Solution:
Secondary sewage treatment is mainly a biological process. In secondary treatment primary effluent is passed into large aeration tanks, where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into floes. Floes is the masses of bacteria associated with fringal filaments to fonn mesh like structures. These microbes consume the major part of the organic matter in the effluent while growing. This significantly leduces the BOD (Biological Oxygen Demand) of the effluent. After the reduction of BOD, effluent is passed in a setting tank called activated sludge, where bacterial floes are allowed to sediment. This sludge is large tank, where bacteria and the fungi are digested by an aerobic bacteria present in the large tank called, anaerobic sludge digester and produce biogas.
