Septic tank
A septic tank being installed in the ground
Position in sanitation chainCollection and storage/treatment (on-site)[1]
Application levelHousehold or neighborhood level (schools, hotels etc.)[1]
Management levelHousehold, public, shared (most common is household level)[1]
Inputsblackwater (waste), greywater, brownwater[1]
OutputsFecal sludge, effluent[1]
TypesSingle tank or multi-chamber septic tanks (potentially with baffles)[1]
Environmental concernsGroundwater pollution, water pollution e.g. during floods[1]

A septic tank is an underground chamber made of concrete, fiberglass, or plastic through which domestic wastewater (sewage) flows for basic sewage treatment.[2] Settling and anaerobic digestion processes reduce solids and organics, but the treatment efficiency is only moderate (referred to as "primary treatment").[2] Septic tank systems are a type of simple onsite sewage facility. They can be used in areas that are not connected to a sewerage system, such as rural areas. The treated liquid effluent is commonly disposed in a septic drain field, which provides further treatment. Nonetheless, groundwater pollution may occur and is a problem.

The term "septic" refers to the anaerobic bacterial environment that develops in the tank that decomposes or mineralizes the waste discharged into the tank. Septic tanks can be coupled with other onsite wastewater treatment units such as biofilters or aerobic systems involving artificially forced aeration.[3]

The rate of accumulation of sludge—also called septage or fecal sludge—is faster than the rate of decomposition.[2] Therefore, the accumulated fecal sludge must be periodically removed, which is commonly done with a vacuum truck.[4]


Schematic of a septic tank[2]
Septic tank and septic drain field

A septic tank consists of one or more concrete or plastic tanks of between 4,500 and 7,500 litres (1,000 and 2,000 gallons); one end is connected to an inlet wastewater pipe and the other to a septic drain field. Generally these pipe connections are made with a T pipe, allowing liquid to enter and exit without disturbing any crust on the surface.[citation needed] Today, the design of the tank usually incorporates two chambers, each equipped with an access opening and cover, and separated by a dividing wall with openings located about midway between the floor and roof of the tank.

Wastewater enters the first chamber of the tank, allowing solids to settle and scum to float. The settled solids are anaerobically digested, reducing the volume of solids. The liquid component flows through the dividing wall into the second chamber, where further settlement takes place. One option for the effluent is the draining into the septic drain field, also referred to as a leach field, drain field or seepage field, depending upon locality. A percolation test is required prior to installation to ensure the porosity of the soil is adequate to serve as a drain field.[5][6]

Septic tank effluent can also be conveyed to a secondary treatment, typically constructed wetlands. Constructed wetlands benefit from the good performance of septic tanks at removing solids, which avoids them getting clogged quickly.

Septic tank effluent can also be conveyed to a centralized treatment facility.

The remaining impurities are trapped and eliminated in the soil, with the excess water eliminated through percolation into the soil, through evaporation, and by uptake through the root system of plants and eventual transpiration or entering groundwater or surface water. A piping network, often laid in a stone-filled trench (see weeping tile), distributes the wastewater throughout the field with multiple drainage holes in the network. The size of the drain field is proportional to the volume of wastewater and inversely proportional to the porosity of the drainage field. The entire septic system can operate by gravity alone or, where topographic considerations require, with inclusion of a lift pump.

Certain septic tank designs include siphons or other devices to increase the volume and velocity of outflow to the drainage field. These help to fill the drainage pipe more evenly and extend the drainage field life by preventing premature clogging or bioclogging.

An Imhoff tank is a two-stage septic system where the sludge is digested in a separate tank. This avoids mixing digested sludge with incoming sewage. Also, some septic tank designs have a second stage where the effluent from the anaerobic first stage is aerated before it drains into the seepage field.

A properly designed and normally operating septic system is odour-free. Besides periodic inspection and emptying, a septic tank should last for decades with minimal maintenance, with concrete, fibreglass, or plastic tanks lasting about 50 years.[7]

Emptying (desludging)

A vacuum truck used to empty septic tanks in Germany

Further information: Fecal sludge management

Waste that is not decomposed by the anaerobic digestion must eventually be removed from the septic tank. Otherwise the septic tank fills up and wastewater containing undecomposed material discharges directly to the drainage field. Not only is this detrimental for the environment but, if the sludge overflows the septic tank into the leach field, it may clog the leach field piping or decrease the soil porosity itself, requiring expensive repairs.

