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

Select the correct sequence with respect to sewage treatment.
$i$. Removal of suspended solid particles by primary treatment.
$ii$. Wastewater screening
$iii$. Anaerobic bacteria digest activated sludge.
$iv$. Treatment by flocs in aeration tank.
$v$. Grit settling

What does sewage water mainly contain?

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 $STPs$,what is the effluent water?

Measurement of the rate of $O_2$ consumption in unit volume of water over a period of time is done to find out