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Septic drain fields, also called leach fields or leach drains, are subsurface
wastewater Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...
disposal facilities used to remove contaminants and impurities from the liquid that emerges after
anaerobic digestion Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to Waste management, manage waste or to produce fuels. Mu ...
in a
septic tank A septic tank is an underground chamber made of concrete, fiberglass, or plastic through which domestic wastewater (sewage) flows for basic sewage treatment. Settling and anaerobic digestion processes reduce solids and organics, but the treatment ...
. Organic materials in the liquid are catabolized by a microbial
ecosystem An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...
. A septic drain field, a septic tank, and associated piping compose a septic system. The drain field typically consists of an arrangement of trenches containing perforated pipes and porous material (often
gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...
) covered by a layer of
soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
to prevent
animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
s (and
surface runoff Surface runoff (also known as overland flow or terrestrial runoff) is the unconfined flow of water over the ground surface, in contrast to ''channel runoff'' (or ''stream flow''). It occurs when excess rainwater, stormwater, meltwater, or other ...
) from reaching the wastewater distributed within those trenches. Primary design considerations are both ''hydraulic'' for the volume of wastewater requiring disposal and ''catabolic'' for the long-term biochemical 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). Sewage farms similarly dispose of
wastewater Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...
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.


Design

Many health departments require a
percolation test A percolation test (colloquially called a perc test) is a test to determine the water absorption rate of soil (that is, its capacity for percolation) in preparation for the building of a septic drain field (leach field) or infiltration basin. Th ...
("perc" test) to establish the suitability of drain field soil to receive septic tank effluent. An
engineer Engineers, as practitioners of engineering, are professionals who Invention, invent, design, build, maintain and test machines, complex systems, structures, gadgets and materials. They aim to fulfill functional objectives and requirements while ...
,
soil scientist Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation t ...
, 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 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 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 A well is an excavation or structure created on the earth by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The ...
or surface water supply. Coarse soils –
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...
and gravel – can transmit wastewater away from the drain field before pathogens are destroyed.
Silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...
and
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...
effectively filter out pathogens but limit wastewater flow rates. 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 Anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved ox ...
septic tank effluent: *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 A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...
of gelatinous slime at the soil interface of the disposal trench. *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 Synthetic fibers or synthetic fibres (in British English; see spelling differences) are fibers made by humans through chemical synthesis, as opposed to natural fibers that are directly derived from living organisms, such as plants like cott ...
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. *Cooking fats or
petroleum Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring un ...
products emulsified by
detergent A detergent is a surfactant or a mixture of surfactants with Cleanliness, cleansing properties when in Concentration, dilute Solution (chemistry), solutions. There are a large variety of detergents. A common family is the alkylbenzene sulfonate ...
s or dissolved by
solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...
s can flow through prior to anaerobic liquefaction when septic tank volume is too small to offer adequate residence time and may congeal as a
hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...
layer on the soil interface of the disposal trench.Hammer, Mark J. "Water and Waste-water Technology" John Wiley & Sons (1975) pp.407-408 *Rising
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
levels may reduce the available
hydraulic head Hydraulic head or piezometric head is a measurement related to liquid pressure (normalized by specific weight) and the liquid elevation above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22, eq.3.2a. It is usually meas ...
(or vertical distance), causing gravitational water flow away from the disposal trench. Initially, effluent flowing downward from the disposal trench might encounter 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 applicable when the effluent leaves the drain field for gravitational force to overcome viscous frictional forces resisting flow through porous soil. Effluent levels near the drain field will rise toward the ground surface to preserve that vertical distance difference if groundwater levels surrounding the drain field approach the effluent level in the disposal trench. *Frozen ground may seasonally reduce the cross-sectional area available for flow or evaporation.


Catabolic design

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 soil
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
s capable of decomposing the anaerobic septic tank's effluent into aerobic water.
Hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...
odors or iron bacteria may be observed in nearby wells or surface waters when effluent has not been completely oxidized before reaching those areas. The biofilm on the walls of the drain field trenches will use atmospheric
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
in the trenches to catabolize organic compounds in septic tank effluent. Groundwater flow is laminar in the aquifer soils surrounding the drain field. Septic tank effluent with soluble organic compounds passing through the biofilm forms a mounded lens atop the groundwater underlying the drain field.
Molecular diffusion Molecular diffusion is the motion of atoms, molecules, or other particles of a gas or liquid at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid, size and density (or their product, ...
controls the mixing of soluble organic compounds into the groundwater and the transport of oxygen from underlying groundwater or the capillary fringe of the groundwater surface to micro-organisms capable of catabolizing dissolved organic compounds remaining in the effluent plume.Perry, Robert H., Chilton, Cecil H. & Kirkpatrick, Sidney D. "Chemical Engineers' Handbook (4th Ed.)" McGraw-Hill Book Company (1963) p.14-13


Biofilter

When a
septic tank A septic tank is an underground chamber made of concrete, fiberglass, or plastic through which domestic wastewater (sewage) flows for basic sewage treatment. Settling and anaerobic digestion processes reduce solids and organics, but the treatment ...
is used in combination with a
biofilter Biofiltration is a pollution control technique using a bioreactor containing living material to capture and biologically degrade pollutants. Common uses include processing waste water, capturing harmful chemicals or silt from surface runoff, a ...
, 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.


Operation and maintenance


Dosing schedules or resting periods

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. The
nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...
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 reduce bioclogging 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.


Inappropriate wastes

Septic tank and drain field microorganisms have very limited capability for catabolizing petroleum products and chlorinated solvents, and cannot remove dissolved
metals A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against no ...
; 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. Laundry
bleach Bleach is the generic name for any chemical product that is used industrially or domestically to remove color from (i.e. to whiten) fabric or fiber (in a process called bleaching) or to disinfect after cleaning. It often refers specifically t ...
may slow or stop microbial activity in the drain field, and sanitizing or deodorizing chemicals may have similar effects. Detergents, solvents, and drain cleaners may transport emulsified, saponified or dissolved fats into the drain field before they can be catabolized into short-chain organic acids in the septic tank scum layer.


See also

*
Blackwater (waste) Blackwater in a sanitation context denotes wastewater from toilets which likely contains pathogens that may spread by the fecal–oral route. Blackwater can contain Human feces, feces, urine, water and toilet paper from flush toilets. Blackwate ...
*
Cesspit Cesspit, cesspool and soak pit in some contexts are terms with various meanings: they are used to describe either an underground holding tank (sealed at the bottom) or a Dry well, soak pit (not sealed at the bottom). A cesspit can be used for ...
* Dry well * French drain *
Groundwater pollution Groundwater pollution (also called groundwater contamination) occurs when pollutants are released to the ground and make their way into groundwater. This type of water pollution can also occur naturally due to the presence of a minor and unwant ...
*
Leachate A leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed. Leachate is a widely used term in the environmental sciences wh ...
*
Onsite sewage facility Onsite sewage facilities (OSSF), also called septic systems, are wastewater systems designed to treat and dispose of effluent on the same property that produces the wastewater, in areas not served by public sewage infrastructure. A septic tank a ...
* Reuse of human excreta * Sewer *
Sewage treatment Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing water p ...


References


External links

* {{Wastewater Environmental engineering Environmental soil science Pollution control technologies Sanitation Sewerage Sewerage infrastructure Waste treatment technology Water pollution