This is followed by a settling tank (usually referred to as "final clarifier" or "secondary settling tank") to allow the biological flocs (the sludge blanket) to settle, thus separating the biological sludge from the clear treated water.

Which tank digests flock?

A large number of aerobic heterotrophic microbes grow in aeration tank. They form floes, which digest a lot of organic matter. As the BOD of waste water is reduced sufficiently, it is passed into setting tank where bacterial floes are allowed to undergo sedimentation.

Which is used to remove sludge?

Dewatering decreases sludge liquid volume as much as 90 percent. Digested sludge is put through large centrifuges that work in the same fashion as a washing machine spin cycle. The spinning centrifuge produces a force that separates the majority of the water from the sludge solid, creating a biosolid substance.