Read the following statements and answer the questions that follow:-
The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pump into it. This allows vigorous growth of useful aerobic microbes into flocs.

What kind of sewage treatment has been discussed in the above statements?

Both II and III.

a- Primary treatment .

b- Secondary treatment.

c- Biological treatment.

d- Both II and III. is the right answer.

Secondary sewage treatment is also known as biological treatment. The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures). While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (biochemical oxygen demand) of the effluent.