Sewage is comprised of about 99.9% water and 0.1% solid or dissolved wastes from households, industries, and stormwater runoff. Sewage undergoes physical, chemical, and biological treatment processes to remove contaminants and produce treated wastewater safe for release. Pretreatment screens and filters remove large solid objects, while primary treatment uses sedimentation to remove about half the total solids. Secondary treatment further breaks down organic matter using trickling filters, activated sludge systems, filter beds, or rotating biological contactors. Membrane bioreactors can also be used for secondary treatment and achieve higher removal rates than conventional activated sludge. The byproduct of sewage treatment is sewage sludge

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SEWAGE TREATMENT
Sewage is used water and the wastes it contains. It is about 99.9% water and about 0.1%
solid or dissolved wastes. These wastes include household wastes (human feces, detergents,
grease, and anything else people put down the drain or garbage disposal unit), industrial wastes
(acids and other chemical wastes and organic matter from food-processing plants), and wastes
carried by rainwater that enters sewers.
Physical, chemical, and biological processes are used to remove contaminants and produce
treated wastewater (or treated effluent) that is safe enough for release into the environment. A
by-product of sewage treatment is a semi-solid waste or slurry, called sewage sludge. The sludge
has to undergo further treatment before being suitable for disposal or application to land.
Sewage treatment may also be referred to as wastewater treatment. However, the latter is a
broader term which can also refer to industrial wastewater. For most cities, the sewer system will
also carry a proportion of industrial effluent to the sewage treatment plant which has usually
received pre-treatment at the factories themselves to reduce the pollutant load. If the sewer
system is a combined sewer then it will also carry urban runoff (stormwater) to the sewage
treatment plant. Sewage water can travel towards treatment plants via piping and in a flow aided
by gravity and pumps. The first part of filtration of sewage typically includes a bar screen to
filter solids and large objects which are then collected in dumpsters and disposed of in
landfills. Fat and grease is also removed before the primary treatment of sewage.
Steps of sewage treatment
Pretreatment
Pretreatment removes all materials that can be easily collected from the raw sewage
before they damage or clog the pumps and sewage lines of primary treatment clarifiers. Objects
commonly removed during pretreatment include trash, tree limbs, leaves, branches, and other
large objects.
The influent in sewage water passes through a bar screen to remove all large objects like cans,
rags, sticks, plastic packets etc. Carried in the sewage stream. This is most commonly done with
an automated mechanically raked bar screen in modern plants serving large populations, while in
smaller or less modern plants, a manually cleaned screen may be used. The raking action of a
mechanical bar screen is typically paced according to the accumulation on the bar screens and/or
flow rate. The solids are collected and later disposed in a landfill, or incinerated. Bar screens or
mesh screens of varying sizes may be used to optimize solids removal. If gross solids are not
removed, they become entrained in pipes and moving parts of the treatment plant, and can cause
substantial damage and inefficiency in the process.

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Grit removal
Grit consists of sand, gravel, cinders, and other heavy materials. It also includes organic
matter such as eggshells, bone chips, seeds, and coffee grounds. Pretreatment may include a sand
or grit channel or chamber, where the velocity of the incoming sewage is adjusted to allow the
settlement of sand and grit. Grit removal is necessary to
 Reduce formation of heavy deposits in aeration tanks, aerobic digesters, pipelines,
channels, and conduits;
 Reduce the frequency of digester cleaning caused by excessive accumulations of grit; and
 Protect moving mechanical equipment from abrasion and accompanying abnormal wear.
The removal of grit is essential for equipment with closely machined metal surfaces such as
comminutors, fine screens, centrifuges, heat exchangers, and high pressure diaphragm pumps.
Grit chambers come in 3 types:
 Horizontal grit chambers,
 Aerated grit chambers and
 Vortex grit chambers.
Vortex type grit chambers include mechanically induced vortex, hydraulically induced vortex,
and multi-tray vortex separators. Given that traditionally, grit removal systems have been
designed to remove clean inorganic particles that are greater than 0.210 millimetres (0.0083 in),
most grit passes through the grit removal flows under normal conditions. During periods of high
flow deposited grit is resuspended and the quantity of grit reaching the treatment plant increases
substantially. It is, therefore important that the grit removal system not only operate efficiently
during normal flow conditions but also under sustained peak flows when the greatest volume of
grit reaches the plant
PRIMARY TREATMENT
As raw sewage enters a sewage treatment plant, several physical processes are used to
remove wastes in primary treatment. Screens remove large pieces of floating debris, and
skimmers remove oily substances. Water is then directed through a series of sedimentation tanks,
where small particles settle out. The solid matter removed by these procedures accounts for
about half the total solid matter in sewage. Flocculating substances can be used to increase the
amount of solids that settle out and thus the proportion of solids removed by primary treatment.
Sludge is removed from the sedimentation tanks intermittently or continuously, depending on the
design of the treatment plant.

