In the waste water treatment process, the treated primary effluent is transferred to large aeration tanks. These tanks are constantly pumped with air by agitation which allows growth of aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures). Microbes use the oxygen supplied to break down the organic content in the effluent which helps in reducing the BOD (the amount of oxygen that would be consumed if all the organic matter in one litre of water is oxidised by bacteria) considerably.

Once the BOD has been reduced to the required level, the treated effluent is passed into settling tank to allow the flocs to settle and form sediments. This sediment is referred as activated sludge rich in microbes. Some portion of the sludge is use as inoculate for next batch of aeration treatment and rest is transferred into sludge digestors where anaerobic microbes grow and digest the sludge content and produce a mixture of gases having methane, hydrogen sulphide and carbon dioxide.