Septic drain fields, also calledleach fields orleach drains, are subsurfacewastewater disposal facilities used to remove contaminants and impurities from the liquid that emerges afteranaerobic digestion in aseptic tank. Organic materials in the liquid arecatabolized by a microbialecosystem.
A septic drain field, a septic tank, and associated piping compose aseptic system.
The drain field typically consists of an arrangement of trenches containing perforated pipes and porous material (oftengravel) covered by a layer ofsoil to preventanimals (andsurface runoff) from reaching the wastewater distributed within those trenches.[1] Primary design considerations are bothhydraulic for the volume of wastewater requiring disposal andcatabolic for the long-termbiochemical oxygen demand of that wastewater. The land area that is set aside for the septic drain field may be called a septic reserve area (SRA).[2]
Sewage farms similarly dispose ofwastewater through a series of ditches and lagoons (often with little or no pre-treatment). These are more often found in arid countries as the waterflow on the surface allows for irrigation (and fertilization) of agricultural land.
Many health departments require apercolation test ("perc" test) to establish the suitability of drain field soil to receive septic tank effluent. Anengineer,soil scientist, or licensed designer may be required to work with the local governing agency to design a system that conforms to these criteria.
A more progressive way[citation needed] to determine leach field sizing is by direct observation of the soil profile. In this observation, the engineer evaluates many features of the soil such as texture, structure, consistency, pores/roots, etc.
The goal of percolation testing is to ensure the soil is permeable enough for septic tank effluent topercolate away from the drain field but fine-grained enough to filter out pathogenic bacteria and viruses before they travel far enough to reach awater well or surface water supply. Coarse soils –sand and gravel – can transmit wastewater away from the drain field before pathogens are destroyed.Silt andclay effectively filter out pathogens but limit wastewater flow rates.[3] 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 receivesanoxic septic tank effluent:[4]
Just as a septic tank is sized to support a community of anaerobic organisms capable of liquefying anticipated amounts of putrescible materials in wastewater, a drain field should be sized to support a community of aerobic soilmicroorganisms capable ofdecomposing the anaerobic septic tank's effluent into aerobic water.Hydrogen sulfide odors oriron bacteria may be observed in nearby wells or surface waters when effluent has not been completely oxidized before reaching those areas.[7] The biofilm on the walls of the drain field trenches will use atmosphericoxygen in the trenches to catabolize organic compounds in septic tank effluent. Groundwater flow islaminar in the aquifer soils surrounding the drain field.[8] Septic tank effluent with soluble organic compounds passing through the biofilm forms a mounded lens atop the groundwater underlying the drain field.Molecular diffusion controls the mixing of soluble organic compounds into the groundwater and the transport of oxygen from underlying groundwater or thecapillary fringe of the groundwater surface to micro-organisms capable of catabolizing dissolved organic compounds remaining in the effluent plume.[9]
When aseptic tank is used in combination with abiofilter, the height and catabolic area of the drain field may be reduced. Biofilter technology may allow higher-density residential construction, minimal site disturbance, and more usable land for trees, swimming pools, or gardens. Adequate routine maintenance may reduce the chances of the drain field plugging up. The biofilter will not reduce the volume of liquid that must percolate into the soil, but it may reduce the oxygen demand of organic materials in that liquid.
A drain field may be designed to offer several separate disposal areas for effluent from a single septic tank. One area may be "rested" while effluent is routed to a different area. Thenematode community in the resting drain field continues feeding on the accumulated biofilm and fats when the anaerobic septic tank effluent is no longer available. This natural cleansing process may reducebioclogging to improve the hydraulic capacity of the field by increasing the available interstitial area of the soil as the accumulated organic material is oxidized. The percolation rate after resting may approach, but is unlikely to match, the original clean water percolation rate of the site.
Septic tank and drain field microorganisms have very limited capability for catabolizing petroleum products andchlorinated solvents, and cannot remove dissolvedmetals; however, some may be absorbed into septic tank sludge or drain field soils, and concentrations may be diluted by other groundwater in the vicinity of the drain field. Cleaning formulations may reduce drain field efficiency. Laundrybleach may slow or stop microbial activity in the drain field, andsanitizing or deodorizing chemicals may have similar effects. Detergents, solvents, anddrain cleaners may transportemulsified,saponified or dissolved fats into the drain field before they can be catabolized into short-chain organic acids in the septic tank scum layer.[7]
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