What is the free water above the primary sludge called?

Which of the following statements regarding sewage is correct?

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.

If a large amount of sewage is discharged into a river,its $BOD$ will ......... .

The measurement of $BOD$ (Biochemical Oxygen Demand) in wastewater provides information about the amount of which of the following?

Secondary treatment of sewage