Sewage treatment plant is the process of removing contaminants from waste water and household sewage. Its objective is to produce an environmentally safe fluid waste stream. Sewage treatment usually work in three stages: primary, secondary, and tertiary. The primary treatment removes heavy solids. Then the secondary treatment involves the addition of micro-organisms that dissolve the suspended matter in the sewage. The final or tertiary stage removes any residual matter and disinfects the water.