Secondary treatment of sewage

Explanation

Secondary treatment or Biological treatment : In secondary treatment, the primary effluent is passed into large aeration tanks where it is constantly 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 th BOD of the effluent. BOD is a measure of the organic matter present in the water. The greater the BOD of waste, more is its polluting potential.
`**" "`BOD refers to the amount of the oxygen that would be consumed, if all organic matter in one litre of water were oxidised by bacteria.
`**" "`One 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 aeration tank to serve as the inoculum.
`**" "` The remaining major part of the sludge is pumped into large tanks called anerobic sludgedigesters. 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 carbon dioxide. These gases from biogas and can be used as source of energy as it is inflammable. the effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.
`**" "`Secondary treatement, 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 biological treatment.