Septic System Dictionary: This document provides definitions for terms used in describing basic as well as advanced or alternative septic systems and onsite wastewater disposal systems.

We provide direct links to in-depth articles which add information for many of the septic terms in this septic glossary.

DEFINITIONS - of Septic System Components & Terms: An Online Dictionary

Some definitions of basic terms associated with septic systems and onsite wastewater disposal systems are listed here. Other terms such as Cesspool, Drywell, Drainfield, are defined where we have online articles about those topics. If you don't immediately see a link to an online article on your term, use the on-site Google search box at the top or bottom of any of our website pages. If you don't find a definition of the term you seek, contact us by email.

A  -  B  -  C  -  D  -  E  -  F  -  G  -  H  -  I  -  J  -  K  -  L  -  M  -  N  -  O  -  P  -  QR  -  S  -  T  -  U  -  V  -  WXYZ

In the following list of septic system terms, numbered items (nn) are adapted, expanded, & elaborated from: New York State Appendix 75-A.1 Definitions. and from various sources [see references at page end] or are our own. Links at many definitions will provide more in-depth explanation about those items.

This article is a section of our online book SEPTIC INSPECTION & TEST GUIDE whose chapters are shown at the end of this article. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author.

Technical review by industry experts has been performed and comments from readers are welcomed. Contributors are listed at the end of each article.

A

• Definition of Absorption Area - an area to which wastewater is distributed for infiltration to the soil.
• Definition of Absorption Field - the area to which sewage is distributed for infiltration to the soil by means of a network of pipes. Synonyms and variations in implementation, trench vs. bed, for example, have given rise to a number of terms which people use loosely to refer to the soil absorption system or "SAS" including
  • drainage trenches
  • septic drainfield,
  • septic drainage field,
  • septic system drainage bed,
  • septic leachfield,
  • septic leaching field,
  • soakaway bed
  • leaching bed,
  • onsite sewage facility
  • seepage bed,
  • and similar terms.
    "An area in which perforated piping is laid in drain rock-packed trenches, or excavations (seepage beds) for the purpose of distributing the effluent from a wastewater treatment unit." [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
    See
    
    SEPTIC DRAINFIELD SIZE
    
    BIOMAT FORMATION & SEPTIC LIFE
    
    
• Definition of Absorption Trench - a long narrow area which includes a pipe for the distribution of septic tank effluent.
• Definition of Additive or septic additive, septic chemical, septic treatment, septic tank additive:
  
  A septic additive is a chemical, bacteria, or other product sold to be placed into a septic tank or septic absorption system, purportedly to improve the function of the system, improve drainfield performance, avoid septic tank pumping, or other repair or maintenance need. In a normal conventional septic system additives are not required, are illegal in some jurisdictions, and in some cases they can actually damage the system leading to costly repairs.
  See
  
  SEPTIC SYSTEM ADDITIVES & Chemicals for Septic Tank and Drainfield "Maintenance," "Un-Clogging," or "Repair" - Septic Tank Treatments.
• Definition of Activated sludge: a wastewater treatment process using special bacteria which has been "activated" or "energized" to treat the wastewater in a reactor vessel or container. Activated sludge systems process wastewater either in a continuous-receipt/mixing process or in a "plug flow" process which separates units of incoming wastewater for individual treatment.
• Definition of Aeration Stabilization Lagoon wastewater treatment systems are an activated sludge treatment method using an earthen berm to form a lagoon to hold and treat wastewater and a pump or blower system to insert oxygen into the system. [See Burks/Minnis]
• Definition of Aerobic Treatment Unit (ATU) - a system that provides for the biological decomposition of the organic portion of the wastewater by mechanical aeration of the wastewater.
  See
  
  AEROBIC SEPTIC SYSTEMS - aerobic septic systems are a common design used to increase the level of sewage treatment within the septic tank and to process & dispose of clarified treated effluent at difficult sites or where soils & space limit wastewater treatment options
• Definition of Aggregate gravel used in septic drainfield or drainage trenches - washed gravel or crushed stone 3/4 - 1 1/2 inches in diameter.
• Definition of Alternative Septic System Design - any small or private onsite waste disposal system design which uses components other than the traditional combination of a septic holding tank and a soil absorption system or drainfield.
  
  "An on-site sewage system other than a conventional gravity system or conventional pressure distribution system. Properly operated and maintained alternative systems provide equivalent or enhanced treatment performance as compared to conventional gravity systems." [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Anaerobic wastewater process: a three step bacterial respiration process that occurs in the absence of oxygen. Heterotrophic bacteria (which do not require oxygen) oxidize material to form CO₂ and water.
  
  The process is more complex than I describe here, involving also autotrophic bacteria and chemical processes in three stages: acid fermentation, acid regression, and alkaline fermentation. [See Burks/Minis]
• Definition of Anabolism: a synthesis process that results in the increase in size and complexity of organic chemical structure. One of the processes in the breakdown of wastewater by microorganisms. Also see metabolism and catabolism.
• Definition of Anoxic wastewater process: a wastewater treatment step or process in which oxygen is not available. For example, the removal of nitrogen and phosphorous are anoxic processes which uses heterotrophic bacteria (which can function at low or no oxygen levels).
  
  An anoxic process such as denitrification will not work if there is too much oxygen present; such processes are also sensitive to the Ph of the wastewater (and want to see a pH between 8 and 7). [See Burks/Minnis].
• Definition of Application Rate [of septic effluent to the soil] - the rate at which septic tank effluent is applied to a subsurface absorption area, for design purposes, expressed in gallons per day per square foot (gpd/sq. ft.).
• Definition of Attached growth wastewater treatment systems pass septic effluent through or over a media onto which bacteria attach and grow, serving to treat the wastewater.
  
  Synthetic textiles and foam cubes used in packaged onsite wastewater treatment systems are examples of attached growth systems. Sand filters are a much older version of attached growth systems which will work only if sand of the proper size and type is used.

B

• Definition of Baffle, septic tank - a flow deflecting device used in septic tanks and distribution boxes to inhibit the discharge of floating solids, reduce the amount of settleable solids that exit, and reduce the exit velocity of the wastewater.
• Definition of Biogas: BIOGAS PRODUCTION & USE describes the production and use of methane produced by sewage or manure - bio-gas or bio gas.
  
