Secondary treatment of sewage

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 bacteria digest the bacteria and fungi present in the sludge during sewage treatment?

In which tanks do the formation of flocs and the sedimentation of flocs occur,respectively?

In anaerobic sludge digesters,what do anaerobic bacteria digest?

$(a)$ What would happen if a large volume of untreated sewage is discharged into a river?
$(b)$ In what way is anaerobic sludge digestion important in sewage treatment?