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

Explanation

Secondary treatment or Biological treatment : In secondary treatment , the primary effluent is passed into large acration tanks where it is contantly agitated mechanically and air is pumped to it . This allows vigorous growth of useful acrobic microbes into flocs. While growing , these microbes consume the major part of the organic matter in the effluent.
This significantly reduces the BOD of the effluent. BOD is a measure of the organic matter present in the water . the greater the BOD of waste water , more is its polluting potential.
BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
Once the BOD of sewage or waste water is reduced significantly the effluent is then passed into a settling tank where the bacteria , flocs, allowed to sediment . this sediment is called activated sludge .A small part of the activated sludge is pumped back into the acration tank to serve as the inooculum.
The remaining major part of the slugde is pumped into lage tanks called anaerobic sludge digesters . Here other kinds of bacteria , which grow anaerobically , digest the bacteria and the fungi in the sludge . During this digestion , bacteria produce a mixture of gases such as methane , hydrogen sulphide and candon dioxide . these gases form biogas and can be used as source of energy as it s inflammable . the efflent from the secondary treatment plant is generally released into natural wator bodies like rivers and streams.
Secondary treatment , of the sewage is also called biological treatment because in secondary treatment , biological factors/organisms play a major role in treatment . So it is called Bioligical treatment.