a. When the BOD of sewage or waste is reduced significantly, the effluent is then passed into a settling tank where the bacterial 'flocs' are allowed sludge.
b. A small part of the activated sludge is pumped back into aeration tanks to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas and can be used as a source of energy as it is inflammable.