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

In the secondary treatment of $STP$,the masses of bacteria associated with fungal filaments are called . . . . . . .

What is activated sludge?

Select the correct statement from the following.

$A$ - $STP$ (Sewage Treatment Plants) purify sewage.
$R$ - In the end of $STP$,the water possesses flocs.

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?