In sewage treatment,which of the following removes floating debris?

Sewage water is purified by ..........

What is the key difference between primary and secondary sewage treatment?

What is the full form of $STPs$?

When does the $BOD$ level in effluent decrease?

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?