Question

Effluent from the primary treatment of sewage is passed for secondary treatment. Explain the process till the water is ready to be released into natural water bodies.

Answer 1

1. The primary effluent is transferred into large aeration tanks where it is constantly agitated mechanically and the air having oxygen is pumped into it which allows fast growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures).
2. While growing these microbes consume the major part of the organic matter which is present in the effluent which significantly reduces the BOD of the effluent.
3. The effluent is then passed into the settling tank where the bacterial 'flocs' are allowed to settle down and this sediment is called Activated sludge.
4. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum(starter).
5. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digestion. Here other kinds of bacteria that grow anaerobically, digest the bacteria and the fungi in sludge.
6. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide, and carbon dioxide. These gases form biogas and can be used as a source of energy.
7. The treated effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.