(a) Two different categories of microbes naturally occurring in sewage water are aerobic and anaerobic microbes. Aerobic microbes require oxygen for their growth while anaerobic microbes do not.
(b) The role of microbes in cleaning sewage water into usable water is as follows:
i) Aerobic microbes grow in the aeration tank of the sewage treatment plant and consumes a major part of the organic matter in the sewage thus reducing the BOD of the effluent.
ii) Anaerobic microbes are responsible for methane fermentation of sewage sludge because they facilitate decomposition of macromolecular organic matter into simpler compounds.
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