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SECONDARY TREATMENT
The effluent from primary treatment flows into secondary treatment systems. These
systems are of following types:
1. Trickling filter system
In a trickling filter system, sewage is sprayed over a bed of rocks about 2 m deep. The
individual rocks are 5 to 10 cm in diameter and are coated with a slimy film of aerobic organisms
such as Sphaerotilus and Beggiatoa. Spraying oxygenates the sewage so that the aerobes can
decompose organic matter in it. Such a system is less efficient but less subject to operational
problems than an activated sludge system. It removes about 80% of the organic matter in the
water
2. Activated sludge system
In an activated sludge system, the effluent from primary treatment is constantly agitated,
aerated, and added to solid material remaining from earlier water treatment. This sludge contains
large numbers of aerobic organisms that digest organic matter in wastewater. However,
filamentous bacteria multiply rapidly in such systems and cause some of the sludge to float on
the surface of the water instead of settling out. This phenomenon, called bulking, allows the
floating matter to contaminate the effluent. The sheathed bacterium sphaerotilus, which
sometimes proliferates rapidly on decaying leaves in small streams and causes a bloom, can
interfere with the operation of sewage systems in this way. Its filaments clog filters and create
floating clumps of undigested organic matter.
3. Filter beds (oxidizing beds)
In older plants and those receiving variable loadings, trickling filter beds are used where
the settled sewage liquor is spread onto the surface of a bed made up of coke (carbonized
coal), limestone chips or specially fabricated plastic media. Such media must have large surface
areas to support the biofilms that form. The liquor is typically distributed through perforated
spray arms. The distributed liquor trickles through the bed and is collected in drains at the base.
These drains also provide a source of air which percolates up through the bed, keeping it aerobic.
Biofilms of bacteria, protozoa and fungi form on the media’s surfaces and eat or otherwise
reduce the organic content. The filter removes a small percentage of the suspended organic
matter, while the majority of the organic matter supports microorganism reproduction and cell
growth from the biological oxidation and nitrification taking place in the filter. With this aerobic
oxidation and nitrification, the organic solids are converted into biofilm grazed by insect larvae,
snails, and worms which help maintain an optimal thickness. Overloading of beds may increase

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biofilm thickness leading to anaerobic conditions and possible bioclogging of the filter media
and ponding on the surface.
4. Rotating biological contactors
Rotating biological contactors (rbcs) are robust mechanical fixed-film secondary
treatment systems capable of withstanding surges in organic load. Rbcs were first installed
in germany in 1960 and have since been developed and refined into a reliable operating unit. The
rotating disks support the growth of bacteria and micro-organisms present in the sewage, which
break down and stabilize organic pollutants. To be successful, micro-organisms need both
oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As
the micro-organisms grow, they build up on the media until they are sloughed off due to shear
forces provided by the rotating discs in the sewage. Effluent from the rbc is then passed through
a secondary clarifier where the sloughed biological solids in suspension settle as a sludge.
5. Membrane bioreactors
Membrane bioreactors are activated sludge systems using a membrane liquid-solid phase
separation process. The membrane component uses low
pressure microfiltration or ultrafiltration membranes and eliminates the need for a secondary
clarifier or filtration. The membranes are typically immersed in the aeration tank; however, some
applications utilize a separate membrane tank. One of the key benefits of an Membrane
bioreactors system is that it effectively overcomes the limitations associated with poor settling
of sludge in conventional activated sludge (cas) processes. The technology permits bioreactor
operation with considerably higher mixed liquor suspended solids (mlss) concentration than cas
systems, which are limited by sludge settling.
 The process is typically operated at mlss in the range of 8,000–12,000 mg/l,
 while conventional activated sludge are operated in the range of 2,000–3,000 mg/l.
The elevated biomass concentration in the Membrane bioreactors process allows for very
effective removal of both soluble and particulate biodegradable materials at higher loading rates.

