Which among the following is pumped back into the aeration tank to serve as the inoculum ?

Activated sludge

After the sewage or waste water undergoes significant reduction in BOD, the resulting effluent is directed to a settling tank. In this tank, the bacterial flocs that have formed during the treatment process settle down as sediment. This sediment is known as activated sludge.

To maintain the growth of beneficial microbes, a small portion of the activated sludge is returned or pumped back into the aeration tank. This serves as an inoculum, providing a population of microbes to continue the treatment process.

The majority of the sludge, containing bacteria and fungi, is pumped into large tanks called anaerobic sludge digesters. In these digesters, different types of bacteria that thrive in anaerobic conditions break down the bacteria and fungi present in the sludge. This digestion process helps further decompose the organic matter and stabilize the sludge.

By separating the activated sludge and subjecting it to anaerobic digestion, the sewage treatment process effectively reduces the organic content and converts it into more stable forms. This aids in the overall treatment and disposal of sewage, ensuring that it is less polluting and can be safely managed.