Write a short note on secondary treatment of sewage.

Explanation

Secondary treatment of sewage is as follows (i) This treatment is also known as biological treatment because it involves the use of microbes or biota for the treatment of sewage. The effluent from primary treatment is passed into large aeration tanks, where it is constantly mechanically agitated and air is pumped into it. (ii) This air helps in the growth of useful aerobic, microbes into flocs (masses of bacteria associated with fungal filament to form mesh like structures). The growth of microbes consumes major part of the organic matter, converting it into microbial biomass and releasing lot of minerals. This significantly reduces the BOD (Biochemical Oxyen Demand) of the water.