The waste water can be treated mainly by the three stages which involve the primary, secondary and tertiary stage. The primary stage involves the process in which sewage is passed through the screens and settling tanks. They remove the solid and organic matter along with the coarse matter which involves the sand and grit. It does not alter the population of pathogens and minerals. The secondary stage involves the process in which sewage is passed through the aerated tanks which decomWrite a short note onposes the organic matter with the help of microorganisms. Then the effluent is filtered and is chlorinated later on. It kills the pathogens and minerals which impose a danger to the eutrophication of rivers and lakes. The clean liquid is returned to the environment after the sewage process is completed. There are still a lot of nitrates and phosphates in this clean liquid. The tertiary stage involves the process in which sewage is passed through the filter. It also goes through the chemical treatment which helps in removing the mineral load. After this process the liquid can be drink. This process is very costly and is left in many sewage treatment plans. The tertiary treatment is also provided by the process of reverse osmosis. It demineralizes the brackish water by removing the salt. The water is passed through the semi permeable membrane. The pressure is very strong in doing so.