Which bacteria are used to treat sewage water?

What is primary sludge?

$BOD$ is concerned with

Given below are two statements with reference to the secondary treatment of sewage.
Statement $I$: The $BOD$ of effluent increases.
Statement $II$: Flocs of bacteria consume the major part of the organic matter present in the effluent.
In the light of the above statements,choose the most appropriate answer from the options given below.

Sewage treatment is carried out by .........

Which process is used to remove grit (soil and small pebbles) from sewage during primary treatment?