L3 - Wastewater Treatment Process_stu

8/16/2019 L3 - Wastewater Treatment Process_stu

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Water Quality,

Wastewater Treatment Process

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Water Pollution Impact Water pollution can be defined as "the presence of a

substance in the environment that because of itschemical composition or quantity prevents thefunctioning of natural processes and producesundesirable environmental and (human)healtheffects.“

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Water Pollution Impact (cont’d) Possible impacts of water pollution:

1. Heavy metals from industrial processes canaccumulate in nearby lakes and rivers. These are toxic

to marine life such as fish and shellfish, and can affectthe rest of the food chain. This means that entireanimal communities can be badly affected by this typeof pollutant.

2. Industrial waste often contains many toxic compoundsthat damage the health of aquatic animals and those who eat them. Some toxins affect the reproductivesuccess of marine life and can therefore disrupt thecommunity structure of an aquatic environment.

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3. Microbial pollutants from sewage often result ininfectious diseases that infect aquatic life andterrestrial life through drinking water. This often

increases the number of mortalities seen within anenvironment.

4. Organic matter and nutrients causes an increase inaerobic algae and depletes oxygen from the watercolumn. This is called eutrophication and causes thesuffocation of fish and other aquatic organisms.

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Water Pollution Impact (cont’d)


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5. Sulfate particles from acid rain change the pH of watermaking it more acidic, this damages the health ofmarine life in the rivers and lakes it contaminates, and

often increases the number of mortalities within anenvironment.

6. Suspended particles can often reduce the amount ofsunlight penetrating the water, disrupting the growthof photosynthetic plants and micro-organisms. Thishas subsequent effects on the rest of theaquatic community that depend on these organisms tosurvive.

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Water Pollution Impact (cont’d)


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Wastewater Treatment

Wastewater comprises liquid waste discharged bydomestic residences, commercial properties, industry,and/or agriculture and can encompass a wide range ofpotential contaminants.

6sewagetreatment.us


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In general treatment schemes are often identified as

Preliminary treatment,

Primary treatment,

Secondary treatment and

Advanced treatment

(tertiary).


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1. Preliminary treatment

Preliminary systems include measurement andregulation of the incoming flow and removal of large

floating solids, grit and perhaps grease.

Influent passes through it before processed by primaryor secondary treatment units;

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Wastewater Treatment (cont’d)


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The removal of wastewater constituents, such as rags,paper, tree limbs, that may cause maintenance oroperational problems with the treatment operations.

Examples are screening and comminution , gritremoval for the elimination of coarse suspendedmatter.

Comminutors are used to macerate solid (rags, paper,

plastic) by revolving cutting bars.

1. Preliminary treatment (cont’d)


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Grit, which is composed primarily of sand and gravel,enters the wastewater collection system due to cracksin pipes, improper or poorly fitting pipe joints, poor

fitting or missing manhole covers and stormwaterrunoff from streets and parking lots.

Grit removal may be accomplished in grit chambers or

by the centrifugal separation of sludge. Grit chambersare designed to remove grit that have specific gravities(S.G. = 2.7) substantially greater that those of theorganic solids in wastewater (S.G. = 1.3)

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1. Preliminary treatment (cont’d)


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2. Primary treatment a portion of the suspended solids and organic matters ,

such as old and grease, are removed from the wastewater by a clarifier or settling tank.

This removal is usually by physical operations such assedimentation.

The effluent from primary treatment will ordinarilycontain considerable organic matter and will have a

relatively high BOD. The solids that collect at the bottom of the tank are

called primary sludge and are pumped from the tankfor treatment elsewhere in the plant.

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2. Primary treatment (cont’d)


originally designed to remove suspended solids in wastewater prior to its discharge.

It usually involves a simple sedimentation process andchemicals are added to assist in removal of finelydivided or colloidal solids.


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3. Secondary treatment Complies biological process for the removal of

biodegradable organics and suspended solids.

Conventional secondary treatment includes biologicaltreatment by activated sludge or trickling filter.

Disinfection process will include frequently after thesecondary treatment if the effluent without further

treatment.

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Secondary Treatment Secondary treatment systems are intended to remove

the soluble and colloidal organic matter which remainsafter primary treatment. The secondary treatment is usually referred as

biological process. Biological treatment consists of application of a

controlled natural process in which micro-organismsremove soluble and colloidal organic material from the waste.

The basic equation of the processes is

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BOD (foodor substrate)

Biomass

(Microbes)

Biomass

(Microbes)

Binaryfission

Primary

Clarifier

Secondary

Treatment SystemSecondary

Clarifier


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Commonly used biological treatment

processes are Activated Sludge Process

Trickling Filters

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Activated Sludge ProcessThe activated sludge process derives its name from the

biological mass formed when air is continuouslyinjected into the wastewater.

Under such conditions, micro-organisms are mixed

thoroughly with the organics under conditions thatstimulate their growth through use of the organics asfood.

As the micro-organisms grow and are mixed by the

agitation of the air, the individual organisms clumptogether (flocculate) to form an active mass ofmicrobes called activated sludge.

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In practice, wastewater flows continuously into anaeration tank where air is injected to mix theactivated sludge with the wastewater and to supplythe oxygen needed for the organisms to break down

the organics.The mixture of activated sludge and wastewater in

the aeration tank is called mixed liquor.

The mixed liquors flows from the aeration tank to a

secondary clarifier where the activated sludge issettled out. Most of the settled sludge is returned tothe aeration tank to maintain a high population ofmicrobes to permit rapid breakdown of organics.

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Activated Sludge Process


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The activated sludge process is controlled by wasting aportion of the micro-organisms each day in order tomaintain the proper amount of bacteria to efficientlydegrade the BOD

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If too much sludge is wasted, the concentration ofbacteria in the mixed liquor will become too low foreffective treatment.

If too little sludge is wasted, a large concentration ofbacteria will accumulate and, ultimately, overflow thesecondary tank and flow into the receiving tank.

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Activated Sludge Process


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Influent

Preliminary

treatment

Primary

clarifier

Aerated activated

sludge tank Secondary

clarifier

Treated

effluent

Return sludge

Secondary

sludge

Activated sludge process


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Trickling Filters The trickling filter consists of a bed of a highly

permeable medium to which micro-organisms areattached and through which wastewater is percolatedor trickled.

The filter media usually consist of either rock or a variety of plastic packing materials.

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Organisms attached to the media in the upperlayer of a bed grow rapidly, feeding on the

abundant food supply. As the wastewater trickles downward, the organic

content decreases to the point where micro-organisms in the lower zone are in a state of

starvation. The majority of BOD is extracted in the upper part

of the filter.

Excess microbial growth sloughing off of the

media is removed from the filter effluent by a finalclarifier.

Purging of a bed is necessary to maintain voids forpassage of wastewater and air.

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Anaerobic Aerobic

Movement ofwastewater

End

products

microbialfilm

Liquid

Organics

DO Air


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Advanced wastewater treatment Treatment required beyond conventional

secondary treatment to remove constituents ofconcern including nutrients, toxic compounds and

increased amounts of organic material andsuspended solids.

Processes frequently employed are chemical

coagulation, flocculation and sedimentationfollowed by filtration and activated carbon.

Less used processes include ion exchange andreverse osmosis for specific ion removal.

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Sludge Treatment and Disposal Sludge treatment and disposal divides into several

operations: thickening, stabilising, heating ,dewatering and drying and ultimately disposing ofthem.

Most small wastewater plants use landfilling to disposeof sludge because land is more readily available and

land prices are lower in rural area.

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L3 - Wastewater Treatment Process_stu