Which gases are produced during the sewage treatment process?

Polluted water can be purified by using

The primary treatment of sewage water involves

What is the correct sequence of processes occurring in sewage treatment plants?

In a sewage treatment plant,primary treatment is

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?