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 efficiency is only moderate (referred to as "primary treatment").
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 can be 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.
The rate of accumulation of sludge—also called
septage or
fecal sludge—is faster than the rate of decomposition.
Therefore, the accumulated fecal sludge must be periodically removed, which is commonly done with a
vacuum truck.
Description
A septic tank consists of one or more concrete or plastic tanks of between 4000 and 7500 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. 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. The excess liquid, now in a generally clear condition, then drains from the outlet 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.
The remaining impurities are trapped and eliminated in the
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
, with the excess water eliminated through
percolation into the soil, through
evaporation, and by uptake through the
root system of
plant
Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
s and eventual
transpiration or entering
groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidat ...
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
A pump is a device that moves fluids ( liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method the ...
. 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.
Emptying (desludging)
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) 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.
Services for desludging should not empty a septic tank completely but leave some settled solids in the septic tank to leave some of the microbial populations in place to continue the anaerobic degradation processes. 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.
Another option is "scheduled desludging" of septic tanks which has been initiated in several Asian countries including the Philippines, Malaysia, Vietnam, Indonesia, and India.
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.
Maintenance
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
* Excessive disposal of cooking oils and grease can cause the inlet drains to block. Oils and grease are often difficult to degrade and can cause odor problems and difficulties with the periodic emptying.
* Flushing non-biodegradable waste items down the toilet such as
cigarette butts
A cigarette is a narrow cylinder containing a combustible material, typically tobacco, that is rolled into thin paper for smoking. The cigarette is ignited at one end, causing it to smolder; the resulting smoke is orally inhaled via the oppo ...
,
cotton buds/swabs or
menstrual hygiene products
Feminine hygiene products are personal care products used during menstruation, vaginal discharge, and other bodily functions related to the vulva and vagina. Products that are used during menstruation may also be called menstrual hygiene produ ...
and
condom
A condom is a sheath-shaped barrier device used during sexual intercourse to reduce the probability of pregnancy or a sexually transmitted infection (STI). There are both male and female condoms. With proper use—and use at every act of inte ...
s can cause a septic tank to clog and fill rapidly, so these materials should not be disposed of in that manner. The same applies when the toilet is connected to a
sewer rather than a septic tank.
* Using the toilet for disposal of
food waste can cause a rapid overload of the system with solids and contribute to failure.
* Certain chemicals may damage the components of a septic tank or kill the bacteria needed in the septic tank for the system to operate properly, such as
pesticides,
herbicides, materials with high concentrations of
bleach or
caustic soda (lye), or any other inorganic materials such as paints or solvents.
* Using
water softeners - the brine discharge from water softeners may harm the bacteria responsible for breaking down the wastewater. Usually, however, the brine is sufficiently diluted with other wastewater that it does not adversely affect the septic system.
Other factors
* Roots from trees and shrubbery protruding above the tank or drainfield may clog and/or rupture them. Trees that are directly within the vicinity of a concrete septic tank have the potential to penetrate the tank as the system ages and the concrete begins to develop cracks and small leaks. Tree roots can cause serious flow problems due to plugging and blockage of drain pipes, and the trees themselves tend to expand extremely vigorously due to the ready supply of nutrients from the septic system.
* Playgrounds and storage buildings may cause damage to a tank and the drainage field. In addition, covering the drainage field with an impermeable surface, such as a driveway or parking area, will seriously affect its efficiency and possibly damage the tank and absorption system.
* Excessive water entering the system may overload it and cause it to fail.
* Very high rainfall, rapid
snowmelt, and
flood
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
ing from rivers or the sea can all prevent a drain field from operating, and can cause flow to back up, interfering with the normal operation of the tank. High winter
water tables can also result in
groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidat ...
flowing back into the septic tank.
* Over time,
biofilms develop on the pipes of the drainage field, which can lead to blockage. Such a failure can be referred to as "biomat failure".