  Biogas is generally a mixture of methane and carbon dioxide produced by bacterial degradation of organic matter (sewage, animal waste, or organic debris & waste) and intended for use as a fuel.
• Definition of Biomat: a bacterial slime layer in the soil below the leachfield and around other wastewater disposal systems. Critical septic effluent treatment occurs in the biomat.
  See
  
  BIOMAT FORMATION & SEPTIC LIFE - what leads to drain field clogging and expensive drainfield repairs
• Definition of Blackwater or septage - waste carried off by toilet, urinal, and (in FL, kitchen drains). The material pumped out of a septic tank is also called "septage".
• Definition of BOD - Biochemical Oxygen Demand - the amount of oxygen necessary to permit microbes (within a septic treatment system) to consume organic material in wastewater.
  
  BOD is normally expressed as BOD-5 or five-day BOD, the amount of oxygen consumed by microbes (for example within a septic treatment systems) over a five day period, for a given volume of wastewater.
  
  BOD is used to describe the quality of untreated wastewater, or in other words, to determine the amount of treatment that wastewater will require before it can be discharged to the environment.
• Definition of Breakout or effluent breakout: Visible movement of septic effluent to the surface of a property. Septic effluent appearing on the surface of a property or in nearby ponds or streams, is incontrovertible evidence of a failure in the septic system.
  
  Breakouts of septic effluent may occur during normal system usage when the absorption bed has failed, when the system has been overloaded, or during a septic loading and dye test.
  See
  
  SPOTTING SEPTIC BREAKOUTS - Spotting Dyed Septic Effluent Breakout Outside - Where Septic Dye is Likely to Show Up During a Septic Dye Test.
• Definition of Building Sewer drain or sewer line or sewer pipe or "septic pipe" - that part of the drainage system which extends from the end of the building drain and conveys wastewater to the sewage system or sewer.

C

• Definition of Capacity of a Septic System: describes the volume of wastewater (blackwater or greywater) which an onsite septic system must be capable of handling. Typically capacity, described as daily volume of wastewater in gallons or liters, is a function of the number of building occupants using the facility, adjusted for other building activities such as laundry, garbage grinders, or other site activities.
  See
  
  SEPTIC SYSTEM DESIGN BASICS - Septic System Design:Choosing Septic Tank Size, Absorption System Size, Basic Design Notes
  
  SEPTIC TANK SIZE: Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow & How to Calculate the Size (in gallons) of a Septic Tank
  
  WATER QUANTITY USAGE TABLES or Daily Usage Volume Requirements
• Definition of Catabolism: the oxidative, exothermic, enzymatic degradation process resulting in release of energy from large organic molecules. One of the processes in the breakdown of wastewater by microorganisms.
• Definition of Centralized Septic System: an onsite wastewater disposal system which collects waste from multiple buildings or facilities for treatment and disposal at a single site or facility.
  
  Centralized septic systems may serve an entire community or a large group of homes such as townhouses or condominiums. Centralized wastewater and septage disposal systems are generally associated with large treatment requirements such as for an entire community.
• Definition of Cesspool: A cesspool combines the septic treatment tank and absorption system into a single component. A cesspool is a stone or concrete block or (safer) pre-cast concrete (photo) lined pit into which sewage is discharged.
  
  Solids remain in the pit, effluent is absorbed into soil below and at the sides of the cesspool. Solids settle to the bottom, floating grease and scum collect at the top, and liquid seeps into the ground, initially through the bottom and most of the time through the side of the cesspool.
  See
  
  CESSPOOLS - What are Cesspools? What Cesspool Maintenance is Needed? How do Cesspools Fail?
• Definition of Chemical Toilets: use a chemically treated reservoir located directly below the toilet seat. The chemicals reduce odors and perform partial (incomplete) disinfection of the waste.
  See
  
  CHEMICAL TOILETS - Are They A Septic System Design Alternative for Difficult Sites?
• Definition of Cleanout, septic tank or holding tank or cesspool or drywell - an opening providing access to part of the sewage system.
• Definition of Clearances or Distances Required for Septic Components: because onsite wastewater treatment systems may discharge septic effluent into local soils or even nearby surface water or ground water, separation distance is required between various septic system components (septic tank, drainfield, piping) and other common property features (buildings, wells, property lines, nearby streams or ponds).
  See
  
  CLEARANCE DISTANCES, SEPTIC SYSTEM: Distances Between Septic System & Wells, Streams, Trees, etc.
• Definition of Cluster Septic System: a type of centralized septic system serving as few as two homes, or just a few homes.
  
  Clustered septic systems may be used in a development of new homes in which small groups of two or three homes are served by individual wastewater treatment systems.
  
  See details at SEWAGE TREATMENT PLANTS, RESIDENTIAL
• Definition of Commissioner - the State or Provincial or other municipality Commissioner of Health or equivalent function
• Definition of Composting Toilets: use natural materials inside of a holding tank to decompose sewage where the decomposed material is retained for later removal.
  See
  
  COMPOSTING TOILETS - Are They A Septic System Design Alternative for Difficult Sites?
• Definition of Common Pump Stop: on a duplexed septic effluent (or other duplexed) pumping system, one float at the lowest (pump down systems) or highest (pump up systems) effluent level will shut off both effluent or septic pumps.
• Definition of Conventional septic system: a traditional onsite wastewater disposal system which uses a (water-tight) septic tank and leach field or drainfield buried in the original site soils.
  
  For contrasting but similar systems see mound systems and raised bed septic systems and for septic systems of advanced design see alternative septic systems or advances septic system products
• Definition of Curtain Drain - a subsurface drain designed and constructed to control groundwater and surface water intrusion into the area of the sewage system. Curtain drains or intercept drains can protect septic drainfields in areas of wet soils or surface and subsurface groundwater.

