Identify the stage of sewage treatment shown below.

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.

Which of the following is used to digest the bacteria and fungi present in sewage sludge?

The key substance between primary and secondary treatment in Sewage Treatment Plants $(STP)$ is?

In how many stages is the sewage treatment process carried out in $STPs$?

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