Question

During the secondary treatment of primary effluents, how does a significant decrease in BOD occur?

Answer 1

Answer:

During secondary treatment, the aeration allows vigorous growth of useful aerobic microbes into flocs, i.e. masses of bacterial cells in association with fungal filaments, forming mesh-like structures.

* As they grow, the microbes consume a major part of the organic matter in the effluent; Therefore, BOD is significantly reduced.

Answer 2

Explanation:

BOD (Biological oxygen demand) is the amount of oxygen required by bacteria to oxidise all the organic matter present in the effluent. ... During the secondary treatment, vigorous growth of bacteria takes place which feeds upon the organic matter leading to decrease of organic matter and hence decrease of BOD.