D

• Definition of Decentralized wastewater treatment: wastewater treatment and disposal systems handling a small volume of effluent, but serving multiple buildings, usually ones which are built close together, such as a small apartment complex.
• Definition of Denitrification: the removal of nitrogen from wastewater, normally by an anoxic process.
• Definition of Design Professional, Septic System or Onsite Wastewater- a person licensed or registered in the State or Province or other authority to design onsite wastewater handling & disposal systems described in the standards for that municipality.
• Definition of Discharge Effluent Pipe: this pipe conducts septic effluent ou of a pumping chamber (under pump power) or septic tank (by gravity).
• Definition of Disinfection Septic Systems for onsite wastewater treatment: some onsite wastewater treatment and disposal systems are required to disinfect the effluent before it can be discharged to the environment.
  
  Sand bed systems, filter systems, and aerobic systems may require disinfection depending on the level of treatment achieved by other wastewater handling components.
  See
  
  AEROBIC Septic Systems: Calcium Hypochlorite for Disinfections In Onsite Aerobic Waste Treatment Septic Systems
• Definition of Distributing valve: A valve that distributes flow to multiple drainfield laterals, zones or locations by automatically rotating upon each pump cycle. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Distribution Device - a device used to uniformly distribute sewage to the absorption area.
  
  Also referred to as "distribution box" or "D-box", this component connects a single effluent line leaving the septic tank or other wastewater treatment component to the network of effluent distribution lines in an absorption area.
  See
  
  Inspecting the D-Box - Inspecting the Septic System Distribution Box or "D-box"
• Definition of Distribution Line - the perforated pipe used to distribute wastewater to the absorption area.
• Definition of Dosing System a septic effuent disposal system using mechanical means (bell siphon, tipping bucket, float valves) or electrically controlled means (electric pumps and pressure distribution systems) to first accumulate septic effluent in a dosing chamber or "distribution chamber" and then, when a specified volume of effluent has been collected, move that "dose" of effluent to the soil absorption system (sand filter bed, drain field, or other system) for final treatment, filtering, and dispersal.
  
  Septic effluent dosing systems, by buffering septic effluent and dispensing it only at when a specified volume has been collected, permit the drain field to "rest" between applications and can permit use of alternating drainfields. Dosing systems may extend the life of a drain field.
  
  Dosing systems may disperse effluent to the absorption system on specific time intervals but more commonly it is dispersed when a specific volume of effluent has been accumulated.
  More reading about dosing systems: see
  
  Gravity / Siphon Dosing Systems
  
  PRESSURE DOSING SYSTEMS
• Definition of Dosing Tank [septic effluent dosing tank]: A tank which collects treated wastewater for period of time and then, periodically, discharges it into another treatment unit or disposal unit, depending upon the needs and design of the particular on-site sewage system. See "Dosing System".
• Definition of Drainfield or drain field or drainage bed or seepage bed or leachfield: see Absorption Area and Leach field. "An area in which perforated piping is laid in drain rock-packed trenches, or excavations (seepage beds) for the purpose of distributing the effluent from a wastewater treatment unit." [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Drinking Water - water whose physical, chemical and biological quality is or is intended to be satisfactory for human consumption, food preparation or culinary purposes.
  
  See WATER, WELLS, WATER TANKS: TESTING GUIDE
• Definition of Drywell: A Drywell: is a pit or hole in the ground, open to soil at its sides and bottoms, intended to receive and dispose of gray water (water from building non-sewage drains such as laundry, showers, sinks).
  
  A drywell, or "seepage pit" is used at some building sites to receive "gray water" from a laundry, sink, or shower. The pit may be site-built of stone or dry-laid concrete block, rubble-filled, or constructed of (safer) pre-cast concrete.
  See
  
  Drywells for onsite wastewater disposal or onsite gray water disposal - What are Drywells, What Drywell Maintenance is Needed? How do Drywells Fail?
• Definition of Duplex Effluent Pumps: For septic system reliability, in a mound or other advanced septic system which requires the use of a pump, two pumps are employed using one of two methods.
  
  (1) the pumps alternate or "take turns" in pumping out effluent under control of a float switch.
  
  This method has the advantage of constantly testing that both pumps work and of distributing the work and thus the wear-out cycle across the equipment.
  
  
  (2) the pumps come on at different levels of effluent in a pumping chamber. This method holds one pump in reserve, causing it to operate only if the effluent level rises above a certain point.
  
  In this design, if the inflow into the system exceeds the capacity of the first pump or if the first pump fails, the second pump goes to work. In good design, if one of the two pumps stops working, an alarm is sounded on the system.
• Definition of Dye Test or "Septic Loading and Dye Test": Septic dye tests involve flushing a special florescent dye down a toilet or other drain. The dye itself does not make anything happen. It is simply a colored indicator that can identify water found outside as having come from the fixture where the dye was introduced.
  
  It's the volume of water introduced into the system that forms the actual "test". If waste water is coming to the surface (an unsanitary condition indicating serious septic failure) one may see dye in that water, provided the septic system is flowing at common rates.
  See
  
  How to Perform a Septic Dye Test - How to Properly Conduct a Septic Loading and Dye Test.

E

• Definition of Effective Grain Size of soils for septic system absorption bed design purposes - a measure of the diameter of soil particles, when compared to a theoretical material having an equal transmission constant.
  
  It is the dimensions of that mesh screen which will permit 10 percent of the sample to pass and will retain 90 percent.
• Definition of Effluent: septic effluent is the clarified, partially treated liquid which leaves a septic tank. Large solids have been separated by settlement, by floating to coagulate in a grease and scum layer, or by filtration or other methods
  
  Septic effluent moves out of a septic treatment tank into an absorption system (or other effluent treatment system) for further treatment and ultimate disposal or discharge to the environment.
• Definition of Evaporation Transpiration Septic Systems: Evaporation-Transpiration (ET) Septic Systems, and Evapo-Transpiration Absorption Septic Systems (ETA) dispose of septic effluent from the septic tank by providing a surface area intended to allow the effluent to evaporate.
  
  ET systems depend entirely on evaporation while ETA systems make use of both evaporation and (limited) soil absorption of septic effluent.
  See
  
  Evaporation-Transpiration & Evapo-Transpiration Absorption - Septic System Design Alternatives for Difficult Sites
• Definition of Expanded wastewater treatment bed is a wastewater treatment system which uses both attached growth and suspended growth treatment processes. [Burks/Minnis]

F

• Definition of Failure of a Septic System: A condition of an on-site sewage system that threatens the public health by inadequately treating sewage or creating a potential for direct or indirect contact between sewage and the public.
  
