During sewage treatment, the primary effluent is first passed into:

The primary effluent is passed into large aeration tanks  where it is constantly agitated mechanically and air is pumped into it. Primary treatment includes filtration and sedimentation of floating debris and grit.A major part of the activated sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge.