A small part of 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, that grow anaerobically, digest the bacteria and fungi in the sludge. During this digestion, bacteria produce a mixture of gases called biogas.
The components of biogas are :
1.Methane
2. Hydrogen sulphide
3. Carbon dioxide