  Examples of failure include:
  
  (a) Sewage on the surface of the ground;
  
  (b) Sewage backing up into a structure caused by slow absorption of septic tank effluent;
  
  (c) Sewage leaking from a septic tank, pump chamber, holding tank, or collection system;
  
  (d) Cesspool or seepage pits where evidence of ground water or surface water quality degradation exists;
  
  (e) Inadequately treated effluent contaminating ground water or surface water.
  
  (f) Noncompliance with standards stipulated on the permit. [Above Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
  See
  SEPTIC FAILURE CRITERIA
  and
  CESSPOOL FAILURE CRITERIA
  
  Cesspool failures (MA)
  
  Failure Causes - Septic Systems for Septic Systems, septic tanks, septic drainfields, cesspools, drywells, distribution piping
  
  Failure Causes - Drainfields: how to inspect septic leach fields & what causes septic field failure
  
  SEPTIC FAILURE SPOTS - Where septic system trouble is likely to show up regardless of septic dye testing
  
  Seepage Pit Collapse or Failure - stages in the life of a drywell, seepage pit, cesspool, and what to do if the pit is damaged
  
  Septic Tank Condition - How to Inspect Septic Tanks and evaluate the septic tank condition, baffles, sludge levels, damage, evidence of septic failure, etc.
  
  Leach Fields: how to inspect and diagnose septic drainfield failures
  
  Soil Absorption System Failures (leach fields, drain fields, seepage pits)
• Definition of Final Treatment/Disposal Unit: That portion of an on-site sewage system designed to provide final treatment and disposal of the effluent from a wastewater treatment unit, including, but not limited to, absorption fields (drainfields), sand mounds and sand-lined trenches. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of FOG - Fats, Oil, Grease a component of sewage which forms in the floating scum layer in septic tanks.
• Definition of Force Main septic or sewer line: a (comparatively) smaller diameter sewage waste line used to move solid waste output from a grinder pump to a waste and wastewater treatment facility.
  
  Grinder pumps are used with "force main" septic systems to move waste products uphill to a private onsite wastewater treatment facility or in larger installations, to move sewage or "blackwater" or waste products to a centralized treatment facility.
  
  Force mains used to carry sewage prepared by a grinder pump will generally be of smaller diameter than waste lines which work by gravity.
  
  See details
  
  at SEWAGE PUMPING STATIONS
  and
  
  at SEWAGE EJECTOR / GRINDER PUMPS - home

G

• Definition of Gas Baffle in a septic tank - a device on the outlet of a septic tank which deflects gas bubbles away from the outlet and reduces the carry over of solid particles from the septic tank.
• Definition of Greenhouse wastewater treatment systems: are structures which enclose a wetland (see below) in order to provide a more controlled (temperature and moisture) environment.
  
  Greenhouses can treat septic effluent to level 3 or better (Jantrania/Gross wastewater system type XI).
• Definition of Greywater or graywater or gray water - domestic wastewater which excludes sewage or "blackwater" from toilets, including bath, lavatory, laundry and sink, excepting kitchen sink (because of food solids) (C)Trap Daniel Friedman
• Definition of Graywater systems or gray water septic systems refer to systems which reduce the liquid effluent load on a septic system by separating greywater (or graywater) from sinks and showers from blackwater (black water) from toilets.
  
  Greywater is wastewater which does not contain sewage, typically coming from building sinks, showers, and laundry facilities.
  See
  
  Greywater or Gray water systems as Components of Alternative Septic Systems for Difficult Sites
• Definition of Groundwater - subsurface water occupying the saturation zone from which wells and springs are fed.
• Definition of Graywater
• Definition of Grain Size, Effective [septic drainfield soil evaluation] - a measure of the diameter of soil particles, when compared to a theoretical material having an equal transmission constant.
  
  It is the dimensions of that mesh screen which will permit 10 percent of the sample to pass and will retain 90 percent.
• Definition of Grinder pumps: a macerating pump capable of grinding up sewage, including the solid waste, so that the waste product can be pumped at pressure to a treatment system.
  
  Grinder pumps are used with "force main" septic systems to move waste products uphill to a private onsite wastewater treatment facility or in larger installations, to move sewage or "blackwater" or waste products to a centralized treatment facility.
  
  Force mains used to carry sewage prepared by a grinder pump will generally be of smaller diameter than waste lines which work by gravity.
  
  See details at SEWAGE EJECTOR / GRINDER PUMPS - home

H

• Definition of Heavy Equipment [related to septic field damage] - all equipment which would result in the compaction of the design absorption area at a depth equivalent to the design depth of the distribution lines.
• Definition of Holding tank septic systems: Septic Holding Tank Systems use a sealed tank to hold household waste and wastewater until the tank can be pumped out by a septic pumping company. Holding tanks have no drains and must be pumped.
  See
  
  Using Septic Holding Tanks as Components of Alternative Septic Systems for Difficult Sites

I

• Definition of Incinerator toilet systems: incinerator toilets use electricity or gas to burn the waste placed into these systems.
  See
  
  Using Incinerating Toilets
• Definition of (20) Infiltration - the flow or movement of water into the interstices or pores of a soil through the soil interface.
• Definition of Infiltrative Surface: In drainfields, the drain rock-original soil interface at the bottom of the trench; in mound systems, the gravel-mound sand and the sand-original soil interfaces; in sand-lined trenches/beds (sand filter), the gravel-sand interface and the sand-original soil interface at the bottom of the trench or bed. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Influent: Wastewater, partially or completely treated, or in its natural state (raw wastewater), flowing into a reservoir, tank, treatment unit, or disposal unit. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Infiltrative Surface: In drainfields, the drain rock-original soil interface at the bottom of the trench; in mound systems, the gravel-mound sand and the sand-original soil interfaces; in sand-lined trenches/beds (sand filter), the gravel-sand interface and the sand-original soil interface at the bottom of the trench or bed. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of (21) Invert - the floor, bottom, or lowest point of the inside cross section of a pipe.

