Explain the biological treatment of primary effluent when passed into the large aeration tanks in a sewage treatment plant (STP).

− The sewage is constantly agitated mechanically and air is pumped into it − This allows the vigorous growth of useful microbes into flocs (masses of bacteria associated with fungal hyphae to form mesh like structures − While growing these microbes consume the major part of the organic matter in the effluent − This significantly reduces the Biochemical Oxygen Demand or BOD