After secondary treatment, a part of activated sludge is passed back to aeration tank as inoculum while most of the part of activated sludge passed to

The correct option is B Anaerobic sludge digesters
Secondary treatment of sewage

During the secondary treatment of sewage, the primary effluent goes through the following stages:-

• Aeration tank
• Settling tank
• Anaerobic digesters

Aeration tank

Aeration tank is the tank where the primary effluent is aerated allowing the growth of flocs. These microbial flocs (a meshwork of fungi and aerobic bacteria) oxidise the organic matter present in the primary effluent, reducing the BOD (Biochemical Oxygen Demand).

Settling tank

The treated effluent is then pumped into the settling tanks where the flocs settle down. This sediment is called activated sludge. Most of the sludge is made to flow into anaerobic digesters while some amount is pumped back into the aeration tank as an inoculum.

Anaerobic digesters

The environment in the digester is anaerobic, hence, most aerobic bacteria cannot survive. Anaerobic bacteria digest the bacteria and fungi (comprising the flocs), thus producing a mixture of gases that can be used as biogas.

Final answer: (a)