Question

During secondary treatment of primary effluent, how does a significant decrease in BOD occur? [2]

Answer 1

Biological Oxygen Demand (BOD) is the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria. [0.5]

After primary treatment, the effluent is passed into large aeration tanks where it is constantly agitated mechanically. This allows the growth of useful aerobic microbes into flocs (bacteria associated with fungal filaments). While growing, these flocs consume a major part of the organic matter in the effluent and hence reduce the BOD. [1.5]