Question

Explain the process of secondary treatment given to the primary effluent up to the point it shows significant change in the level of biological oxygen demand (BOD) in it. [2]

Answer 1

Secondary treatment refers to the treatment process that uses biological processes to convert dissolved suspended and colloidal organic wastes to more stable solids that can either be removed by settling or discharged to the environment without causing harm. In this process, supernatant from the primary treatment is passed into large aeration tanks. In these tanks, the effluent is agitated mechanically and air is pumped into it. This causes vigorous growth of bacteria that leads to the formation of flocs. While growing, these microbes consume a major part of organic matter in the effluent, significantly reducing the biochemical oxygen demand (BOD). [2]