Q.Describe the secondary treatment process of sewage treatment.

Primary treatment only removes solids, so secondary treatment is the biological stage that uses aerobic microbes to consume the dissolved organic matter and lower the effluent's BOD. …

Secondary treatment (Biological treatment) of sewage:

The effluent from the primary treatment is taken for secondary treatment, which is carried out as follows:

1. Aeration in aeration tanks – The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This promotes the vigorous growth of useful aerobic microbes into flocs — masses of bacteria associated with fungal filaments forming mesh-like structures.

2. Reduction of BOD – While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (Biochemical Oxygen Demand) of the effluent. (BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria; the greater the BOD, the more the polluting potential.)

3. Settling / sedimentation – Once the BOD of the sewage is reduced significantly, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment. This sediment is called activated sludge.

4. Use of the sludge:

   • A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. …