Sewage is characterized in terms of its physical, chemical, and biological components. It brings to the sewage treatments plant all the wastes sent into the sewer from drains and toilets: industrial wastes, hospital wastes, commercial wastes, human excreta, stormwater runoff, and every other kind of hazardous, toxic, and biological waste material produced in a municipality and carried away from its source via the sewer.
Not all sewers discharge treated wastewater, but the vast majority in industrialized countries do, using regulatory policy to establish effluent limitations determined by treatment levels.
Sewage treatment is focused on reducing in wastewater discharges so-called conventional pollutants: oil, grease, organics like nitrogen and phosphorous (measured by biological oxygen demand), total suspended solids, and settleable matter. Most chemical removal is incidental, dictated by volatility, solubility, and hydrophobic properties, not treatment processes. The result is that many of the 42 billion pounds of synthetic organic chemicals produced in or imported to the United States for commercial and industrial uses each day are now found in the treated wastewater and sewage sludge (whether "treated" or not) from most municipalities and communities.
What is not removed by treatment or partitioned into the wastewater is found in sewage sludge. Sewage sludge is a heterogeneous mix of hazardous materials. The U.S. Environmental Protection Agency’s National Targeted Sewage Sludge Survey (2009) detected “high concentrations of toxic contaminants in sewage sludge,” specifically of “toxic contaminants with heavy metals, steroids and pharmaceuticals, including the antibacterials, triclocarban and triclosan…antibiotics, disinfectants, antimicrobials, steroids, endocrine disrupting chemicals and other anthropogenic drugs.”
The majority of sewage sludge produced in wastewater treatment plants in the United States is disposed of on land, primarily agricultural land. This highly controversial practice is supported by the federal government and regulated by the EPA.