1. Preliminary treatment . The removal and disintegration of gross solids , the removal of grit and the separation of oil and grease .

2. Primary treatment . The first major stage of stp after preliminary treatment , which usually involves the removal of settleable solids , which are separated as sludge.

3. Secondary (biological ) treatment . The dissolved and colloidal organic matter is oxidized by micro organisms.

4. Tertiary Treatment . Further treatment of a biologically treated effluent to remove remaining BOD biochemical oxygen demand, suspended solids , bacteria , specific toxic compounds or nutrients

5. Sludge treatment . The dewatering , stabilization and disposal of sludge. Depending on the quality of the final effluent required , not all the stages will be required.