“Sewage comprises waste water generated from households, hotels and may include industrial waste water.”
What type of waste does sewage contain?
Sewage usually contains a high quantity of organic wastes and may also consist of
inorganic wastes. It is essential to treat sewage before its entry into any water
body. Why so? Sewage, if allowed to enter water sources without treatment, it will
contaminate them; which is why it is essential to treat sewage properly before
letting it into rivers or streams for example.
Sewage Treatment Plant process
Sewage can be treated by designing a sewage treatment plant (STP) which involves
three stages: primary/preliminary, secondary and tertiary.
A. Primary treatment
In a sewage treatment plant, sewage water is first allowed to pass through screens
or grit chamber where large solids are removed. This step is followed by
aeration/mixing in a tank and then primary sedimentation where suspended solids
settle down. Primary treatment involves addition of a coagulant and aims at
removing grits, coarse solids, oil and grease if any present. Preliminary treatment
results in sedimentation of solids (sludge) and formation of an effluent that floats
above sludge. Effluent from primary treatment consists of 45-50 % unstable
organic matter. The effluent then undergoes secondary treatment where fine
solids settle out by means of bacterial activities.
B. Secondary or Biological Treatment
This stage converts organic matter in sewage into stable forms through biological
activities, resulting in secondary sedimentation. Common approaches are trickling
filters and activated sludge method.
Trickling filters comprise an enclosed tank with a bed of bricks and a layer of
microorganisms. The effluent enters the tank through an inlet and trickles over the
bed layer by sprinklers. Microbial activities oxidize the organic matter in the
effluent, resulting in the removal of fine solids, formation of sludge and an
effluent with less organic solids.
Activated sludge method involves the addition of a mixture of active
microorganisms to sewage, followed by aeration and agitation. The aerobic
conditions and microbial actions oxidize the organic solids resulting in
coagulation, flocculation and settling of solids. Activated sludge produces a clear
liquid without foul Odor.
C. Tertiary Treatment
The effluent then undergoes tertiary treatment/disinfection by incorporation of
UV radiation or chlorination. Other methods such as sand filters and reverse
osmosis may also be used instead for this stage of treatment, depending upon the
nature of the sewage and the effluent from secondary treatment.
