1.

Explain the different steps involved in the secondary treatment of sewage.

Answer»
  • Primary effluent is passed into large aeration tanks with constant mechanical agitation and air supply. Useful aerobic microbes grow rapidly and form flocs.
  • Flocs while growing consume organic matter and thus reduce the biochemical oxygen demand (BOD), the effluent is passed into settling tank. 
  • The bacterial flocs settle at the bottom of the tank and it forms activated sludge, a small part this is used as an inoculum in the aeration tank and the remaining part is passed into large tanks called anaerobic sludge digesters.


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