Question

Explain the process of sewage water treatment before it can be discharged into natural water bodies. Why is this treatment essential?

Answer 1

Sewage mainly includes human excreta and contains large amounts of organic waste and pathogenic microbes. Thus it is necessary that waste should be made less polluted before discharging into water bodies.

The process of sewage treatment is explained in the following steps:

a) Primary treatment - In this treatment, filtration and sedimentation process is applied to remove large and small particles. Filtration process removes the floating debris by passing it through mesh wire screens. Then sedimentation removes the soil and small pebbles. The sediment is known as primary sludge and the supernatant is known as primary effluent. The effluent then enters the secondary treatment.

b) Secondary treatment - In this treatment, large aeration tanks containing primary effluent are constantly agitated with a continuous supply of air. Thus aerobic microbes grow vigorously consuming a major part of the organic effluent reducing biological oxygen demand (BOD) of the effluent. The sewage is then passed into the settling tank where bacterial flocs settle and now it is known as activated sludge. Some of the activated sludge is again passed into aeration tank to serve as inoculum while rest of it enters into large tanks called anaerobic sludge digesters. Here anaerobic bacteria grow to produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide to form biogas. This effluent can then be released into natural water bodies.