3.0 Sewage

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Sewage - Origin

Origin Sewage is generated by residential, institutional, and

commercial and industrial establishments.

It includes household waste liquid from toilets, baths,showers, kitchens, sinks and so forth that is disposed ofvia sewers.

In many areas, sewage also includes liquid waste fromindustry and commerce.

The separation and draining of household waste intogrey-water and black-water is becoming morecommon. Greywater being permitted to be used forwatering plants or recycled for flushing toilets.

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Domestic wastewater will contain both solid & dissolvedpollutants including faecal matter, paper, urine, sanitaryitems, and a variety of other contaminants.

The sewage travels through the sewer system andultimately to a sewage works where it receivestreatment before discharge of the treated effluent to astream, river, estuary or the sea.

Sewage Origin

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Sewage treatment can be of two types:

A de-centralized system (in septic tanks, biofilters oraerobic treatment systems)

A centralized system (collected and transportedthrough a network to a treatment plant)

Sewage Treatment

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Sewage Treatment

Sewage treatment involves:Preliminary TreatmentPrimary Treatment

Secondary Treatment

Tertiary Treatment

Sludge Treatment

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Sewage treatment generally involves three stages, called

primary, secondary and tertiary treatment.Primary treatment consists of holding the sewagein a tank, so that heavy solids can settle to thebottom while oil, grease and lighter solids float to the

surface. The settled and floating materials areremoved and the remaining liquid is subjected tosecondary treatment.

Secondary treatment removes dissolved and

suspended biological matter. Secondary treatment istypically performed by water-borne micro-organisms in a managed habitat.Secondary treatment may require a separationprocess to remove the micro-organisms from thetreated water prior to discharge or tertiary treatment.

Sewage Treatment Overview

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Tertiary treatment is sometimes defined asanything more than primary and secondarytreatment in order to allow discharge into a highlysensitive or fragile ecosystem (estuaries, low-flowrivers, coral reefs,...).

Treated water is sometimes disinfected chemically orphysically.

If it is sufficiently clean, it can also be used forgroundwater recharge or agricultural purposes.

Sewage Treatment - Overview

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ScreeningGrit removal

Fat & Grease RemovalFlow equalization

Preliminary treatment

Primary treatment Sedimentation or ClarifiertanksSecondary treatment Activated sludge

Surface-aerated basins (Lagoons)Filter beds (oxidizing beds)Constructed wetlandsSoil bio-technologyBiological aerated filtersRotating biological contactorsMembrane bioreactorsSecondary sedimentation

Tertiary treatmentFiltrationLagooningNutrient removal

Sewage Treatment (Stage & Types)

Sludge treatmentAnaerobicAerobicCom ostin

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Screening

Large solids (plastics, rag, toilet paper residues) areremoved first by mechanical screens.

Traditionally, screening was used to remove only largesolid material in order to protect downstream

operations. Nowadays, much finer screens are commonly

employed to remove smaller inert solids.

The material retained is usually washed to removeorganic matter and then compressed for disposal tolandfill or to an incinerator.

Sewage Treatment (Preliminary)

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Grit Removal

At the next preliminary stage, fine mineral matter (grit andsand), originating mainly from road runoff, is allowed todeposit in long channels or circular traps. The retainedsolids are removed and usually sent to landfill for disposal.

A grit chamber is usually installed before primarysedimentation tanks and it should be placed well beforewastewater pumps.

There are basically three different types of grit chambers:the horizontal flow type , another is the aerated grit chamber and the last one is the vortex type .


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Clarifiers and mechanized secondary treatment aremore efficient under uniform flow conditions .

Basins provide a place to temporarily hold incomingsewage during plant maintenance and a means ofdiluting and distributing toxic or high-strengthwaste which might otherwise inhibit biologicalsecondary treatment.

The basin should be downstream of screening and gritremoval

Flow equalization


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Fat and grease is removed by passing the sewage througha small tank where skimmers collect the fat floating on thesurface.

Air blowers in the base of the tank may also be used tohelp recover the fat as a froth. Many plants, however, useprimary clarifiers with mechanical surface skimmers for fatand grease removal.

Fat and grease removal


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Primary treatment

Sewage flows through large tanks, commonly calledprimary sedimentation tanks or primary clarifiers.

The tanks are used to settle sludge while grease andoils rise to the surface and are skimmed off.

Primary settling tanks are usually equipped withmechanically driven scrapers that continually drive thecollected sludge towards a hopper in the base of thetank where it is pumped to sludge treatment facilities.

A typical sedimentation tank may remove from 50 to 70percent of suspended solids, and from 30 to 35 percentof biochemical oxygen demand (BOD) from the sewage.

Sewage Treatment (Primary)

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Secondary treatment systems are classified as fixed-filmor suspended-growth systems.

