List out the events that reduce Biological Oxygen Demand (BOD) of a primary effluent during sewage treatment

Explanation

Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen required for the microbial breakdown of biodegradable organic matter. Aerobic organisms use a lot of oxygen and as a result , there is a sharp decline in Dissolved Oxygen (DO) in the water body. This can cause death of fishes and other aquatic species. Determination of BOD is thus, an important parameter in suggesting the quality of a water body. Bod is higher in polluted water and lesser in clean water.
Secondary sewage treatment includes following steps :
(a) It is a biological process in which bacteria occuring naturally in the sewage are used.
(b) Effluent obtained from the primary treatment is passed into large aeration tank, Here, it is constantly agitated and air is pumped into it.
(c) Due to this , rapid growth of aerobic bacteria occur into flocs. These consume the organic matter of the sewage and reduce the BOD
(d) Effluent is passed into settling tank, where the flocs are allowed to settle forming the activated sludge.
(e) A small amount of activated sludge is pumped back into the aeration tank as inoculum
(f) The remaining major part of the activated sludge is pumped into the anaerobic sludge digesters, where the anaerobic bacteria, digest organic matter and produce mathane , hydrogen sulphide and carbon dioxide which forms the biogas .
(g) Effluent is then allowed to pass into the water boby