Primary treatment : These treatment steps involve physical removal of particles from the sewage through filtration and sedimntation. These are removed in stages; initially, floating debris is removed by sequential filtration. Then the grit are removed by sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.
During secondary treatment, the primary effluent is passed into large aeration tanks. Air is also passed into these tanks and agitated. This enables the growth of aerobic microbes into flocs. While growing, the bacteria consume majority of the organic matter present in the effluent. This causes a significant reduction in the biochemical oxygen demand BOD of the effluent.
Essential to control pollution in natural water bodies, to check water borne diseases / pathogenic organism.