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.