Sewage is municipal waste water released from household, commercial and industrial establishments. It carries many organisms most of which are pathogenic, organic matter and other household waste including human excreta, detergents, greases, heavy metals, plastics, etc. It consists of about 99.9% water and about 0.1% solid or dissolved wastes. The treatment of sewage is done using heterotrophic microbes mainly consists of three steps:
(i) In primary treatment, solid wastes are removed by physical means, such as filtration and sedimentation. (ii) In secondary treatment, liquid effluent solid wastes that remain after primary treatment are treated by biological means (actions of decomposers). An activated sludge is generated by aeration of sewage which consists of organic material containing large number of microorganisms consisting of bacteria (Coliform Clostridium, Pseudomonas, etc.) and fungi that form a filamentous mesh-like filter and are called floc.
(iii) In tertiary treatment, an effluent of water pure enough to drink is produced by chemical and physical means