When a septic tank is emptied, the accumulated sludge (septage, also known as fecal sludge[8]) is pumped out of the tank by a vacuum truck. How often the septic tank must be emptied depends on the volume of the tank relative to the input of solids, the amount of indigestible solids, and the ambient temperature (because anaerobic digestion occurs more efficiently at higher temperatures), as well as usage, system characteristics and the requirements of the relevant authority.

Some health authorities require tanks to be emptied at prescribed intervals, while others leave it up to the decision of an inspector. Some systems require pumping every few years or sooner, while others may be able to go 10–20 years between pumpings. An older system with an undersize tank that is being used by a large family will require much more frequent pumping than a new system used by only a few people. Anaerobic decomposition is rapidly restarted when the tank is refilled.

An empty tank may be damaged by hydrostatic pressure causing the tank to partially "float" out of the ground, especially in flood situations or very wet ground conditions.[9]

Another option is "scheduled desludging" of septic tanks which has been initiated in several Asian countries including the Philippines, Malaysia, Vietnam, Indonesia, and India.[10] In this process, every property is covered along a defined route and the property occupiers are informed in advance about desludging that will take place.


The maintenance of a septic system is often the responsibility of the resident or property owner. Some forms of abuse or neglect include the following:

User's actions

Other factors

Septic tank additives

Further information: Pit additive

Septic tank additives have been promoted by some manufacturers with the aim to improve the effluent quality from septic tanks, reduce sludge build-up and to reduce odors. These additives—which are commonly based on "effective microorganisms"—are usually costly in the longer term and fail to live up to expectations.[14] It has been estimated that in the U.S. more than 1,200 septic system additives were available on the market in 2011.[15] Very little peer-reviewed and replicated field research exists regarding the efficacy of these biological septic tank additives.[15]

Environmental concerns

A septic tank before installation, with manhole cover on top
The same tank partially installed in the ground

While a properly maintained and located septic tank poses no higher amount of environmental problems than centralized municipal sewage treatment,[16] certain problems could arise with a septic tank in an unsuitable location, and septic tank failures are typically more expensive to fix or replace than municipal sewer.[16] Since septic systems require large drainfields, they are unsuitable for densely built areas.

Odor, gas emissions and carbon footprint

Some constituents of wastewater, especially sulfates, under the anaerobic conditions of septic tanks, are reduced to hydrogen sulfide, a pungent and toxic gas. Nitrates and organic nitrogen compounds can be reduced to ammonia. Because of the anaerobic conditions, fermentation and methanogenesis processes take place, which may generate carbon dioxide and/or methane. Both carbon dioxide and methane are greenhouse gases, with methane having a global warming potential about 25 times larges than carbon dioxide. This makes septic tanks potential greenhouse gas emitters. The same methane can be burnt to produce energy for local usage.[17]

Nutrients in the effluent

Septic tanks by themselves are ineffective at removing nitrogen compounds that have potential to cause algal blooms in waterways into which affected water from a septic system finds its way. This can be remedied by using a nitrogen-reducing technology,[18] such as hybrid constructed wetlands, or by simply ensuring that the leach field is properly sited to prevent direct entry of effluent into bodies of water.[citation needed]

The fermentation processes cause the contents of a septic tank to be anaerobic with a low redox potential, which keeps phosphates in a soluble and, thus, mobilized form. Phosphates discharged from a septic tank into the environment can trigger prolific plant growth including algal blooms, which can also include blooms of potentially toxic cyanobacteria.

The soil's capacity to retain phosphorus is usually large enough to handle the load through a normal residential septic tank. An exception occurs when septic drain fields are located in sandy or coarser soils on property adjacent to a water body. Because of limited particle surface area, these soils can become saturated with phosphates. Phosphates will progress beyond the treatment area, posing a threat of eutrophication to surface waters.[19]


Diseases extremely dangerous to human contact such as E. coli and other coliform bacteria are often reported following failures of septic tanks.[20]

A properly functioning septic system, on the other hand, provides significant reduction of pathogens compared to direct discharge due to settling (in the tank) and soil absorption (in the drain field). Log reductions of 4–8 for coliform bacteria, 0–2 for viruses are achieved in the effluent. Parasitic worm eggs are also removed. Additional filters may be added to improve removal performance although they will need to be replaced periodically.[21]

Groundwater pollution

In areas with high population density, groundwater pollution beyond acceptable limits may occur. Some small towns experience the costs of building very expensive centralized wastewater treatment systems because of this problem, due to the high cost of extended collection systems. To reduce residential development that might increase the demand to construct an expensive centralized sewerage system, building moratoriums and limitations on the subdivision of property are often imposed. Ensuring existing septic tanks are functioning properly can also be helpful for a limited time, but becomes less effective as a primary remediation strategy as population density increases.

