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Sewage collection and disposal systems transport sewage through cities and other inhabited areas to sewage treatment plants to protect public health and prevent disease. Sewage is treated to control water pollution before discharge to surface waters. ==Collection== *A sewage system may convey the wastewater by gravity to a sewage treatment plant. *Where pipeline excavation is difficult because of rock or there is limited topographic relief (i.e., due to flat terrain), gravity collection systems may not be practical and the sewage must be pumped through a pipeline to the treatment plant. *In low-lying communities, wastewater may be conveyed by vacuum sewer. Pipelines range in size from pipes of six inches (150 mm) in diameter to concrete-lined tunnels of up to thirty feet (10 m) in diameter. *Community sewage can also be collected by an effluent sewer system, also known as a STEP system (Septic Tank Effluent Pumping). At each home, a buried collection tank is used to separate solids from the liquid effluent *Only the liquid portion is then pumped through small diameter pipe (typically 1.5" to 4") to downstream treatment. *Because the wastestream is pressurized, the pipes can be laid just below the ground surface along the land's contour. *Sewage can also be collected by low pressure pumps and vacuum systems. *A low pressure system uses a small grinder pump located at each point of connection, typically a house or business. *Vacuum sewer systems use differential atmospheric pressure to move the liquid to a central vacuum station. *Typically a vacuum sewer station can service approximately 1,200 homes before it becomes more cost-effective to build another station. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sewage collection and disposal」の詳細全文を読む スポンサード リンク
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