In a sewage treatment plant,primary treatment is

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

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

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?

In how many stages is the process of sewage treatment plants carried out?

$A$: Microbes are used in $STPs$.
$R$: $STPs$ are used to prevent water pollution.