Surface water pollution

In areas adjacent to water bodies with fish or shellfish intended for human consumption, improperly maintained and failing septic systems contribute to pollution levels that can force harvest restrictions and/or commercial or recreational harvest closures.


Septic tank system in rural Southern Minnesota

In the United States, the 2008 American Housing Survey indicated that about 20 percent of all households rely on septic tanks,[22] and that the overwhelming majority of systems are located in rural (50%) and suburban (47%) areas.[22] Indianapolis is one example of a large city where many of the city's neighborhoods still rely on separate septic systems.[23] In Europe, septic systems are generally limited to rural areas.[citation needed]


European Union

In the European Union the EN 12566 standard provides the general requirements for packaged and site assembled treatment plants used for domestic wastewater treatment.

Part 1 (EN 12566-1) is for septic tanks that are prefabricated or factory manufactured and made of polyethylene, glass reinforced polyester, polypropylene, PVC-U, steel or concrete. Part 4 (EN 12566-4) regulates septic tanks that are assembled on site from prefabricated kits, generally of concrete construction. Certified septic tanks of both types must pass a standardized hydraulic test to assess their ability to retain suspended solids within the system. Additionally, their structural adequacy in relevant ground conditions is assessed in terms of water-tightness, treatment efficiency, and structural behaviour.[24]


In France, about 4 million households (or 20% of the population) are using on-site wastewater disposal systems (l’assainissement non collectif),[25] including septic tanks (fosse septique). The legal framework for regulating the construction and maintenance of septic systems was introduced in 1992 and updated in 2009 and 2012 with the intent to establish the technical requirements applicable to individual sewerage systems.[26] Septic tanks in France are subject to inspection by SPANC (Service Public d’Assainissement Non Collectif), a professional body appointed by the respective local authorities to enforce wastewater collection laws, at least once in four years. Following the introduction of EN 12566, the discharge of effluent directly into ditches or watercourses is prohibited, unless the effluent meets prescribed standards.[27]


According to the Census of Ireland 2011, 27.5% of Irish households (i.e. about 440,000 households), with the majority in rural areas, use an individual septic tank.[28]

Following a European Court of Justice judgment made against Ireland in 2009 that deemed the country non-compliant with the Waste Framework Directive in relation to domestic wastewaters disposed of in the countryside, the Water Services (Amendment) Act 2012 was passed in order to regulate wastewater discharges from domestic sources that are not connected to the public sewer network and to provide arrangements for registration and inspection of existing individual domestic wastewater treatment systems.[29][30]

Additionally, a code of practice has been developed by the Environmental Protection Agency to regulate the planning and construction of new septic tanks, secondary treatment systems, septic drain fields and filter systems.[31] Direct discharge of septic tank effluent into groundwater is prohibited in Ireland, while the indirect discharge via unsaturated subsoil into groundwater, e.g. by means of a septic drain field, or the direct discharge into surface water is permissible in accordance with a Water Pollution Act license.[31] Registered septic tanks must be desludged by an authorized contractor at least once a year; the removed fecal sludge is disposed of, either to a managed municipal wastewater treatment facility or to agriculture provided that nutrient management regulations are met.[31]

United Kingdom

Since 2015, only certain property owners in England and Wales with septic tanks or small packaged sewage treatment systems need to register their systems, and either apply for a permit or qualify for an exemption with the Environment Agency.[32] Permits need to be granted to systems that discharge more than a certain volume of effluent in a given time or that discharge effluent directly into sensitive areas (e.g., some groundwater protection zones).[33] In general, permits are not granted for new septic tanks that discharge directly into surface waters. A septic tank discharging into a watercourse must be replaced or upgraded by 1 January 2020 to a Sewage Treatment Plant (also called an Onsite sewage facility), or sooner if the property is sold before this date, or if the Environment Agency (EA) finds that it is causing pollution.

In Northern Ireland, the Department of the Environment must give permission for all wastewater discharges where it is proposed that the discharge will go to a waterway or soil infiltration system. The discharge consent will outline conditions relating to the quality and quantity of the discharge in order to ensure the receiving waterway or the underground aquifer can absorb the discharge.[34]

The Water Environment Regulations 2011 regulate the registration of septic tank systems in Scotland. Proof of registration is required when new properties are being developed or existing properties change ownership.[35]


In Australia, septic tank design and installation requirements are regulated by State Governments, through Departments of Health and Environmental Protection Agencies. Regulation may include Codes of Practice[36][37] and Legislation.[38] Regulatory requirements for the design and installation of septic tanks commonly references Australian Standards (1547 and 1546). Capacity requirements for septic tanks may be outlined within Codes of Practice, and can vary between states.