JKL

• Definition of Leach field or leaching bed or soil absorption system or drainfield: a conventional septic effluent treatment and absorption system which typically consists of a network of perforated pipes buried in gravel-filled trenches. E
  
  ffluent enters the pipes, seeps out of perforations into the soil, where a bacteria layer (see "biomat") performs further effluent treatment before the liquid is then discharged into the soil.
  See
  
  How Big Should the Leach Field Be? - How Big Should the Septic Leach Field Be?
  
  Septic System Drainfield Absorption System Biomat.
• Definition of Lift Pump: a lift pump is used to move liquid effluent from a lower pumping chamber or effluent tank to a higher level tank or possibly out of an effluent tank up to a mound system, sand bed, or other elevated effluent treatment system.
• Definition of Local Health Department - a city, county, or part-county department of health or a State Department of Health District Office.

M

• Definition of Media filter septic systems: Media filter septic systems use a conventional septic tank followed by any of several methods to further filter and treat septic effluent before it is discharged to the soil, soil surface, or waterway.
  See
  
  Using Septic Media Filters.
• Definition of Metabolism: The sum of all of the biochemical processes employed in the breakdown of organic compounds (catabolism) and in the building up of cell protoplasm (anabolism). Metabolic processes convert chemically-bound energy into energy forms that can be used to support life.
  
  One of the processes in the breakdown of wastewater by microorganisms. Also see anabolism and catabolism.
• Definition of Mound or Septic Mound System:
  
  An effluent treatment disposal system composed of fill and a network of perforated pipes in which effluent treatment occurs within the filled bed.
  
  Also called the Wisconsin Mound System. Mounds are constructed of a variety of fill though Burks/Minnis describe mound systems as a single-pass sand filter.
  See
  
  Using Septic Mounds as Components of Alternative Septic Systems for Difficult Sites
  
  Using a Raised Bed Effluent Disposal System.

N

• Definition of Net free area or effective septic tank working volume The "net free area" (DJF's term) or "effective septic tank volume" is the actual tank interior volume minus the space occupied by settled sludge and floating scum.
  
  Net free area or effective septic tank working volume is discussed when considering effluent retention time since a small net free area reduces the effluent retention time in the septic tank.
  See
  Net Free Area and effective septic tank working volume are discussed at "retention time" in our septic tank pumping guide.
• Definition of Non-potable well - water used for irrigation, etc, but not for human consumption
• Definition of Obstructed land - areas on property used for such purposes as pools, concrete slabs, buildings, driveway, parking and similar areas which would prohibit, hinder, or affect the installation, operation, or maintenance of onsite sewage disposal system

O

• Definition of On-site Sewage System (OSS): [or Onsite Wastewater Treatment System or Septic System] An integrated arrangement of components for a residence, building, industrial establishment or other places not connected to a public sewer system which:
  
  (a) Convey, store, treat, and/or provide subsurface soil treatment and disposal on the property where it originates, upon adjacent or nearby property; and
  
  (b) Includes piping, treatment devices, other accessories, and soil underlying the disposal component of the initial and reserve areas. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of OTL - Onsite [Wastewater] Treatment Level: the level of treatment of wastewater by an onsite facility before the wastewater is discharged to the environment. Jantrania/Gross describe five levels with higher numbers indicating a higher level of treatment (making the water less polluted).
  
  A typical conventional septic tank and drainfield treat effluent to Jantrania/Gross's OTL-1, meaning that less than 45% of contaminants have been removed from the wastewater by the septic tank.
• Definition of Overall Treatment Level of wastewater (OTL) describes the degree of sanitization of wastewater that occurs as wastewater passes through a treatment system. It is a scale of the level of water pollution which ranges from an OTL of 0% (or 10 on a pollution scale) (untreated raw sewage effluent) to an OTL of 100% (or 0 on a pollution scale).
  
  An OTL of 100% means that the output of the treatment system has produced water of the same quality as drinking water. While drinking water standards vary among various states, provinces, and countries, drinking water standards specify the level of allowable bacteria (such as less than one CFU per 100 ml of water) as well as the allowable levels of nitrites, nitrates, and a long list of common chemical contaminants.
  
  The U.S. EPA National Primary Drinking Water Standards list 87 contaminants that must be tested in approving water for human consumption.
  
  All of these need to be addressed by the wastewater treatment system. The typical OTL of an onsite wastewater treatment system is required to discharge effluent which is at least as clean as normally-occurring groundwater. Some treatment systems produce a lower OTL and require disinfection. Other wastewater treatment systems produce water which is cleaner than local groundwater.

P

• Definition of Percolation / soil percolation rate or "perc" for septic drainfield testing - the movement of water through the pores of a soil or other porous medium following infiltration through the soil interface.
  See
  
  Soil Percolation Requirements and Soil Depth Requirements for Septic Absorption Systems / Septic System Drainfields
• Definition of Perc Test / Soil Percolation Test:
  
  A hole, 5-7 feet deep is dug in an area to be tested for future use as a drain field, or near the drain-field area in representative soils. Water is poured into the hole and and the soils or septic engineer or contractor observes the rate at which soil absorbs the water by noting the time that it takes for the level of water in the hole to drop one inch (for example).
  
  More precise "perc tests" may involve using a specific quantity of water or a hold of specific dimensions to make these observations.
  
  Usually two holes are dug, 50' to 100' apart in order to evaluate the proposed septic leachfield area. Evidence of the seasonal high water table is noted (possibly based on changes in soil color at various depths). For safety, perc test holes must be re-filled after the test is complete.
  
  If the hole must be left open and unattended during the test it should be barricaded to prevent anyone from falling in. Here are some hints from Callum County, Washington:
  • Dig the perc test hole downhill from the house site if possible.
  • Stay away from swales and drainage ways, and areas that are seasonally wet.
  • Keep 100 feet away from all wells and surface water, including irrigation ditches.
  • Septic systems cannot be located on slopes in excess of 45% (24 degrees).
  • It is helpful if property boundaries and the proposed house site are marked ahead of time.
  "Soil percolation or perc tests are used to determine the ability of a soil to transmit wastewater effluent through the soil profile. The soil percolation rate is the amount of time water takes to move through soil, measured in minutes per inch. Finer textured soils have slower percolation rates; it takes longer for water to drain from a test hole.
  
