The goal of wastewater management is to clean and protect water. This means that water must be clean enough so that it can be used by people for drinking and washing, and by industry for commercial purposes. It also must be clean enough to release into oceans, lakes, and rivers after it has been used. The treatment of wastewater is part of the overarching field of sanitation. Sanitation also includes the management of human waste and solid waste as well as storm water (drainage) management. By-products from wastewater treatment plants, such as screenings, grit and sewage sludge may also be treated in a wastewater treatment plant. Wastewater contains a wide range of contaminants. The quantities and concentrations of these substances depend upon their source. Pollutants are typically categorized as physical, chemical, and biological. Common pollutants include complex organic materials, nitrogen- and phosphorus-rich compounds, and pathogenic organisms ( bacteria , viruses , and protozoa ). Synthetic organic chemicals, inorganic chemicals, microplastics, sediments, radioactive substances, oil, heat, and many other pollutants may also be present in wastewater.