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 as:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Activated sludge
In secondary (biological) treatment of sewage, aerobic microbes grow in aeration tanks as flocs (bacteria held together by fungal filaments) and eat up the organic matter, so the BOD falls. The flocs are then let settle.
Option (1) Biogas — Incorrect. Biogas is made later, when the sludge is digested by anaerobic bacteria in sludge digesters.
Option (2) Activated sludge — Correct. The settled flocs form the activated sludge. A small part goes back to the aeration tank as inoculum.
Option (3) Biofertiliser — Incorrect. Biofertilisers are organisms that enrich soil nutrients, such as Rhizobium and cyanobacteria.
Option (4) Primary sludge — Incorrect. Primary sludge is the solid that settles in primary treatment, before any microbial action.
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.”