In the treatment of waste water discharge,which treatment stage involves biological treatment?

In the first stage of sewage treatment,physical particle matter is removed by which process?

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?

Identify the wrong statement regarding the process in sewage treatment.

What is the full form of $STPs$?

What does $STPs$ stand for?