Sewage is the combination of both
black water and grey water. Black water carries wastewater that is released through the toilets
including urine and excreta, kitchen and bathroom. Grey water carries
wastewater from bathrooms and kitchen. Release of sewage into public sewer
system is compulsory in the highly urbanize area. Release of sewage without
proper treatment into surface water sources and ground water sources cause
serve health, economic and environmental problems.
Sources of sewage: Toilets, bathrooms and kitchen are
the sources of releasing sewage.
Composition of sewage: wastewater from toilets contains
pathogens in large quantities. Biodegradable
organics (BOD) releases from both toilets and kitchens. It causes the depletion
of oxygen in the water sources and affects the aquatic plant and animal life. Excessive
amount of nutrients (N, P, K) also release through both toilets and kitchen.
They are able to accelerate the growth of algae. The sudden increase of algal
growth is referred to as the eutrophication. Hot
and cold water release through the kitchens and bathrooms. It affects the
temperature stability of the aquatic bodies. Detergents releases through the
cleaning of bathrooms and toilets. Sediments are another pollutant that reduces the light
penetration through aquatic bodies.
Collection of the sewage: Later sewers collect sewage from
individual houses into primary gully. Then sewage from primary gullies is flowed
into secondary gullies by using branched sewers. Then according to the slope,
sewage from all secondary gullies is drained along the road into the main
gully. The main gullies
are connected to the treatment plant via large sewer. Here collection of the sewage is done by
using gravity instead of pumping. Therefore sewers are constructed with slope.
Thereby it reduces the cost of treatment of sewage.
The
treatment process is operated as flowing diagram.
Incoming
sewage is passed through a metallic bar screen. It removes the large materials
including sanitary napkins, grits, glass pieces and straws. Those are sent and
disposed in the sanitary land fill.
Thereby it helps to protects the pumps and other equipment from wrapping
or damage them.
Sedimentation tank: Then water is transferred into a
sedimentation tank by using pump system. Here it is allowed water to settle
down under gravity. Large floatable materials are collected at the bottom of
the sedimentation tank. They are removed and directed to the central sludge
treatment facility.
Aeration tank: Water is transferred into a tank
with continuous aeration. Here biodegradable organics are removed by using
digestion of aerobic respiratory bacteria. They oxidize organic matter
completely into carbon dioxide and water without leaving any residues. .
Retention time of wastewater is 4-8 hours in the aeration tank. The amount of
sediments can be reduced by extending the retention time from 1-2 days.
Mechanical aerators are employed for aeration.
Secondary clarifier: Then wastewater is transferred into
another settling tank. Here as microorganisms are higher than the available
food, they tend to form flocs via attaching to each other. They settle down
under gravity at the bottom of the settling tank. They are removed as the
sludge. Part of the sludge is removed as the activated sludge. Rest is
transferred into the central sludge treatment facilities to dispose in a proper
manner.
Disinfection: Chlorine, ozone and UV are the common
method of disinfection. But chlorine is the economical viable method. Chorine
solution is added drop wisely by using specialized machine. Excess chorine
generates disinfection byproducts (dbp) including chloroform. They may cause
serve health problems therefore excess chlorine have to be removed before
discharge water into natural water bodies. Sodium metabisulfate is added to
remove excess chorine. . But both ozone and UV do not leave resides that need
to be removed before discharges. But use of ozone as a disinfection method is
costly because ozone has to be produced in-site due to the instability. It
requires highly specialized expensive equipment to generate ozone. Therefore
use of UV is environmental friendly and cost effective.
Wetland system: Even after primary and secondary
treatment, wastewater still contains excess nutrients including phosphate and
nitrate, sediments and non-degraded organic matter. If wastewater is released
at this level of treatment, it may pollute the both groundwater table and
surface water sources. So construction of artificial wetland is the economical
viable method as a tertiary treatment. It removes pollutant from water via
biological, physical and chemical methods.
Reuse and disposal of treated
wastewater: After
treatment, water can be reused within the plant. Flushing toilets and watering
plants are done by using treated water. If water is not reused, it can be
discharged into natural water bodies, after tastings for the national and
international standards. COD, BOD and suspended solid are checked once every
week. Parameters in the natural water bodies also tested recently.
Sludge treatment: Sludge collected from primary and
secondary clarifier is transferred to a tank. It is similar to typical
sedimentation tank in the design. Here sludge is settled down under gravity. So
it increases the thickness of gravity. Then sludge is collected and directed
into either anaerobic digester or drying bed. Sludge is degraded into carbon
dioxide by using anaerobic respiratory bacteria. Then this carbon dioxide is
reduced into methane which is combustible gas. Methane can be used for
electricity generation or directly use in cookers. In drying beds, sludge is filtered through a
sand and gravel filter. Remaining liquid fraction is accumulated as the
leachate at the bottom. The leachate is collected via pipe systems into central
treatment facilities and treats. The liquid fraction of sludge is further reduced
by using filtration, drying via sunlight and wind. The sludge is collected from
the surface f the drying beds. It can be reused or disposed in sanitary land
fill.
Reuse and disposal of sludge: Sludge can be used as a resource.
Sludge is used for compost production. Plant materials are added to adjust the
proper carbon to nitrogen ratio for microbial decomposition. It can be applied as a soil conditioner and
fertilizer. It as a fertilizer, it adds primary nutrients such as nitrogen,
phosphorous and potassium. Thereby it increases the growth of plants. It as a
soil conditioner, it improves the biological, chemical and physical properties.
If sludge is not reused, it is needed to be disposed in a sanitary landfill
because sludge contains the most of the worst materials of wastewater.
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