First stage is sludge concentration, the main purpose is to reduce initial capacity of sludge to reduce the volume of subsequent treatment structures,
Second stage is sludge digestion, which is mainly to decompose the organic matter in sludge;
Third stage is Sludge Dewatering for further reducing the sludge capacity;
Finally stage is sludge disposal, in which the final sludge is absorbed by a certain way.
The supermatant or fitrate produced in the above stages still contains a large amount of pollutants and should be returmed to the sewage treatment system for treatment.