Question

Explain the biological treatment of primary effluent when passed into the large aeration tanks in a sewage treatment plant (STP).

Hint:

In secondary treatment, aerobic microbes in aeration tanks form flocs and reduce BOD by breaking down organic waste.

Answer 1

Step 1: The primary effluent obtained after sedimentation is transferred to large aeration tanks where it is subjected to secondary (biological) treatment.
Step 2: In these tanks, air is pumped to facilitate the vigorous growth of aerobic microbes, especially bacteria and protozoa, which form flocs (masses of bacteria and fungal filaments).
Step 3: These microbes digest the organic matter in the effluent, significantly reducing the biochemical oxygen demand (BOD), which is an indicator of organic pollution.
Step 4: Once the BOD is sufficiently reduced, the effluent is passed into a settling tank where the flocs settle as activated sludge.
Step 5: Part of this activated sludge is recycled back to the aeration tank as inoculum, and the rest is sent for anaerobic digestion or dried and used as manure.