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. Gravity sewers can be either sanitary sewers, combined sewers, storm sewers or effluent sewers. Gravity sewer systems typically resemble the regional runoff pattern with large trunk sewers in each valley receiving flow from smaller lateral sewers extending up hillsides. Sewer systems within comparatively level terrain require careful planning and construction to minimize energy losses in free falls, sharp bends, or turbulent junctions. Every reach of the sewer should routinely experience the minimum velocity necessary to maintain solids in suspension and avoid blockage from solids deposition in low-velocity areas. Sewers in hilly areas, however, may require energy dissipation features to avoid sewer damage from high fluid velocities and the scouring effects of gritty solids in turbulent flow. Covered sewers are buried below the frost line to avoid freezing, and deep enough to receive gravity flow from anticipated wastewater sources. Long gravity sewers may require significant excavation depths or tunneling to maintain acceptable gradients near the sewer outfall. Availability of reliable pumps allows lifting accumulations of water into gravity sewers from collection sumps in excavations like mines or building foundations; but the cost of pumping surface runoff discourages use of lift stations in storm drains or combined sewers.