A combined sewer is a type of gravity sewer with a system of pipes, tunnels, pump stations etc. to transport sewage and urban runoff together to a sewage treatment plant or disposal site. This means that during rain events, the sewage gets diluted, resulting in higher flowrates at the treatment site. Uncontaminated stormwater simply dilutes sewage, but runoff may dissolve or suspend virtually anything it contacts on roofs, streets, and storage yards. As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. Combined sewers may also receive dry weather drainage from landscape irrigation, construction dewatering, and washing buildings and sidewalks.
Photo of the interior of a combined sewer in Brighton, England.
Combined sewer outflow into the Anacostia River in Washington, D.C.
A medieval waste pipe in Stockholm Old Town formerly deposited sewage on the street to be flushed away by rain.
Sewage canal of a medieval house as depicted in 1447 St. Barbara Altarpiece in the National Museum in Warsaw.
A gravity sewer is a conduit utilizing the energy resulting from a difference in elevation to remove unwanted water. The term sewer implies removal of sewage or surface runoff rather than water intended for use; and the term gravity excludes water movement induced through force mains or vacuum sewers. Most sewers are gravity sewers because gravity offers reliable water movement with no energy costs wherever grades are favorable. Gravity sewers may drain to sumps where pumping is required to either force sewage to a distant location or lift sewage to a higher elevation for entry into another gravity sewer, and lift stations are often required to lift sewage into sewage treatment plants. Gravity sewers can be either sanitary sewers, combined sewers, storm sewers or effluent sewers.
Gravity sewer installation illustrating the significant excavation depth often required to maintain a favorable gradient.
Gravity sewer flow as seen looking down into an open sanitary manhole.
Old brick gravity sewers like this one are typically replaced by reinforced concrete.