What kind of process is primary treatment in sewage treatment?

Polluted water can be purified by using

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

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?

The microorganisms involved in floc formation during sewage treatment are