CBSE Class 12-science Answered

(a) As the BOD of sewage or waste water is reduced to about 10-15%, the effluent is passed into a settling tank where the bacterial flocs undergo sedimentation. The sediment is called activated sludge.

(b) A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining part of the activated sludge is pumped into large tanks called anaerobic sludge digesters. The aerobic microbes get killed and the anaerobic bacteria digest the organic matter as well as the aerobic bacteria and fungi in the sludge and produce a mixture of gases like methane, hydrogen sulphide and CO₂ which constitute biogas.