Mainly because of water leaching from the effluent drains of a lot of closely spaced septic systems,[39] many council districts (e.g. Sunshine Coast, Queensland) have banned septic systems, and require them to be replaced with much more expensive small-scale sewage treatment systems that actively pump air into the tank, producing an aerobic environment.[citation needed] Septic systems have to be replaced as part of any new building applications, regardless of how well the old system performed.[citation needed]

United States

According to the US Environmental Protection Agency, in the United States it is the home owners' responsibility to maintain their septic systems.[40] Anyone who ignores this requirement will eventually experience costly repairs when solids escape the tank and clog the clarified liquid effluent disposal system.

In Washington, for example, a "shellfish protection district" or "clean water district" is a geographic service area designated by a county to protect water quality and tideland resources. The district provides a mechanism to generate local funds for water quality services to control non-point sources of pollution, such as septic system maintenance. The district also serves as an educational resource, calling attention to the pollution sources that threaten shellfish growing waters.[41]

Slang usage

The term "septic tank", or more usually "septic", is used in some parts of Britain as a slang term to refer to Americans,[42] from Cockney rhyming slang septic tank equalling yank.[43] This is sometimes further shortened to "seppo" by Australians .[44]

See also


  1. ^ a b c d e f g "Sanitation Systems – Sanitation Technologies – Septic Tank". SSWM. 19 June 2018. Retrieved 31 October 2018.
  2. ^ a b c d Tilley, Elizabeth; Ulrich, Lukas; Lüthi, Christoph; Reymond, Philippe; Zurbrügg, Chris (2014). "Septic tanks". Compendium of Sanitation Systems and Technologies (2nd ed.). Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag). ISBN 978-3906484570.
  3. ^ "Septic Systems for Waste Water Disposal". American Ground Water Trust. Archived from the original on 2008-05-17. Retrieved 2008-05-20.
  4. ^ "What is a septic system? How do I maintain one?". National Environmental Services Center. nesc. Archived from the original on 26 April 2016. Retrieved 4 March 2015.
  5. ^ Section H2 Building Regulations
  6. ^ Gustafson, David; Machmeier, Roger E. "How to run a percolation test". University of Minnesota. Archived from the original on 19 April 2016. Retrieved 26 February 2016.
  7. ^ "Septic Tanks: The Real Poop". University of California Extension. Archived from the original on 3 March 2009. Retrieved 28 December 2015.
  8. ^ Arbogast, Brian (November 18, 2013). "Why Fecal Sludge Management is Serious Business". The Water Blog, The World Bank.
  9. ^ "Septic Systems - What to Do after the Flood". United States Environmental Protection Agency. 18 November 2015. Retrieved 14 August 2018.
  10. ^ Mehta, Meera; Mehta, Dinesh; Yadav, Upasana (2019). "Citywide Inclusive Sanitation Through Scheduled Desludging Services: Emerging Experience From India". Frontiers in Environmental Science. 7: 188. doi:10.3389/fenvs.2019.00188. ISSN 2296-665X.
  11. ^ "What can make my system fail?" (PDF). United States Environmental Protection Agency. EPA. Retrieved 28 December 2015.
  12. ^ Roberts, Mark (9 March 2016). "Septic Tanks – The Guide". Septicleanse. Retrieved 18 July 2016.
  13. ^ "water-softener-fact-sheet" (PDF). Retrieved 18 October 2022.
  14. ^ Foxon, K., Still, D. (2012). "Do pit additives work?" Water Research Commission (WRC), University of Kwazulu-Natal, Partners in Development (PiD), South Africa
  15. ^ a b S. Pradhan, Michael T. Hoover, G.H. Clark, M. Gumpertz, C. Cobb, J. Strock (2011) "Impacts of biological additives; Part 2 Septic Tank Effluent Quality and Overall Additive Efficacy" Archived 2016-03-04 at the Wayback Machine, Journal of Environmental Health, Volume 74, Number 5, p. 22–28
  16. ^ a b Arnold, Rory. "Sewer vs septic system: which is better for the environment?". Retrieved 16 October 2020.
