Question

Answer the following question.
Secondary treatment of the sewage is also called Biological treatment. Justify this statement and explain the process.

Answer 1

Secondary treatment or Biological treatment :

• Primary effluent is passed into large aeration tanks with constant mechanical agitation and air supply.
• Useful aerobic microbes grow rapidly and form flocs.
• Flocs are masses of bacteria and consume organic matter and thus reduce the biochemical oxygen demand (BOD).
• When BOD of sewage has reduced, the effluent is passed into the Settling tank.
• Here, the bacterial flocs settle and sediment is called activated sludge.
• A small part of the sludge is used as an inoculum in the aeration tank and the remaining part is passed into large tanks called anaerobic sludge digesters.
• In the digesters, heterotrophic microbes anaerobically digest bacteria and fungi in sludge producing a mixture of gases such as methane, H2S, CO2which forms the biogas.