Which of the following statements is correct?

What is the process of treating wastewater by municipalities in cities called?

What does $STPs$ stand for?

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?

The removal of floating and suspended solids from sewage through filtration and sedimentation is the part of

In which tanks do the formation of flocs and the sedimentation of flocs occur,respectively?