Secondary treatment of sewage is also called Biological treatment. During the sewage treatment :
(i) Primary effluent is passed into large aeration tanks with constant mechanical agitation and air supply.
(ii) Useful aerobic microbes grow rapidly and form flocs.
(iii) Flocs are masses of bacteria and consume organic matter and thus reduce the biochemical oxygen demand (BOD).
(iv) When BOD of sewage has reduced, the effluent is passed into settling tank.
(v) Here, the bacterial flocs settle and sediment is called activated sludge.
(vi) A small part of the sludge is used as an inoculum in the aeration tank and the remaining part is passed into large tanks called anaerobic sludge digesters.
(vii) In the digesters, heterotrophic microbes anaerobically digest bacteria and fungi in sludge producing mixture of gases such as methane, H2S, CO2 which forms the biogas