  17. ^ Boiocchi, Riccardo; Mainardis, Matia; Rada, Elena Cristina; Ragazzi, Marco; Salvati, Silvana Carla (January 2023). "Carbon Footprint and Energy Recovery Potential of Primary Wastewater Treatment in Decentralized Areas: A Critical Review on Septic and Imhoff Tanks". Energies. 16 (24): 7938. doi:10.3390/en16247938. hdl:11390/1267670. ISSN 1996-1073.
  18. ^ "Residential nutrient reduction" (PDF). Archived from the original on June 2, 2011. Retrieved 2010-12-22.((cite web)): CS1 maint: bot: original URL status unknown (link)
  19. ^ Craig G. Cogger. "eb1475 Septic System Waste Treatment in Soil". College of Agriculture and Home Economics, Pullman, Washington. Archived from the original on 2006-08-23. Retrieved 2006-07-11.
  20. ^ Dare, Don (March 21, 2018). "Residents' well water contaminated by septic failure at Bean Station slaughterhouse". WATE-TV. Retrieved February 8, 2021.
  21. ^ Vinnerås, B.; Nordin, A.; Niwagaba, C.; Nyberg, K. (2019-02-08). "Septic Systems | Global Water Pathogen Project". Water Research. 42 (15). 4067–4074. doi:10.1016/j.watres.2008.06.014. PMID 18718625. Retrieved 2022-08-07.
  22. ^ a b "American Housing Survey for the United States: 2007" (PDF). American Housing Survey. U.S. Department of Housing and Urban Development. p. 6. Retrieved 2018-08-08.
  23. ^ "Septic Tank Elimination Program". Citizens Energy Group. Retrieved 2016-02-16.
  24. ^ Santala, E. Finnish regulations, European standards and testing of small wastewater treatment plants. Finnish Environment Institute.
  25. ^ "Portail interministériel sur l'assainissement non collectif". 13 December 2018. Retrieved 18 October 2022.
  26. ^ "Arrêté du 7 mars 2012 modifiant l'arrêté du 7 septembre 2009 fixant les prescriptions techniques applicables aux installations d'assainissement non collectif recevant une charge brute de pollution organique inférieure ou égale à 1,2 kg/j de DBO5 – Legifrance". Retrieved 7 September 2015.
  27. ^ "Fosse Septique France". The Good Life France. 20 July 2012. Retrieved 7 September 2015.
  28. ^ "Census 2011 Profile 4 The Roof over our Heads – Housing in Ireland". CSO – Central Statistics Office.
  29. ^ Water Services (Amendment) Act 2012 Archived 2015-09-24 at the Wayback Machine, Department of the Environment, Community and Local Government, Ireland.
  30. ^ Case C-188/08, Judgment of the Court (Second Chamber) of 29 October 2009. Commission of the European Communities v Ireland. Failure of a Member State to fulfil obligations – Directive 75/442/EEC – Waste – Domestic waste waters discharged through septic tanks in the countryside – Waste not covered by other legislation – Failure to transpose.
  31. ^ a b c Code of Practice: Wastewater Treatment Systems for Single Houses Archived 2012-06-25 at the Wayback Machine, 2010. Environmental Protection Agency, Ireland.
  32. ^ Standard Note SN06059 (2014). Septic tanks: new regulations, House of Commons Library
  33. ^ "Septic tanks and treatment plants: permits and general binding rules". Retrieved 7 September 2015.
  34. ^ Department of the Environment (Northern Ireland) Septic Tanks and Domestic Discharges Archived 2015-06-13 at the Wayback Machine
  35. ^ Scottish Environment Protection Agency (SEPA) The Water Environment (Controlled Activities) Regulations 2011: A practical guide
  36. ^ "On-site Wastewater Systems Code" (PDF). SA Health. SA Health, Government of South Australia. April 2013. Retrieved 11 February 2017.
  37. ^ "Code of practice - onsite wastewater management" (PDF). EPA Victoria. July 2016. Archived from the original (PDF) on 10 April 2017. Retrieved 11 February 2017.
  38. ^ "Health (Treatment of Sewage and Disposal of Effluent and Liquid Waste) Regulations 1974" (PDF). Government of Western Australia. 7 October 2005.
  39. ^ "How to Solve The Biggest Problems With Septic Tanks and Systems". Coerco. Retrieved 2021-05-11.
  40. ^ "A Homeowner's Guide to Septic Systems" (PDF). United States Environmental Protection Agency. EPA. Retrieved 28 December 2015.
  41. ^ Beatrice. "Shellfish - Marine" (PDF). Marine. Archived from the original (PDF) on 15 January 2014. Retrieved 7 September 2015.
  42. ^ "Online slang dictionary".
  43. ^ "Collins Dictionary".
  44. ^ "Macquarie Dictionary". 25 October 2023.