Gravity sewer

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.