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Effluent sewer systems, also called septic tank effluent gravity (STEG), solids-free sewer (SFS), or septic tank effluent drainage (STED) systems, have septic tanks that collect sewage from residences and businesses, and the liquid fraction of sewage that comes out of the tank is conveyed to a downstream receiving body such as either a centralized sewage treatment plant or a distributed treatment system for further treatment or disposal away from the community generating the sewage. Most of the solids are removed by the interceptor tanks, so the treatment plant can be much smaller than a typical plant and any pumping for the supernatant can be simpler without grinders (sometimes water pumps are sufficient).
An alternative effluent sewer which is similar to the STEG system is the STEP system. Because of the vast reduction of solid wastes and the capture of fats, oils and grease (FOG) within the interceptor tank, a pumping system can be used to move the wastewater under pressure rather than a gravity driven conveyance system.