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Minnesota <br />Pollution <br />Control <br />Agency <br />wq-wwists 1-11 <br />Soil -Based Sewage <br />Treatment Systems <br />Facts About Subsurface Sewage <br />Treatment Systems <br />Introduction <br />Wastewater/Individual Sewage Treatment Systems #1.11 • June 2008 <br />a soil -based treatment system, such as a <br />mound, trench or at -grade drainfield. <br />Subsurface SewageTreatment Systems <br />(SSTSs) are commonly known as septic <br />systems. They are soil -based treatment <br />systems used by homes and businesses <br />which are not connected to municipal <br />sewer. SSTS were formerly called <br />Individual Sewage Treatment Systems <br />(ISTS). Even though their name has <br />changed, their purpose has not; to treat and <br />dispose of the wastewater generated on site <br />each day by non -municipal homes and <br />businesses. <br />The wastewater contains sewage, which in <br />turn contains bacteria, viruses, parasites, <br />nutrients and some chemicals. Therefore, <br />proper treatment and disposal is necessary <br />to minimize the potential for disease <br />transmission and environmental <br />contamination from the sewage. <br />How SSTSs treat sewage <br />SSTSs treat sewage through a combination <br />of biological, physical and chemical <br />processes. They are designed to account for <br />the daily wastewater flow, the type of <br />distribution system (gravity or pressure), <br />soil conditions of the site, and need the <br />development of a biological layer (a <br />biomat) for proper wastewater treatment. <br />When properly designed, constructed and <br />maintained they provide a high degree of <br />sewage treatment and are a proven method <br />of controlling the negative environmental <br />effects of untreated sewage. <br />A typical SSTS consists of a septic tank <br />followed by one of many different types of <br />The septic tank <br />A buried, watertight septic tank is the first <br />component of a SSTS. Sewage is piped <br />from a home or business to the septic tank, <br />which is sized to retain wastewater for 24 <br />to 36 hours. This retention time allows <br />three distinct layers to develop inside the <br />tank: <br />• The heavier solids sink to the bottom. <br />• The lighter greases, fats, and soaps float to <br />the top. <br />• The remaining middle layer (effluent) <br />flows out to the drainfield for final <br />treatment. <br />o The amount of effluent that flows out to <br />the drainfield will equal the amount that <br />flows into the tank each day. <br />Baffles inside the tank at the inlet and <br />outlet connections help prevent the heavier <br />and lighter layers traveling to the <br />drainfield, where they can clog the <br />distribution pipes and cause premature <br />drainfield failure. Over time, these heavier <br />and lighter layers will accumulate, and <br />must be removed at regular tank pumping <br />intervals. <br />Minnesota Pollution Control Agency • 520 Lafayette Rd. N., St. Paul, MN 55155-4194 • www.pca.state.mn.us <br />651-296-6300 • 800-657-3864 • TTY 651-282-5332 or 800-657-3864 • Available in alternative formats <br />