Septic tank additives
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
Effective microorganisms (EM) are various blends of common predominantly anaerobic microorganisms in a carbohydrate-rich liquid carrier substrate (molasses nutrient solution) of ''EM Research Organization, Inc.'',''Effective Microorganisms'' ''EM'' ...
"—are usually costly in the longer term and fail to live up to expectations.
[Foxon, K., Still, D. (2012)]
Do pit additives work?
Water Research Commission (WRC), University of Kwazulu-Natal, Partners in Development (PiD), South Africa It has been estimated that in the U.S. more than 1,200 septic system additives were available on the market in 2011.
Very little peer-reviewed and replicated field research exists regarding the efficacy of these biological septic tank additives.
[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
Journal of Environmental Health, Volume 74, Number 5, p. 22-28
Environmental concerns
While a properly maintained and located septic tank poses no higher amount of environmental problems than centralized municipal sewage treatment,
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.
Since septic systems require large
drainfields, they are unsuitable for densely built areas.
Odor and gas emissions
Some constituents of wastewater, especially
sulfates, under the anaerobic conditions of septic tanks, are reduced to
hydrogen sulfide, a
pungent and
toxic gas.
Methane may also be released.
Nitrates and
organic nitrogen compounds can be reduced to
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...
. Because of the anaerobic conditions,
fermentation processes take place, which may generate
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
and/or
methane.
Nutrients in the effluent
Septic tanks by themselves are ineffective at removing
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...
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,
or by simply ensuring that the leach field is properly sited to prevent direct entry of effluent into bodies of water.
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.
Pathogens
Diseases extremely dangerous to human contact such as
E. coli and other
coliform bacteria are often reported following failures of septic tanks.
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.
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.
Use
In the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...
, the 2007 American Housing Survey indicated that about 20 percent of all households rely on septic tanks,
and that the overwhelming majority of systems are located in
rural (50%) and
suburban (47%) areas.
[ Indianapolis is one example of a large city where many of the city's neighborhoods still rely on separate septic systems. In Europe, septic systems are generally limited to rural areas.
]
Regulations
European Union
In the European Union
The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been ...
the EN 12566
EN 12566 - Small wastewater treatment systems for up to 50 PT refers to a set of European Committee for Standardization, European standards which specify the general requirements for packaged and/or site assembled Wastewater treatment, wastewater ...
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
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including ...
, glass reinforced polyester
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
, polypropylene, PVC-U, steel or concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...
. 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.
France
In France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of overseas regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans. Its metropolitan ar ...
, about 4 million households (or 20% of the population) are using on-site wastewater disposal systems (''l’assainissement non collectif''), 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. 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.
Ireland
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.
Following a European Court of Justice judgment made against Ireland
Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...
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.
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.[Code of Practice: Wastewater Treatment Systems for Single Houses](_blank)
2010. Environmental Protection Agency, Ireland. 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. 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.
United Kingdom
Since 2015, only certain property owners in England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe ...
and Wales
Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...
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. 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). 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
Northern Ireland ( ga, Tuaisceart Éireann ; sco, label=Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. North ...
, 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.
The Water Environment Regulations 2011 regulate the registration of septic tank systems in Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to th ...
. Proof of registration is required when new properties are being developed or existing properties change ownership.
Australia
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 and Legislation. 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, 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. Septic systems have to be replaced as part of any new building applications, regardless of how well the old system performed.
United States
According to the US Environmental Protection Agency, in the United States it is the home owners' responsibility to maintain their septic systems. 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.
Slang usage
The term "septic tank", or more usually "septic", is used in Britain and Australia as a slang term to refer to Americans, from Cockney rhyming slang septic tank = yank. This is sometimes further shortened to "seppo".
See also
* Cesspit
* Fecal sludge management
* Grease trap
* Sanitation
Sanitation refers to public health conditions related to clean drinking water and treatment and disposal of human excreta and sewage. Preventing human contact with feces is part of sanitation, as is hand washing with soap. Sanitation syste ...
References
External links
Septic Systems - U.S. Environmental Protection Agency
{{Authority control
Sewerage infrastructure
Storage tanks
Sanitation