Question

Explain the process of secondary treatment given to the primary effluent up to the point it shows a significant change in the level of biological oxygen demand (BOD) in it.

Answer 1

Secondary treatment

1. In this treatment large aeration tanks containing primary effluent are constantly agitated with a continuous supply of air.

2. Thus aerobic microbes grow vigorously consuming a major part of the organic effluent reducing Biological Oxygen Demand (BOD) of the effluent.

3. The sewage is then passed into the settling tank where bacteria settle and now it is known as activated sludge.

4. 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.

5. Here anaerobic bacteria grow to produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide to form biogas. The treatment goes on till the BOD is reduced to certain levels. This effluent can then be released into natural water bodies.