Microbes play a dual role when used for sewage treatment as they not only help to retrieve usable water but also generate fuel. Write how this happens?

[prasanna]

Heterotrophic microbes naturally present in sewage are used, and vogorous growth of aerobic microbes as flocs uses up organic matter in effluent and reduce BOD of waste water; other kinds of bacteria grow in it anaerobically and digest the bacteria and fungi called flocs. As they digest flocs, a mixture of carbon dioxide or biogas is evolved, which can be used as a fuel.