question_answer 32)

  Activated sludge should have the ability to settle quickly so that it can (a) be rapidly pumped back from sedimentation tank to aeration tank (b) absorb pathogenic bacteria present in waste water while sinking to the bottom of the settling tank (c) be discarded and anaerobically digested (d) absorb colloidal organic matter

Answer:

                  (a)In the secondary treatment of waste water, when BOD of sewage has reduced, the effluent is passed into setting tank. Here, the bacterial floes settle and the sediment is called activated sludge. A small part of the sludge is used as an inoculums in the aeration tank and the remaining part is passed into large tanks called anaerobic sludge digesters. That's why activated sludge should have the ability to settle quickly.