下水処理場
Municipal wastewater (sewage) is composed of human waste and other residential waste streams. In the United States, approximately 34 billion gallons of wastewater are collected every day and sent to sewage treatment plants. Wastewater is collected through combined sewers, which are used for sanitary waste and stormwater runoff, or in separate sanitary sewers. Sewage treatment plants include physical removal processes, such as screens and settling tanks, and biological processes to remove organic matter and pathogens from water. Treatment plants have strict permit requirements to ensure that their discharges will not cause harm to the environment or public health. Some plants have enhanced treatment processes to control nutrient pollution before discharge.