Two different categories of microbes naturally occurring in sewage water are aerobic and anaerobic microbes.
(i) Role of aerobic microbes in cleaning water Aerobic microbes grow excessively in the aeration tank and it consumes a major part of the organic matter. It helps in reducing the BOD of the effluent.
(ii) Role of anaerobic microbes in cleaning water In the settling-tank, anaerobic microbes digest the aerobic bacteria and fungi and produce gases like $CH_4$, $C0_2$, $H_2S$, etc. It makes the water less polluted to be released in the water bodies.