Explain the various stages in the secondary treatment of sewage.

Explanation

(1) The primary effluent is passed into large aeration tanks. This allows vigorous growth of useful aerobic microbes into flocs. Flocs is a mass of bacteria associated with fungal filaments to form mesh like structures.
(2) The growth of microbes consumes the major part of the organic matter in the effluent. This significantly reduces the BOD [Bio chemical oxygen demand] of the effluent.
(3) Once the BOD of sewage reduces significantly, the effluent is then passed into the settling tank, where the bacterial flocs are allowed to sediment. This sediment is called activated sludge.
(4) Small part of activated sludge is pumped back to aeration tank to serve as the TH inoculums. The remaining sludge is pumped into anaerobic sludge digester.
(5) In the anaerobic sludge digester there are other kinds of bacteria which grow anaerobically and digest the bacteria and fungi in the sludge.
(6) During the digestion, bacteria produce biogas (mixture of methane, hydrogen sulphide and carbon-dioxide). This biogas can be used as a source of energy as it is inflammable.
(7) The effluent from the secondary treatment plant is released into natural water body like river and streams.