Water pollution consists of any physical or chemical change in water that adversely affects the health of and other organisms Sewage is the release of wastewater from drains or sewers for example : from toilets ,washing machines and showers. It also includes human wastes such as soap and detergents. Disease causing agent for example : bacteria, viruses, protozoa and parasitic worms are transmitted in sewage. Sediment pollution, primarily from soil erosion, increase water turbidity, thereby reducing photosynthetic productivity in the water. Inorganic plant and algal nutrients for example: nitrogen and phosphorus , contribute to enrichment . The fertilization of a body of water. Many organic compounds for example : pesticides ,pharmaceuticals, solvents and industrial chemical. These organic are quite toxic. Inorganic chemical also includes toxic such as lead and mercury. Radioactive Substance include the wastes from mining, refining, and using radioactive metals. Thermal pollution occurs when heated water, produced during many industrial, is released into waterways. Sewages supplies nutrients that contribute to eutrophication and a high biochemical oxygen demand .Eutrophication , the nutrient enrichment of lakes ,estuaries or slow moving streams results in high photosynthetic productivity and supporting an overpopulation of algae. Eutrophication also kills fishes and causes a decline in water quality as these algae dies and decomposed. Biochemical oxygen demand is the amount of oxygen needed by microorganisms to decompose biological wastes inti carbon dioxide such water and minerals. A large amount of sewage generates a high biochemical oxygen demand , which also lowers the lever of dissolved oxygen in the water. Most municipal water supplies are treated before used so that the water is safe to drink. Water is usually treated with aluminum sulfate to cause suspended particles to clump and settle out. Its also filtered through sand and disinfected by adding chlorine. There is concern that chlorine in drinking water creates health problem. So in the case UV includes radiation and zonation and their being developed. There are two types of tertiary treatment for wastewater such as primary and secondary. Primary treatment is treating wastewater by removing suspended and floating particales by mechanical processes. Secondary treatment is treating wastewater biological to decompose suspended organic material. Secondary treatment reduces the water’s biological oxygen demand. Primary treatment and secondary treatment generate primary and secondary sludge which also consist of the solids remaining after sewage treatment has been completed.