Fixed-film or attached growth systems includetrickling filters, bio-towers, and rotating biologicalcontactors, where the biomass grows on media andthe sewage passes over its surface.

Sewage Treatment (Secondary)

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Suspended-growth systems include activatedsludge , where the biomass is mixed with thesewage and can be operated in a smaller spacethan trickling filters that treat the same amount ofwater.

Fixed-film systems are more able to cope withdrastic changes in the amount of biological materialand can provide higher removal rates for organicmaterial and suspended solids than suspendedgrowth systems.


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The Activated Sludge Process

Microorganisms remove organic materials for food.

In this process microorganisms are suspended inwastewater in the aeration tank.

Mixing is accomplished through injection of air throughdiffusers located near the bottom of the aeration tank orthrough mechanical agitation with propeller type mixingdevices.

The process also produces a large amount of wastesludge that must be stabilized and disposed of andrequires a great deal of energy to process thewastewater.


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The Activated Sludge Process

The process also produces a large amount of wastesludge that must be stabilized and disposed of andrequires a great deal of energy to process thewastewater.


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Surface-aerated basins (Lagoons) The basins may range in depth from 1.5 to 5.0 metres

and use motor-driven aerators floating on the surface ofthe wastewater.

In an aerated basin system, the aerators provide twofunctions: they transfer air into the basins required bythe biological oxidation reactions, and they provide themixing required for dispersing the air and for contactingthe reactants (that is, oxygen, wastewater and

microbes). 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.


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Filter beds (oxidizing beds) (trickling filters) In Trickling filter, sewage is spread onto the bed made

up of coke, limestone chips or plastic media.

Such media must have large surface areas to supportthe biofilms that form.

The sewage is typically distributed through perforatedspray arms. The distributed sewage trickles through thebed and is collected in drains at the base.

These drains also provide a source of air whichpercolates up through the bed, keeping it aerobic.

Biological films of bacteria, protozoa and fungi form onthe medias surfaces and eat or otherwise reduce theorganic content.


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Constructed wetlands

They can be surface or subsurface flow, horizontal orvertical flow.

They provide a high degree of biological improvement &depending on design, act as a primary, secondary and

tertiary treatment. They are known to be highlyproductive systems as they copy natural wetlands.

Robust and reliable, their treatment capacities improveas time go by, at the opposite of conventional treatmentplants whose machinery age with time.

They are being increasingly used, although adequateand experienced design are more fundamental than forother systems and space limitation may impede theiruse.


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( d )

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Soil bio-technology A new process called soil bio-technology (SBT)

developed at IIT Bombay has shown tremendousimprovements in process efficiency enabling total waterreuse, due to extremely low operating power

requirements Unlike conventional treatment plants, SBT plants

produce insignificant amounts of sludge, precluding theneed for sludge disposal areas that are required by

other technologies.


S (S d )

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S T (S d )

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S T (S d )

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S T (S d )

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S T t t (S d )

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Biological aerated filters

Biological Aerated or Anoxic Filter or Biofilters combinefiltration with biological carbon reduction, nitrification orde-nitrification.

BAF usually includes a reactor filled with a filter media.

The media is either in suspension or supported by agravel 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 occurs in

aerobic mode while nitrate conversion occurs in anoxicmode.

BAF is operated either in up flow or down flow


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S g T t t (S d )

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Rotating biological contactors (RBCs)

The rotating disks support the growth of bacteria andmicro-organisms present in the sewage, which breakdown and stabilize organic pollutants.

To be successful, micro-organisms need both oxygen tolive and food to grow. Oxygen is obtained from theatmosphere as the disks rotate.

As the micro-organisms grow, they build up on themedia until they are sloughed off due to shear forcesprovided by the rotating discs in the sewage.

Effluent from the RBC is then passed through finalclarifiers where the micro-organisms in suspensionsettle as a sludge. The sludge is withdrawn from theclarifier for further treatment.


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http://en.wikipedia.org/wiki/File:Rotating_Biological_Contactor.png

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Membrane bioreactors

Membrane bioreactors (MBR) combine activated sludgetreatment with a membrane component that uses lowpressure micro-filtration membranes and eliminates theneed for clarification and tertiary filtration.

The membranes are typically immersed in the aerationtank

The cost of building and operating an MBR is usuallyhigher than conventional wastewater treatment.Membrane filters can be blinded with grease or abradedby suspended grit and lack a clarifier's flexibility to passpeak flows.

The small footprint of MBR systems, and the highquality effluent produced, make them particularly usefulfor water reuse applications.



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Secondary Sedimentation

The final step in this secondary treatment stage is tosettle out the biological floc or filter material through asecondary clarifier and to produce sewage watercontaining low levels of organic material and

suspended matter.


Sewage Treatment (Tertiary)

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The purpose of tertiary treatment is to provide a finaltreatment stage to raise the effluent quality before it isdischarged to the receiving environment (sea, river, lake,ground, etc.).

