The domestic sewage in large cities

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(B) : Sewage water can be purified by passing it through sewage treatment plants with the action of heterotrophic microorganisms. There are three stag

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is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plants $(STPs)$

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(B) : Sewage water can be purified by passing it through sewage treatment plants with the action of heterotrophic microorganisms. There are three stages of this treatment - primary,secondary,and tertiary. Primary treatment removes floating and suspended solids from sewage through two processes of filtration and sedimentation. First,floating matter is removed through sequential filtration. The filtrate is kept in large open settling tanks where grit settles down. The sediment is called primary sludge,while the supernatant is called effluent. In secondary treatment,the primary effluent is taken to aeration tanks. $A$ large number of aerobic heterotrophic microbes grow in the aeration tank and form flocs. The microbes digest a lot of organic matter,reducing the $BOD$ of the waste. The sediment of the settling tank is called activated sludge. The remaining is passed into a large tank called an anaerobic sludge digester,where anaerobic microbes digest the organic mass and produce gases like methane,$H_2S$,and $CO_2$.

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