Wastewater is water that has been used and contaminated through residential, commercial, industrial, or institutional activities, requiring treatment before safe discharge or reuse. Municipal wastewater includes sewage from toilets, greywater from sinks and showers, and water from commercial and industrial processes connected to municipal systems. Wastewater contains organic matter, nutrients, pathogens, and potentially harmful chemicals. Proper wastewater management protects public health and the environment. Municipal wastewater systems collect wastewater through sewer networks, transport it to treatment facilities, process it to meet discharge standards, and release treated effluent. Stormwater may be combined with wastewater in older combined sewer systems or handled separately in modern systems. Wastewater treatment is a core municipal responsibility with significant infrastructure and operating costs.