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.