Measuring of $BOD$ in sewage water gives an estimation of . . . . . .

Sewage or municipal waste should not be directly passed into rivers,streams and other water bodies because
$I.$ it contains human excreta and other organic waste
$II.$ it contains a number of pathogenic microbes
Which of the statement given above is/are correct?

$A$ sewage treatment process in which a part of decomposer bacteria present in the wastes is recycled into the starting of the process is called:

In sewage treatment,what is removed by sedimentation?

Statement $P$: $A$ major portion of organic matter in water is consumed by chemical processes.
Statement $Q$: Microorganisms,including bacteria,consume organic matter present in water.

In the sewage treatment,bacterial flocs are allowed to sediment in a settling tank. This sediment is called as