Explanation

Primary treatment is the physical removal of large and small particals from sewage. Secondary treatment of the liquid effluent from the primary settling-tank is purely a biological treatment involving microbial activity. In the anaerobic sludge digesters, heterotrophic microbes anaerobically digest bacteria and fungi in sludge producing mixture of gases such as methane, hydrogen sulphide and $CO_{2}$, which form the biogas