Secondary treatment of sewage is also called biological treatment. Justify this statement and explain the process.
Secondary treatment of sewage is a biological process that employs the heterotrophic bacteria naturally present in the sewage. The effluent from the primary treatment is passed into large aeration tanks, where it is constantly agitated and the air is pumped into it. This allows the rapid growth of aerobic microbes into ‘floes’ which consume the organic matter of the sewage and reduce the biological oxygen demand (BOD). The greater is the BOD of wastewater, the more is its polluting potential. When the BOD of sewage is reduced significantly, the effluent is passed into a settling tank, where the ‘floes’ are allowed to sediment forming the activated sludge. A small part of the activated sludge is pumped back into the aeration tanks. The remaining major part of the sludge is pumped into anaerobic sludge digesters, where the anaerobic bacteria digest the bacteria and fungi in the sludge-producing methane, hydrogen sulfide, and carbon dioxide, i. e. biogas. This is why secondary treatment of sewage is also called biological treatment. The effluent after secondary treatment is released into water-bodies like streams or rivers.
