Measurement of the rate of $O_2$ consumption in unit volume of water over a period of time is done to find out

Secondary treatment of sewage

Three water samples namely river water,untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to $BOD$ test. The samples were labelled $A, B$ and $C$; but the laboratory attendant did not note which was which. The $BOD$ values of the three samples $A, B$ and $C$ were recorded as $20\, mg/L, 8\, mg/L$ and $400\, mg/L$ respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?

Measuring of $BOD$ in sewage water gives an estimation of . . . . . .

In which of the following are flocs formed?

Which of the following in sewage treatment removes suspended solids?