Which among the following is pumped back into the aeration tank to serve as the inoculum during sewage treatment?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Activated sludge
In secondary (biological) treatment of sewage, the effluent is aerated so that useful aerobic microbes grow into masses called flocs. When the BOD has fallen enough, the flocs are allowed to settle in a settling tank. This sediment is the activated sludge. A small part of it is pumped back into the aeration tank, where its microbes act as the inoculum for the next batch. The rest goes to anaerobic sludge digesters.
NCERT Extract:
Microbes in Sewage Treatment – “… Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters.”