Where are methanogens found in a sewage treatment plant?

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?

Sewage water can be purified for recycling with the action of

Before discharging municipal wastewater into natural water bodies like rivers and streams,it is passed through which of the following?

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

If a large amount of sewage is discharged into a river,its $BOD$ will ......... .