Wastewater is any water that has been contaminated by human activity. Municipal wastewater is sanitary sewage from houses, offices, schools, and factories that is collected and treated at a central facility. It often includes process wastewater from small factories and businesses. Process wastewater from large factories, refineries, food processing facilities, and power plants is usually not discharged to a sanitary sewer. Instead, the water is treated onsite and discharged.
Municipal wastewater treatment involves several steps. At a minimum, treatment removes suspended solids and reduces the concentration of organic matter. Other steps may remove nutrients, such as nitrogen and phosphorus. Treatment must also kill disease-causing microorganisms. Chlorine gas and sodium hypochlorite are common disinfectants.
Treated wastewater can be discharged into another body of water or it can be reused. Because chlorine is harmful to many aquatic and marine animals, including fish, discharged water must contain very low levels of chlorine. The discharge permit for the plant specifies the allowed concentration. To meet discharge limits, dechlorination using chemicals such as sulfur dioxide and sodium bisulfite is often necessary. Wastewater intended for reuse, that is for irrigation, injection into an aquifer, or injection into seawater barrier wells, is usually not dechlorinated.