  These soil types need larger drainfields than soils with faster percolation rates, such as sandy soils, to handle a given amount of wastewater. Soils with very slow percolation rates may not be suitable for drainfields. In Nebraska, if soils perc at a rate slower than 60 minutes per inch, consider installing a lagoon system if the lot is at least 3 acres.
  
  Otherwise, an engineer must design a specialized system. Soils with very fast percolation rates, less than 5 minutes per inch, must be modified by adding a loamy sand liner to the drainfield, so that proper treatment can occur." [Univ. of Nebraska, Lincoln Extension, Institute of Agricultural & Natural Resources, "Residential Onsite Wastewater Treatment: Site Evaluation]
• Definition of (24) Permeability - a measure of the rate of movement of liquid through soil.
• Definition of Pollution (of water or wastewater): contamination of water (or air or other substances) with unwanted and potentially harmful substances, making the water (or air or other) unsuitable for human consumption, harmful to animals, and (depending on whose definition) unsuitable for agricultural, industrial, or recreational use.
  
  [Paraphrased from Jantrania/Gross p.35]. Where conventional septic tank and drainfield onsite wastewater treatment is in use the chief pollutants released by the system are bacteria (fecal coliform) and nitrates.
  
  Under current practices, the level of sewage treatment which a given wastewater treatment system provides to its effluent prior to discharge is generally described as primary, secondary, or tertiary, with the last term representing the most thorough treatment. J
  
  antrania/Gross have proposed a more finely graduated 0-10 scale to describe the level of pollution of effluent, corresponding to the level of treatment ranging from 10=no treatment, or raw sewage, to 0=equivalent to drinking water or 100% treatment of sewage.
• Definition of Ponds for wastewater treatment: a large basin which holds and treats wastewater by bacteria and/or algae which form in the treatment pond.
  
  Treatment ponds are usually site-constructed and may use a pond bottom liner in addition to earth berms to form the treatment container.
• Definition of Potable well - water used for drinking
  See
  
  Drinking Water Supply, Contamination Levels, Water Testing Procedures
  
  Water Supply & Drain Piping, Wells, Pumps, Water Supply Equipment, other Building Pipes and Plumbing
• Definition of Pressure dosing septic systems:
  
  Pressure-dosed Drainfield Septic Systems use a separate pumping chamber and pump, located downstream from the septic tank, to move effluent into a drainfield which in turn distributes effluent through a pressure-fed network of distribution pipes.
  See
  
  Using Pressure Dosing as a Component of Alternative Septic Systems for Difficult Site
• Definition of Pressure sewer: see Force Main
• Definition of Septic Effluent Pumping Chamber:
  
  A tank or compartment following the septic tank or other pretreatment process, which contains a pump, floats, and volume for storage for effluent. If a siphon is used, in lieu of a pump, this is called a "siphon chamber". [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]

QR

• Definition of Raised Bed Septic Systems: a wastewater absorption trench system which has been constructed in soil-fill material which has been placed on top of the natural soil on a building lot. Raised septic bed systems make at least partial use of existing soils for wastewater treatment.
  See
  
  Using a Raised Bed Effluent Disposal System.
  
  Using Septic Mounds.
• Definition of Receiving Environment (RE): the environment, generally soils or nearby waterways, which receive effluent which has been treated by an onsite wastewater treatment system.
  
  A general objective of wastewater treatment systems is to discharge into the receiving environment water which is as clean or cleaner of pollutants than the naturally occurring groundwater in the same locale.
• Definition of Retention time in septic tanks: the length of time (hours or days) that septic effluent remains in the septic tank before moving out to the treatment or absorption system.
  See
  
  Effluent Retention Time - Septic Effluent Retention time and Effective Septic Tank Volume - Why pump a septic tank before it is "full" of solids and grease?
• Definition of Rotating biological contractor wastewater treatment systems use a series of plastic disks which rotate dipped in wastewater, thus exposing wastewater on the upper portion of the disk to air. Bacteria form a biomat on the disks and treat the effluent.
• Definition of RUCK wastewater treatment system: separate sewage (blackwater) from other wastewater (greywater from kitchens, laundry, showers) and treat each type of effluent separately.
  
  RUCK systems are used to remove additional nitrogen and phosphorous, up to 90%, from wastewater.

S

• Definition of Sand bed filter septic systems: In a sand filter septic system, the septic tank or aerobic unit effluent is intermittently spread across the surface of a bed of sand through a network of distribution lines.
  
  A biomat forms in the upper 9-18" of sand to perform the actual treatment function.
  
  Collector pipes beneath the filter collect treated effluent after it has passed through the sand. Sand filter beds to treat wastewater have been in use for more than 100 years. Periodic maintenance (raking the biomat so that it does not clog the system) is required.
  See
  
  Using a Sand Bed Effluent Disposal System
• Definition of Saturated vs. Non-Saturated Wastewater Treatment Systems: A wastewater treatment system such as an aerobic treatment unit (ATU), because it involves a tank filled with wastewater and forced oxygenation of that wastewater, is a type of saturated wastewater treatment system.
  
  Other non-saturated wastewater treatment systems such as trickling filter beds use passively-infused air to support their oxygen-supported microorganisms. Unlike ATUs, non-saturated systems allow passive air contact with effluent as it moves through the media. Air is not being pumped. Both types of systems make use of aerobic microorganisms.
• Definition of Scum [in the septic tank] or floating scum layer: the wastewater material which is less dense than water and floats on top of the water.
  See
  Residential Septic Tank Sludge and Scum - what do they contain?
  MEASURE SEPTIC TANK SCUM & SLUDGE - How and Why to Measure Septic Tank Scum and Sludge in the Septic Tank
• Definition of Secondary Effluent Treatment: the level of septic effluent treatment provided by a centralized wastewater treatment plant, (according to Jantrania et als,) obtains about 85% reduction BOD and TSS or 30 mg/l.
• Definition of Septage - waste carried off by toilet, urinal, (and in FL, kitchen drains). The material pumped out of a septic tank is also called "septage".
• Definition of Septic System: A set of components to receive, treat, and dispose of blackwater or sewage at a residential or other property, typically including a tank to receive and hold solid waste and a treatment system to sanitize and dispose of clarified septic effluent or wastewater, such as a septic leach field or drainfield, or an advanced or alternative wastewater treatment system such as a septic mound, raised bed septic system, or an aerobic septic system.
  See
  
  What is a Septic System
  
  Home Buyer's Detailed Guide to Septic Systems - Buying a Home With a Septic Tank
  
  Septic Systems - An Engineer's View.
  
