Secondary treatment of primary effluent is called Biological treatment because secondary treatment involves biological organism such as aerobic and anaerobic microorganisms such as bacteria and fungi digest organic waste.
In the biological treatment, primary effluent is passed into aeration tank where aerobic microbes grow rapidly to form flocs. BOD reduced, effluent is passed into settling tank where flocs sediment to produce activated sludge, sludge is pumped to anaerobic sludge digester to digest bacteria and fungi.
In the digesters, heterotrophic microbes anaerobically digest bacteria and fungi in sludge producing mixture of gases such as CH4, H2S, CO2 which forms the biogas.
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