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

The major contribution of bacteria in the carbon cycle includes ..........

When does the $BOD$ level in effluent decrease?

In sewage treatment,what is the water that separates from the primary tank called?

Flocs = $.........$

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.