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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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STP: Secondary treatment
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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STP: Secondary treatment
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