More than one tertiary treatment process may be used atany treatment plant.

If disinfection is practiced, it is always the final process. It isalso called "effluent polishing."

FiltrationSand filtration removes much of the residual suspendedmatter. Filtration over activated carbon, also called carbon adsorption, removes residual toxins.



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LagooningLagooning provides settlement and further biologicalimprovement through storage in large man-made ponds orlagoons.

These lagoons are highly aerobic and colonization bynative macrophytes, especially reeds, is often encouraged.

Small filter feeding invertebrates such as Daphnia and

species of Rotifera greatly assist in treatment by removingfine particulates.



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Nutrient removal

Wastewater may contain high levels of the nutrientsnitrogen and phosphorus.

Excessive release to the environment can lead to abuildup of nutrients, called eutrophication , which can

in turn encourage the overgrowth of weeds, algae, andcyanobacteria (blue-green algae).

This may cause an algal bloom, a rapid growth in thepopulation of algae.

The algae numbers are unsustainable and eventuallymost of them die.


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Nitrogen removal

The removal of nitrogen is effected through Biological oxidation of nitrogen from ammonia to

nitrate ( Nitrification )

De-nitrification -reduction of nitrate to nitrogen gas.

Nitrogen gas is released to the atmosphere andthus removed from the water.

Denitrification requires anoxic conditions.

Sand filters, lagooning and reed beds can all be usedto reduce nitrogen, but the activated sludge processcan do the job the most easily.

The conversion of ammonia to nitrate alone is referred

to as tertiary treatment.


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Phosphorus removal

Phosphorus removal can also be achieved by chemicalprecipitation, usually with salts of iron, aluminum, orlime.

This may lead to excessive sludge production as

hydroxides precipitates and the added chemicals canbe expensive.

Chemical phosphorus removal requires significantlysmaller equipment footprint than biological removal, is

easier to operate and is often more reliable thanbiological phosphorus removal.

Another method for phosphorus removal is to usegranular laterite.



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Disinfection

The purpose of disinfection of waste water is to reducethe number of microorganisms in the water to bedischarged into the environment for the later use ofdrinking, bathing, irrigation, etc.

The effectiveness of disinfection depends on The quality of the water,

The type of disinfection,

The disinfectant dosage (concentration and time),

Other environmental variables.

Common methods of disinfection include ozone,chlorine, ultraviolet light , or sodium hypochlorite.

After multiple steps of disinfection, the treated water isread to be released.



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Chlorination remains the most common form of wastewater disinfection due to its low cost and long-termhistory of effectiveness. One disadvantage is thatresidual chlorinated-organic compounds that may beharmful to the environment. Therefore treated effluentmust also be chemically dechlorinated.

Ultraviolet (UV) light - the treated water has noadverse effect on organisms that later consume it. Thekey disadvantages of UV disinfection are the need forfrequent lamp maintenance and the need for a highlytreated effluent, so that the microorganisms are notshielded from the UV radiation (i.e., any solids presentin the treated effluent may protect microorganisms fromthe UV light).



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Ozone (O 3) is generated by passing oxygen (O 2)through a high voltage potential resulting in a thirdoxygen atom becoming attached and forming O 3.

Ozone is very unstable and reactive and oxidizes mostorganic material it comes in contact with, therebydestroying many pathogenic microorganisms.

Ozone is considered to be safer than chlorine because,unlike chlorine which has to be stored on site (highlypoisonous in the event of an accidental release), ozoneis generated onsite as needed.

Ozonation also produces fewer disinfection by-productsthan chlorination.

Disadvantage - the high cost of the ozone generation

equipment and the requirements for special operators.



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Odor control

Odors emitted by sewage treatment are typically anindication of an anaerobic or "septic" condition.

Early stages of processing will tend to produce foulsmelling gases, with hydrogen sulfide being most

common Large plants will often treat the odors with carbon

reactors, a contact media with bio-slimes, small dosesof chlorine, or circulating fluids to biologically capture

and metabolize the obnoxious gases. Other methods of odor control exist, including addition

of iron salts, hydrogen peroxide, calcium nitrate, etc. tomanage hydrogen sulfide levels. High-density solids

pumps are suitable to reduce odors by conveyingslud e throu h closed i e-work.



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Sludge treatment and disposal

The sludges accumulated in a wastewater treatmentprocess must be treated and disposed of in a safe andeffective manner.

The equipment used in treatment plants are thefollowing:

The sludge is passed trough a pre-thickener or mainsludge thickener. This dewaters the sludge.

After this, the actual digestion is done in a tank,

Then the remaining solid is moved off. Composting is most often applied to small-scale plants

with aerobic digestion for midsized operations, andanaerobic digestion for the larger-scale operations.


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3.0 Sewage