What is the water obtained after the sedimentation process in sewage treatment called?

$(a)$ 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?

High amount of $E. coli$ in water indicates:

Secondary sewage treatment is primarily a/an ...... process.

$A$ sewage treatment process in which a part of decomposer bacteria present in the wastes is recycled into the starting of the process is called:

$A$: More sewage treatment plants are required in proportion to the human population.
$R$: Biogas,which is a fuel energy,is produced in $STPs$.