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Septic drain fields, also called leach fields or leach drains are used to remove contaminants and impurities from the liquid that emerges from the septic tank. A septic tank, the septic drain field, and the associated piping compose a complete septic system. The septic drain field is effective for disposal of organic materials readily catabolized by a microbial ecosystem. The drain field typically consists of an arrangement of trenches containing perforated pipes and porous material (often gravel) covered by a layer of soil to prevent animals and surface runoff from reaching the wastewater distributed within those trenches.〔Steel, E.W. & McGhee,Terence J. "Water Supply and Sewerage"McGraw-Hill Book Company (1979) isbn=0-07-060929-2 pp.576-577〕 Primary design considerations are hydraulic for the volume of wastewater requiring disposal and catabolic for the long-term biochemical oxygen demand of that wastewater. ==Hydraulic design== Many health departments require a percolation test ("perc" test) to establish suitability of drain field soil to receive septic tank effluent. An engineer or licensed designer may be required to work with the local governing agency to design a system that conforms to these criteria. Wastewater from toilets is assumed to contain bacteria and viruses capable of causing disease. Disinfection methods used prior to surface disposal of municipal sewage cannot be used with septic tanks because disinfection would prevent wastewater treatment by killing the septic tank and soil ecosystems catabolizing the putrescible contents of the wastewater. A properly functioning drain field holds and deactivates pathogens before they leave the drain field soil. The goal of percolation testing is to ensure the soil is permeable enough for septic tank effluent to percolate away from the drain field, but fine grained enough to filter out pathogenic bacteria and viruses before they travel far enough to reach a water well or surface water supply. Coarse soils – sand and gravel – can transmit wastewater away from the drain field before pathogens are destroyed. Silt and clay effectively filter out pathogens but allow very limited wastewater flow rates.〔Alth, Max & Charlotte "Constructing and Maintaining your Well & Septic System" Tab Books (1984) isbn=0-8306-0654-8 pp.166-167〕 Percolation tests measure the rate at which clean water disperses through a disposal trench into the soil. Several factors may reduce observed percolation rates when the drain field receives anoxic septic tank effluent:〔Alth, Max & Charlotte "Constructing and Maintaining your Well & Septic System" Tab Books (1984) isbn=0-8306-0654-8 p.217〕 *Microbial colonies catabolizing soluble organic compounds from the septic tank effluent will adhere to soil particles and reduce the interstitial area available for water flow between soil particles. These colonies tend to form a low-permeability biofilm of gelatinous slime at the soil interface of the disposal trench.〔Alth, Max & Charlotte "Constructing and Maintaining your Well & Septic System" Tab Books (1984) isbn=0-8306-0654-8 pp.164-165&219〕 *Insoluble particles small enough to be carried through the septic tank will accumulate at the soil interface of the disposal trench; non-biodegradable particles like synthetic fiber lint from laundry, mineral soil from washing, or bone and eggshell fragments from garbage disposals will remain to fill interstitial areas formerly available for water flow out of the trench.〔Alth, Max & Charlotte "Constructing and Maintaining your Well & Septic System" Tab Books (1984) isbn=0-8306-0654-8 p.219〕 *Cooking fats or petroleum products emulsified by detergents or dissolved by solvents can flow through prior to anaerobic liquifaction when septic tank volume is too small to offer adequate residence time, and may congeal as a hydrophobic layer on the soil interface of the disposal trench.〔Hammer, Mark J. "Water and Waste-water Technology" John Wiley & Sons (1975) isbn=0-471-34726-4 pp.407-408〕 *Rising groundwater levels may reduce the available hydraulic head (or vertical distance) causing gravitational water flow away from the disposal trench. Effluent initially flowing downward from the disposal trench ultimately encounters groundwater or impermeable rock or clay requiring a directional shift to horizontal movement away from the drain field. A certain vertical distance is required between the effluent level in the disposal trench and the water level where the effluent is leaving the drain field for gravitational force to overcome viscous frictional forces resisting flow through porous soil. Effluent levels in the vicinity of the drain field will appear to rise toward the ground surface to preserve that vertical distance difference if groundwater levels surrounding the drain field approach the level of effluent in the disposal trench.〔 *Frozen ground may seasonally reduce the cross-sectional area available for flow or evaporation. 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「septic drain field」の詳細全文を読む スポンサード リンク
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