It is the process of mechanically removing the solid materials present in water through metal screening. Grit chambers and sedimentation. Metal screening removes large floating objects such as small piece of woods, rags, masses of garbage and death insects and animals

It is a biological oxidation of organic matter. It is achieved by filter method or by sludge process. In the filter method, the waste water is sprinkled over the surface of a bed of small stones of one to two metres deep. When the water percolates through the stone bed, a very complex biological growth of algae, fungi, protozoa and bacteria occurs. By these formation, the waste water gets oxidised. The oxidised waste water is then passed into the sedimentation tanks.

The residue from earlier two treatment process still leave about 10 percent of suspended solid bodies, 10 percent of the oxygen demanding wastes, 30 percent of toxic metal compounds, 50 percent of Nitrogen and 70 percent of phosphorous. This Tertiary Treatment method is an advanced form of chemical and physical process