevent eutrophication beforedischarging effluent into lakes, reservoirs, and estuaries. Phosphorus can be removedbiologically in a process called enhanced biological phosphorus removal. In thisprocess, specific bacteria, called polyphosphate accumulating organisms (PAOs), areselectively enriched and accumulate large quantities of phosphorus within their cells(up to 20% of their mass). When the biomass enriched in these bacteria is separatedfrom the treated water, these biosolids have a high fertilizer value.

Phosphorus removal can also be achieved by chemical precipitation, usually with saltsor iron, alum, or lime. This may lead to excessive sludge productions as hydroxidesprecipitates and the added chemicals can be expensive. Despite this, chemicalphosphorus removal requires a significantly smaller equipment footprint than biologicalremoval, is easier to operate, and is often more reliable thatn biological phosphorusremoval.


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5. Disinfection

The purpose of disinfection in the treatment of wastewater is to substantially reduce

the number of microorganisms in the water to be discharged back into theenvironment and is almost always the final step in the treatment process regardless ofthe level or type of treatment used. The effectiveness of disinfection depends on thequality of the water beign treated (e.g., cloudiness, pH, ammonia content, etc.), thetype of disinfection being used, the disinfectant dosage (concentration and time), andother environmental variables. Cloudy water will be treated less successfully since solidmatter can shield organisms. Generally, short contact times, low doses, and high flowsall prevent effective disinfection. Common methods of disinfection include ozonation,chlorine, and ultraviolet light.

Chlorination remains the most common form of wastewater disinfection due to its lowcost and long-term history of effectiveness. One disadvantage is that chlorination ofresidual organic material can generate chlorinated-organic compounds that may becarcinogenic or harmful to the environment. Residual chlorine or chloramines (formedby the combination of chlorine and ammonia) may also be capable of chlorinatingorganic material in the natural aquatic environment. Further, because residual chlorineis toxic to aquatic species, the treated effluent must also be chemically dechlorinatedadding to the complexity and cost of treatment.

Ultraviolet (UV) light can be used instead of chlorine. Because no chemicals are used,the treated water has no adverse effect on organisms that later consume it. UVradiation causes damage to the genetic structure of bacteria, viruses, and otherpathogens making them incapable of reproduction. The key disadvantages of UV

disinfection are the need for frequent lamp maintenance and replacement, and theneed for a highly treated effluent to ensure that the target microorganisms are notshielded from the UV radiation.

Ozonation is also becoming a popular alternative to chlorine. Ozone (O3) is generatedby passing oxygen (O2) through a high voltage potential resulting in a third oxygenatom becoming attached and forming O3. Ozone is very unstable and reactive andoxidizes most organic material it comes in contact with thereby destroying manypathogenic microorganisms. Ozone is considered to be safer than chlorine because itis generated onsite as needed and does not have to be stored. Ozonation alsoproduces fewer disinfection by-products. A disadvantage of ozone disinfection is the

high cost of the ozone generation equipment and the requirements for specialoperators.

Ozone is also useful at reducing the concentrations of iron, manganese, and sulfur byoxidizing these metals in water to form insoluble metal oxides or elemental sulfur. Theinsoluble particles are then removed by filtration. Ozonation is also effective atreducing or eliminating most taste and odor problems.


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6. Solids Handling

Primary treatment and secondary biological processesconcentrate waste organics into a sludge. Methodsfor processing raw sludge include anaerobic digestion

and mechanical dewatering by either belt-filterpress ing or centrifugation. Conventional methods ofdisposal are apllication as a fertilizer or soil conditioneron agricultural land, landfilling in a dedicated disposalsite, or codisposal with municipal solid waste. .

2. Biochemical Oxygen Demand

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