Microbes are used in
I. primary treatment of sewage.
II. secondary treatment of sewage.
III. anaerobic sludge digesters.
IV. production of biogas.
Choose the correct option.

Primary treatment does not involve microbes. It is the physical removal of large and small particles from sewage. However, secondary treatment of the effluent from the primary settling tank is purely a biological treatment involving microbial activity. Further, in the anaerobic sludge digesters, heterotrophic microbes anaerobically digit bacteria and fungi in the sludge producing mixture of gases such as methane, hydrogen sulphide and CO₂, which form biogas. Thus, microbes are used in secondary treatment, anaerobic sludge digesters and biogas production.