l7-sewage treatmentb 1

7/31/2019 L7-Sewage TreatmentB 1

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Sewage Treatment


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Purpose of sewage treatment

Protect public health

Protect the environment

Convert waste water into stable end-products

Dispose of end-products in a safe manner

Recover & recycle materials if possible

Provide a service which is reliable & regular

Operate without nuisance or offence

Provide an economic system

Comply with appropriate standards & legislation


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Sewage treatment processes

Techniques & processes for sewage treatment

can be grouped in terms of their operating

principles under the following types:

Physical processes

Chemical processes

Biological processes


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Sewage treatment processes

Physical processes- eg. Screening, sedimentation,

flotations, filtration, centrifugation, reverse

osmosis, micro filtration

Chemical processes- eg. Neutralisation,precipitation, oxidation-reduction, ion-exchange

Biological processes- eg. Biological filtration,

activated sludge, stabilisation ponds, anaerobic

digestion


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Sewage treatment plants (STP)

Large scale sewage treatment

Involve a combination of units that areinstalled to carry out particular treatment

processes The exact combination of treatment plant

units depend upon the nature of the input &the objectives for quality of the water system

that is receiving the effluent from thetreatment plant


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Sewage treatment plants: stages

Preliminary treatment

Primary treatment

Secondary treatment

Tertiary treatment


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STP: Preliminary Treatment

Needed to remove large solids & grit which

may cause blockages & damage to other parts

of the treatment plant

Commonly uses screens made of steel barsabout 25mm apart to remove solids such as

rags, paper, wood & plastic

Finer screens of stainless steel wire mesh are

used when needed


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STP: Preliminary Treatment

To remove grit such as sand, gravel & small fragmentsof glass & metal, a process of rapid sedimentation isused

It allows the smaller solids to remain suspended & passto the next stage

This next stage is carried out by equipment such asvortex separators which uses centrifugal force to throwthe grit to the sides of a circular chamber

Other forms of treatment may remove oil & grease ifthey are present in large quantities

Outputs from some industries require pre-treatmentbefore they enter a treatment system


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STP: Primary treatment

Uses sedimentation tanks in which the waste

water is moved at a velocity which allows the fine

solids to fall out of suspension by gravity

these solids form a removable sludge at the baseof the tank

Also allow floatable materials such as oil & grease

to form a scum on the surface

Scrapper blades or skimming devices move acrossthe surface of the tank & separate the scum


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STP: Primary treatment

Can reducebiochemical oxygendemand (BOD)measurement byup to 40% & reducethe suspendedsolids by up to 70%

The outputcontains onlydissolved material& fine suspendedmaterial


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STP: Secondary treatment

The aim is expose the settled waste to

biochemical reactions under aerobic

conditions

The purified waste water is then separated bysecondary settlement tanks

a variety of plants can be used for the process


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Two common choices:

Biological filtration- the waste water is trickledthrough a bed of inert material on which abiomass of micro-organisms develops. Aeration

occurs naturally

Activated sludge- the sewage is aerated in tanksby mechanical agitators which keep oxygen levelshigh. New biomass or sludge develops & some of

it is returned to the aeration tank to seed oractivate the raw sewage


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Biological filter bed


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Final settling tanks


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In both cases theaerobic bacteriaare able to grow& purify thesewage

2ndary

sedimentationtanks separatethe biomass orhumus from theeffluent, in whichBOD reading isusually less than

20mg/l


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STP: Tertiary treatment

Used, if the quality of effluent is not good enough tomeet requirements or standards (eg. The level ofsuspended solids in the effluent may be too high)

Methods:

Settlement in lagoons Irrigation onto land

Filtration through beds of sand or gravel, fine metal filters

Effluents may also be disinfected if there is a danger ofpathogens affecting nearby activities such as bathing

Techniques used are the same as for the treatment ofwater supplies ie


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Sludge treatment


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Sludge treatment

Output of sewage treatment: Liquid effluent

Sludge solids

After treatment to suitable stds, the effluent can

be safely returned to surface water system (riversetc)

Sludge however has to be further treated andthen transported for disposal

Sludge treatment can represent 50% of the

capital & operating costs for sewage treatment


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Sludge treatment

Sludge from primary & 2ndary treatment hastypically 90% water content and needstreatment to reduce this

The exact methods depends on the nature ofthe sludge and the proposed disposal method

Common processes used:

Thickening

Stabilisation dewatering


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Sludge treatment

Thickening-process of reducing volume of sludge, often bygravity settlement. As the sludge concentration increases,the volume typically decreases by 3-5 times

Stabilisation- a process which prevents anaerobicbreakdown of the sludge & subsequent offensive odours.

(Anaerobic digestion- method which utilises the organicmatter in the sludge). Chemical stabilisation is analternative method involving the addition of lime (calciumhydroxide) to the sludge, creating an alkaline environmentunsuitable for micro-organisms.

Dewatering- process of reducing water content of sludge

by physical methods incl. drying, filtration, squeezing,centrifugal action & natural compaction


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Sludge Disposal

Increasing volume becomes a technical &

environmental challenge. Options for disposal

are:

Landfill

Land disposal

Sea disposal

incineration


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Sludge Disposal

Landfill Disposal directly to landfill sites has been a common practice, with

undesirable characteristics: Contamination of waters by leaching fr the landfill

Risk to public health by pathogen transfer

Increased methane production during decomposition

Environmental pollution by odour, flies & transport

Land disposal

applied to farmland as a soil conditioner & fertilizer.

