Sewage is water that contains waste matter produced by human beings.  It is also called wastewater.  It contains about a tenth of 1 per cent solid matter.  Sewage comes from the sinks and toilets of homes, restaurants, office buildings, and factories.  It contains dissolved material that cannot be seen, plus bits of such solid matter as human waste and ground-up garbage.  Some sewage may also contain ground and surface water runoff that occurs after storms or floods.  Most sewage also includes harmful chemicals and disease-producing bacteria.

Most sewage eventually flows into lakes, oceans, rivers, or streams.  In the United States, almost all sewage is treated in some way before it goes into the waterways as a semiclear liquid called effluent.  Untreated sewage looks and smells foul, and it kills fish and aquatic plants.

Even treated sewage can harm water.  For example, most methods used to treat sewage convert organic wastes into inorganic compounds called nitrates, phosphates, and sulfates.  Some of these compounds may serve as food for algae and cause large growths of these simple aquatic organisms.  After algae die, they decay.  The decaying process uses up oxygen.  If too much oxygen is used, fish and plants in the water will die.

A system of pipes that carries sewage from buildings is called a sanitary sewerage system.  There are two main types of sanitary sewerage systems: (1) urban sewerage systems and (2) rural sewerage systems.