  Alternative Septic Designs - Onsite Wastewater Treatment Methods for Difficult Sites
• Definition of Septic Tank:
  
  A septic tank is a closed container which receives blackwater or sewage from a building, normally as the first step in onsite wastewater treatment.
  
  The purpose of the treatment tank or "septic tank" is to contain solid waste and to permit the beginning of bacterial action to process sewage into a combination of clarified effluent, settled sludge, or floating scum in the tank.
  
  An intact, un-damaged septic tank is normally always filled with these materials.
  See:
  
  SEPTIC TANK INSPECTION PROCEDURE - How to Inspect the Condition of A Septic Tank.
  
  Septic Tank Pumping Guide: When, Why, How to Pump A Septic Tank.
  
  How to Find The Septic Tank - tips on locating septic tanks for inspection and septic tank pumping.
  
  Septic System Design Basics - Choosing Septic Tank Size, Absorption System Size, Basic Septic Design Notes.
  
  
• Definition of Septic Tank: A watertight pretreatment receptacle receiving the discharge of sewage from a building sewer or sewers, designed and constructed to permit separation of settleable and floating solids from the liquid, detention and anaerobic digestion of the organic matter, prior to discharge of the liquid." [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Sequencing batch reactor: wastewater treatment systems use activated sludge to treat wastewater in a single tank for all of the treatment functions and steps. [See Burks/Minis]
• Definition of Sewage - the combination of human and household waste with water which is discharged to the home plumbing system including the waste from a flush toilet, bath, sink, lavatory, dishwashing or laundry machine, or the water-carried waste from any other fixture, equipment or machine.
  See:
  
  Sewage & Septic Backups and what to do about sewage spillage in buildings
  
  Sewage Contaminants which may be released from sewage spills from residential septic systems and wastewater disposal systems
  
  Septic Tank Sludge and Floating Scum: what makes up the contents of residential sewage
  "Sewage: Any urine, feces, and the water carrying human wastes, including kitchen, bath, and laundry wastes from residences, building, industrial establishments or other places.
  
  For the purposes of typical BOH guidelines, "sewage" is generally synonymous with domestic wastewater. Also see "residential sewage." [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Sludge, septic: settled solid waste at the bottom of a septic tank. Sludge is semi-solid organic waste.
  For more details see
  
  Residential Septic Tank Sludge and Scum - what do they contain? and
  
  Measure Scum & Sludge - How and Why to Measure Septic Tank Scum and Sludge in the Septic Tank
• Definition of Soil Absorption System or Subsurface Soil Absorption System - "SSAS": A system of trenches three feet or less in width, or beds between three feet and ten feet in width, containing distribution pipe within a layer of clean gravel designed and installed in original, undisturbed soil for the purpose of receiving effluent and transmitting it into the soil. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Soil Auger: "... Like a probe, an auger provides a column of soil for viewing when extracted. Auger diameters are typically larger than probes, and extensions can be added to access deeper into the soil.
  
  An auger produces larger samples and is more effective in rocky areas than a probe, although it still may be difficult to use due to rocks.
  
  Some disadvantages are that the auger is slower and more labor intensive, and the disturbed nature of the sample may not reveal faint mottles, cemented layers, or structure." [Univ. of Nebraska, Lincoln Extension, Institute of Agricultural & Natural Resources, "Residential Onsite Wastewater Treatment: Site Evaluation]
• Definition of Soil Pit: "... a dug-out area near the perimeter of an expected drainfield area.
  
  Do not dig a pit within the proposed drainfield area, as soil can settle after the system has been installed, disrupting the distribution lines. Sunshine should reach the sidewall during the observation period so subtle differences in soil color will be most visible.
  
  Soil pits provide the best method for viewing both undisturbed soil and how soil varies over the depth of the pit. Pits may be the only reliable method to determine depth to bedrock.
  
  However, digging a pit requires considerable manual labor or a backhoe, which can be costly. Safety is another consideration. Pits are especially hazardous in sandy soil or if excavated below the current water table depth. When digging, construct walls with a step-type configuration for safe entry and exit.
  
  Take appropriate safety precautions when working around the hole to prevent a cave-in, especially if sidewall soil is unstable or sandy. Heavy equipment or large objects such as boulders should not be allowed to rest on the surface immediately next to the pit sidewalls.
  
  Pits should be fenced to avoid falls or unauthorized entry and backfilled as quickly as possible after use." [Univ. of Nebraska, Lincoln Extension, Institute of Agricultural & Natural Resources, "Residential Onsite Wastewater Treatment: Site Evaluation]
• Definition of Soil Probe: "... a hollow tube that, when pushed into the soil and extracted, gives an undisturbed column of soil for viewing.
  
  Probes vary in length and diameter. Usually extensions can be added to probe deeper into the soil. It is the quickest method of looking at soil, and also allows you to detect faint soil mottling or cemented layers.
  
  Disadvantages are the relatively small diameter of the sample and inability to penetrate soil in rocky areas or under very dry conditions." [Univ. of Nebraska, Lincoln Extension, Institute of Agricultural & Natural Resources, "Residential Onsite Wastewater Treatment: Site Evaluation]
• Definition of Suspended Growth vs. Attached Growth Aerobic Treatment Systems: Oxygen-supported (aerobic) bacteria in the mixed liquor perform the primary treatment in the system.
  
  As the bacteria themselves die off they remain suspended in the mixed liquor - a "suspended growth aerobic treatment system". Alternatively, a media, such as synthetic fabrics, may be suspended in the treatment tank, permitting the bacteria to attach to the media surfaces - an "attached growth aerobic treatment system".
• Definition of Stabilized Rate of Percolation - the rate corresponding to two consecutive equal or near equal percolation test results.
• Definition of Suspended growth wastewater treatment systems use activated sludge wastewater treatment to process waste: completely-mixed activated sludge (continuously receiving input), plug flow activated sludge systems (wastewater is treated in individual batches), sequencing batch reactors, aerated stabilization lagoons, attached growth systems, trickling filters, rotating biological contractors, and sand filters.