The value of the sludge depends on the treatment it receives, but itis rich in organic matter & nutrients such as nitrogen & phosporus.

The sludge may need to be balanced with other nutrients & cannotbe spread onto farmland all year long


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Sludge treatment & disposal


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Sludge treatment & disposal


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Sludge Disposal

Sea disposal

In the past, it was convenient for some communities near the sea todispose of sewage sludge by loading it onto special boats & discharging itout at sea

EC do not permit this method

Incineration

Sludge contains considerable quantities of combustible materials & oncedried, can be burnt

During incineration, the organic & volatile components (incl toxiccompounds) are destroyed

The inert ash that remains is a hazardous waste that still needs to bedisposed of via a controlled waste management site

Incinerators have a high capital cost but are increasingly being used by

large cities There are opportunities to mix sludge with refuse material , burn it and

use it as a source of energy


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Options for sewage treatment

Other alternatives for sewage treatment

Share the same physical, chemical & biological processesbut may be more appropriate for the following situations: Single buildings

Isolated groups of buildings

Buildings & communities which lack the resources for bldg ormaintaining treatment plants

Communities who wish to use more natural methods of sewagetreatment

Communities who have enough unused land to put aside foralternative treatments

Initial treatments of strong industrial waste water Tertiary treatment of effluent from some treatment plants


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Cesspool

Is a watertight underground container used

for the storage of household sewage

No treatment of sewage occurs & the tank

must be periodically emptied & the sewagetaken away for treatment

Constructed in variety of ways incldg in situ

concrete, prefabricated plastic & fibreglass set

into concrete


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Cesspool

Usually used for single

dwellings or small groups

of houses & require no

resources or monitoring

apart from emptying However, running cost of

emptying the tank is high

& large underground

tanks can be expensive to

construct or install


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SepticTank

Is a small scale sewage treatment plant in theform of an underground tank in which treatmenttakes place & from which there is continuousdischarge of liquid

The tank contains separate zones which allowsolids to settle & form a sludge layer whereanaerobic processes decompose the sewage

Lighter materials may form a scum layer which

helps prevent oxygen transfer & provides usefulthermal insulation


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Septic Tank

The tank acts as a primary treatment & the

effluent fr the tank needs further treatment such

as 2ndary filter or percolation into the soil via a

system of percolated drainage pipes

Usually rectangular in shape, if constructed on

site or may be supplied as rounded prefabricated

tanks in plastic or glass-reinforced plastic

Usually require no particular maintenance apartfr removal of sludge


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Septic Tank


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Septic Tank

After desludging, a small percentage of sludgeis kept in the tank to seed the new sludge &the tank is filled with water

Commonly used for isolated single dwellingsbut can also be used for small ruralcommunities such as a village

A reed bed might be used following a bank ofseptic tanks arranged so they can bemaintained without interrupting operation


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To Subsoil drainage pipes or biological

filter


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Stabilisation ponds

Stabilisation ponds or lagoons is any enclosed

body of water where organic waste is allowed

to oxidise by natural activity

Ponds can simply be contained by earthembankments & used in combination if

necessary

Easiest to operate where there is plenty of

sunshine & land available


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Stabilisation ponds

Simple to maintain & very effective at

removing dangerous pathogens

Have been used since early times, are the

most common form of treatment indeveloping countries & are of increased

interest to all communities

The operation of the ponds can be grouped

according to their method of operation


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Stabilisation ponds

Methods of

operation:

Anaerobic

ponds

Oxidation ponds

Aeration ponds


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Stabilisation ponds

Anaerobic ponds-formation of sludge & a topcrust creates good anaerobic conditions.Particularly suitable for stronger thicker wastes

Oxidation ponds-an aerobic system where the

oxygen is taken from the atmosphere & alsosupplied by the activity of algae in the ponds. Theponds are kept shallow (1-2m) to allow maximumpenetration of sunlight

Aeration ponds- ponds in which the oxygen issupplied by aerators & not by action of algae


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Wetlands & reedbeds

Uses the natural processes associated with the growthof aquatic plants (submerged algae, floating plants eghyacinth & emergent plants eg reeds)

The development of micro-organisms around the plant

& their root systems allows both aerobic & anaerobicbacteria to develop & digest the sewage product

These plants can be encouraged to form in artificialwetlands or reed beds & these can be used for 2ndaryor tertiary treatment of sewage, treatment of storm

water & stabilisation of sludge


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Wetlands & reedbeds

The plants are grown in beds of soil or gravel which areretained by an impervious lining

The by-products of the biological treatment processesare taken up by the plants & removed when the plantsare harvested

Metals, if present get removed into the mud at thebottom of the bed

the beds do not need daily attention but the waterlevels, plant growth & harvesting need to be managed

They are being used to treat industrial waste & are also

well-suited for the treatment of domestic sewage fromsmall isolated developments


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Wetlands & reedbeds


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How lucky we r

London nightmen: Cesspool sewerman

l7-sewage treatmentb 1

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