Domestic sewage in large cities:
is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plant (STPs)
when treated in STPs, does not require the aeration step as the sewage contains adequate oxygen
has very high amounts of suspended solids and dissolved salts
has a high BOD as it contains both aerobic and anaerobic bacteria
Secondary treatment (Biological treatment) of sewage involves two main stages involving microbes. First, the primary effluent is passed into large aeration tanks where vigorous growth of useful aerobic microbes (flocs) consumes the major part of the organic matter. Subsequently, the sediment (activated sludge) is pumped into anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in the sludge , .
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