“Microbes play a dual role when used for sewage treatment as they not only help to retrieve usable water but also generate fuel”. Explain.

• During the secondary treatment of sewage, the microbes present in the sewage water are employed.

• The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped in to it.

• Vigorous growth of useful aerobic microbes into flocs takes place. While growing these microbes consume major part of the organic matter of the effluent which reduces its BOD.

• The sewage water is treated till BOD is reduced.

• Then the effluent is passed into a settling tank, where the flocs are allowed to sediment forming the activated sludge.

• A major part of this activated sludge is pumped into anaerobic sludge digesters, where the anaerobic bacteria digest microbes in the activated sludge.

• During this digestion bacteria produce a mixture of gases like methane, hydrogen sulphide and carbon dioxide, which form the biogas and can be used as a source of energy. The effluent is generally released into rivers and streams.