Secondary sewage treatment is mainly a

$A$: Microbes are used in $STPs$.
$R$: $STPs$ are used to prevent water pollution.

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.

$X$ - There are two phases in sewage treatment plant.
$Y$ - Microbes are involved in this process.

What is the correct sequence of processes in a sewage treatment plant?

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