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Thus increased sludge retention times, usually exceeding 15 days, ensure complete nitrification
even in extremely cold weather.
The cost of building and operating an Membrane bioreactors is often higher than conventional
methods of sewage treatment. Membrane filters can be blinded with grease or abraded by
suspended grit and lack a clarifier's flexibility to pass peak flows. The technology has become
increasingly popular for reliably pretreated waste streams and has gained wider acceptance
where infiltration and inflow have been controlled, however, and the life-cycle costs have been
steadily decreasing. The small footprint of Membrane bioreactors systems, and the high quality
effluent produced, make them particularly useful for water reuse applications.
6. Aerated lagoons
An aerated lagoon (or aerated pond) is a simple wastewater treatment system
consisting of a pond with artificial aeration to promote the biological oxidation of wastewaters.
There are many other aerobic biological processes for treatment of wastewaters. They all have in
common the use of oxygen (or air) and microbial action to reduce the pollutants in wastewaters.
TERTIARY TREATMENT
The effluent from secondary treatment contains only 5% to 20% of the original quantity
of organic matter and can be discharged into flowing rivers without causing serious problems.
However, this effluent can contain large quantities of phosphates and nitrates, which can increase
the growth rate of plants in the river.
Tertiary treatment is an extremely costly process that involves physical and chemical methods.
 Fine sand and charcoal are used in filtration.
 Various flocculating chemicals precipitate phosphates and particulate matter.
Denitrifying bacteria convert nitrates to nitrogen gas.
 Finally, chlorine is used to destroy any remaining organisms.

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Water that has received tertiary treatment can be released into any body of water without danger
of causing eutrophication. Such water is pure enough to be recycled into a domestic water
supply. However, the chlorine-containing effluent, when released into streams and lakes, can
react to produce carcinogenic compounds that may enter the food chain or be ingested directly
by humans in their drinking water. It would be safer to remove the chlorine before releasing the
effluent, but this is rarely done today, although the cost is not great. Ultraviolet lights are now
replacing chlorination as the final treatment of effluent. They destroy microbes without adding
carcinogens to our streams and waters. Likewise, especially in europe, the treatment of effluent
with ozone is replacing chlorination. Ozone generators are simple and not very costly, and they
do not add carcinogens to natural waterways.
Septic tanks
Rural families which do not have access to city sewer connections or their treatment
facilities rely on backyard septic tank systems. Homeowners must be careful not to flush or put
materials such as poisons and grease down the drain, as these might kill the beneficial microbes
in the septic tank that decompose sludge solids that accumulate there. This would necessitate
immediate pumping of the tank by a vehicle known as the ‘‘honey wagon’’ to prevent sewage
from backing up into the house. Even with normal operation, it is occasionally necessary to
pump the sludge from the tank and haul it to a sewage treatment plant. Soluble components of

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the sewage continue out of the septic tank into the drainage (leaching) field. There they seep
through perforated pipe, past a gravel bed, and into the soil, which filters out bacteria and some
viruses and binds phosphate. Soil bacteria decompose organic materials. Drainage fields must be
placed where they will not allow seepage into wells, a difficult problem on hills or in densely
populated areas. Drainage fields cannot be used where the water table is too high or the soil is
insufficiently permeable, such as in rocky areas.
Sludge treatment and disposal
The sludges accumulated in a wastewater treatment process must be treated and disposed
of in a safe and effective manner. The purpose of digestion is to reduce the amount of organic
matter and the number of disease-causing microorganisms present in the solids.
The most common treatment options include anaerobic digestion, aerobic digestion,
and composting. Incineration is also used, albeit to a much lesser degree. The use of a green
approach, such as phytoremediation, has been recently proposed as a valuable tool to improve
sewage sludge contaminated by trace elements and persistent organic pollutants.
Sludge treatment depends on the amount of solids generated and other site-specific conditions.
Composting is most often applied to small-scale plants with aerobic digestion for mid-sized
operations, and anaerobic digestion for the larger-scale operations.
The sludge is sometimes passed through a so-called pre-thickener which de-waters the sludge.
Types of pre-thickeners include centrifugal sludge thickeners, rotary drum sludge thickeners and
belt filter presses. Dewatered sludge may be incinerated or transported offsite for disposal in a
landfill or use as an agricultural soil amendment.

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