T

• Definition of Treatment Component: A type of on-site sewage system component that modify and/or treat sewage or effluent prior to the effluent being transmitted to another treatment component or a disposal component.
  
  Treatment occurs by a variety of physical, chemical, and/or biological means.
  
  Constituents of sewage or effluent may be removed or reduced in concentrations. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of [Septic] Tilt Switch: a mercury switch used to turn a septic pump on and off.
  
  Typically the septic tilt switch is located in a steel container or "can" (to prevent interference from solid waste) or may be encased in an epoxy block (to protect the switch and make it waterproof] and is used to control a septic effluent pump.
  
  Mercury tilt switches work by gravity (and changes in the liquid level in a tank) to turn a pump on or off. Where the mercury tilt switch is affixed to a levered float which moves up or down (tilts) in response to changes in the liquid level in a tank, it may also be called a mercury float switch.
  
  A drop of mercury enclosed in a metal tube (the mercury reservoir) moves to close or open an electrical circuit by either touching two electrical contacts (wire ends inside the tube) or, as the tube slopes to put the wire ends in the "up" position, the droplet of mercury rolls away from the electrical wire ends and thus opens or turns "off" the circuit.
  
  In onsite wastewater pump systems this is a special application of a widely-used switch which simply turns a circuit on or off depending on which way the mercury tube is tilted. (For example the same switch was used for decades in Honeywell (R) heating thermostats.
• Definition of Trickling filter wastewater treatment systems employ a porous material on which microorganisms become attached to form a biomat (or "slime layer").
  
  Wastewater treatment systems using gravel, slag, or even chopped plastic scrap are used in these systems. Effluent is collected after passing through the system by a piping system which permits oxygen to enter the filter and which collects solids which have passed through. (Compare with attached growth and media filter systems.
• Definition of TSS - Total Suspended Solids the total amount of suspended solid material in sewage - matter that has not either settled to the bottom of a septic tank as sludge or coagulated at the top of a septic tank as the floating scum layer.

U

• Definition of usable Soil - [relates to soil percolation tests and drainfield location] for septic system drainfields, leaching beds, absorption beds, soakaway beds, drainage trenches: unless otherwise stated a soil with a percolation rate of faster than 60 min/in with a compatible soil classification.

VWXYZ

• Definition of Vertical Separation: The depth of unsaturated, original, undisturbed soil of Soil Types 1B - 6 between the bottom of a disposal component and the highest seasonal water table, a restrictive layer, or Soil Type 1A. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
  
  Also see Massachusetts Title 5 for definition and specification of allowable vertical separation between the bottom of the drainfield trench and the seasonal high water table.
  
  This space of unsaturated soil is required to permit biomat formation and filtration and treatment of septic effluent before it is discharged to the area's groundwater.
• Definition of Wastewater: Water-carried human excreta and/or domestic waste from residences, buildings, industrial establishments or other facilities. (See SEWAGE.) [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Wastewater Treatment Unit: A unit designed, constructed, and installed to stabilize liquid waste by biochemical and physical action. [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Wastewater Design Flow: The volume of wastewater predicted to be generated by occupants of a structure.
  
  For residential dwellings, this volume is calculated by multiplying the number of bedrooms by either 120 or 150 GPD (gallons per day). [Quoted from "Dosing Gravity Drainfield Systems" Washington State DOH, 12/31/1998]
• Definition of Water Softener: Water softeners remove unwanted minerals from the water supply using one of several methods such as ion exchange. A high level of minerals in water, referred to as "hard" water, can lead to clogged pipes and other plumbing problems as well as aesthetic concerns such as unpleasant bathing (difficult to obtain a soap lather) or tastes in water.
  See
  
  WATER SOFTENERS & CONDITIONERS - operation, maintenance, adjustment, and water softener effect on drinking water and on septic systems.
• Definition of Wastewater - any water discharged from a house through a plumbing fixture to include, but not limited to, sewage and any water or waste from a device (e.g., water softener brine) which is produced in the house or property.
• Definition of Wastewater Treatment: "The process of removing pollutants and pathogens from wastewater, discharging the water to the environment, ... , and disposing of the byproducts of the treatment process." Onsite Wastewater Treatment Systems, Burks & Minnis, cited below.
  See
  
  The Basics of Onsite Wastewater Treatment
  
  Septic Design Books, Septic Installation Manuals, Septic Standards & Codes
• Definition of Watercourse - a visible path through which surface water travels on a regular basis. Drainage areas which contain water only during and immediately after a rainstorm shall not be considered a watercourse. (31) Watershed - an area of drainage for a body of water that serves as a source of drinking water and for which watershed rules and regulations have been adopted by the commissioner.
• Definition of Waterless septic systems: do not exist, but this term may be used to describe waterless toilets, composting toilets, chemical toilets, and incinerating toilets to receive blackwater or sewage from a building.
  
  I assert that a true "waterless" system does not exist because even these special toilets depend on greywater (from sinks, showers, laundry) having been separated and disposed-of by other means. The toilet types listed will require maintenance and disposal of their contents.
  See
  
  Products - for Alternative Septic System Designs aerobic system pumps, media filters, gravelless systems, other advanced wastewater treatment products including waterless and low water toilets, chemical toilets, incinerator toilets.
• Definition of Well head area - the area surrounding a well which includes the cone of influence (where the drawdown of groundwater causes groundwater flow).
• Definition of Wetland - an area(s) of marshes or swamps which have been designated as such by the State Department of Environmental Conservation or other agency having jurisdiction. Marshes or swamps that have not been classified by an agency as a wetland shall not be treated for design purposes as a wetland. Wetlands may be pre-existing swamps or wetlands or they may be constructed anew. In either case they are considered a "natural" system used to treat septic effluent.
  See
  
  Using a Private Wetlands Effluent Disposal System as a Component of Alternative Septic Systems for Difficult Sites.

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