Sewage, the mixture of water and waste products carried off through a drainage system of underground pipes, or sewers. The waste products consist of both organic and inorganic matter, including human wastes, mineral salts, and garbage. In most large cities sewage also contains liquid wastes from various industrial processes.

The organic matter in sewage decomposes rapidly, giving off foul-smelling and hazardous gases. Disease-causing organisms are passed into sewage through the feces and urine of infected persons. The wastes from industrial processes are often toxic. The safe disposal of sewage is therefore essential to the health of a community.

The collection and treatment of sewage is one of the most important municipal services. In the United States, most cities and towns have sewer systems that carry sewage to a sewage treatment plant. At the plant the sewage is treated to destroy disease organisms and to remove substances that can cause harmful or other undesirable effects in the water. The treated sewage is then discharged into nearby streams, lakes, or coastal waters. In rural areas, the treated sewage is usually disposed of in the soil.

The disposal of untreated or inadequately treated sewage directly into a stream or body of water can result in serious water pollution. Disease organisms endanger water supplies and swimming areas. Various chemicals may poison the water, killing fish and other wildlife, while certain nutrients in the sewage can cause an excessive growth of aquatic plants. As wastes decompose, they can deplete the oxygen supply in the water, making it unfit for all forms of aquatic life.

The facilities for collecting, treating, and disposing of sewage are called a sewerage system. In cities and towns, sewerage systems are designed to serve the entire community. Sewage from individual buildings flows into collecting sewers, which carry the waste to a central plant for treatment and disposal. Sewers that only carry domestic sewage—that is, sewage from residential and commercial buildings—are called sanitary sewers. Storm sewers are designed specifically for carrying runoff from rain and melted snow. In many systems, both domestic sewage and runoff water are carried in combined sewers. Combined sewers are generally undesirable, however, because overflow caused by heavy rains often makes it necessary to discharge untreated sewage from the sewage treatment plant.

Sewers are usually made of clay, concrete, or plastic. Sewerage systems are usually built so that gravity will carry the waste through the pipes. Where this is not possible, the sewage must be pumped.

In rural areas and in communities not served by municipal sewerage systems, sewage is disposed of through the use of septic tanks or, less commonly, cesspools. The privy, or outdoor toilet, is commonly used for disposing of human wastes where there is no indoor plumbing.