A septic tank is a key component of the septic system, a small-scale sewage treatment system common in areas with no connection to main sewage pipes provided by local governments or private corporations. (Other components, typically mandated and/or restricted by local governments, optionally include pumps, alarms, sand filters, and clarified liquid effluent disposal means such as a septic drain field, ponds, natural stone fiber filter plants or peat moss beds.) Septic systems are a type of On-Site Sewage Facility (OSSF). In North America, approximately 25% of the population relies on septic tanks; this can include suburbs and small towns as well as rural areas (Indianapolis is an example of a large city where many of the city's neighborhoods are still on separate septic systems). In Europe, they are in general limited to rural areas only. Since a septic system requires a drainfield that uses a lot of land area, they are not suitable for densely built cities.
The term "septic" refers to the anaerobic bacterial environment that develops in the tank which 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 artificial forced aeration.
Periodic preventive maintenance is required to remove the irreducible solids that settle and gradually fill the tank, reducing its efficiency. In most jurisdictions this maintenance is required by law, yet often not enforced. According to the Environmental Protection Agency, in the United States it is the home owner's responsibility to maintain their septic system. Those who disregard the requirement will eventually be faced with extremely costly repairs when solids escape the tank and destroy the clarified liquid effluent disposal means. A properly maintained system, on the other hand, can last for decades or possibly even a lifetime.
A septic tank generally consists of a tank (or sometimes more than one tank) of between 4000 and 7500 liters (1,000 and 2,000 gallons) in size connected to an inlet wastewater pipe at one end and a septic drain field at the other. In general, these pipe connections are made via a T pipe, which allows liquid entry and exit without disturbing any crust on the surface. Today, the design of the tank usually incorporates two chambers (each of which is equipped with a manhole cover), which are separated by means of a dividing wall that has openings located about midway between the floor and roof of the tank.
Waste water 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, with the excess liquid then draining in a relatively clear condition from the outlet into the leach field, also referred to as a drain field or seepage field, depending upon locality. A percolation test is required to establish the porosity of the local soil conditions for the drain field design.
The remaining impurities are trapped and eliminated in the soil, with the excess water eliminated through percolation into the soil (eventually returning to the groundwater), through evaporation, and by uptake through the root system of plants and eventual transpiration. 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 leach 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 methods of increasing the volume and velocity of outflow to the drainage field. This helps to load all portions of the drainage pipe more evenly and extends the drainage field life by preventing premature clogging.
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.
Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank, or else the septic tank fills up and undecomposed wastewater discharges directly to the drainage field. Not only is this bad 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.
How often the septic tank has to be emptied depends on the volume of the tank relative to the input of solids, the amount of indigestible solids, and the ambient temperature (as anaerobic digestion occurs more efficiently at higher temperatures). The required frequency varies greatly depending on jurisdiction, usage, and system characteristics. Some health authorities require tanks to be emptied at prescribed intervals, while others leave it up to the determination of the 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 re-fills.
A properly designed and normally operating septic system is odor-free and, besides periodic inspection and pumping of the septic tank, should last for decades with no maintenance.
A well designed and maintained concrete, fiberglass, or plastic tank should last about 50 years.
Potential problems 
- Excessive dumping 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 items such as cigarette butts and hygiene products such as sanitary napkins, tampons, and cotton buds/swabs will rapidly fill or clog a septic tank; these materials should not be disposed of in this way.
- The use of garbage disposals for disposal of waste food can cause a rapid overload of the system and early failure.
- Certain chemicals may damage the components of a septic tank, especially pesticides, herbicides, materials with high concentrations of bleach or caustic soda (lye) or any other inorganic materials such as paints or solvents.
- Other chemicals can destroy septic bacteria itself, most notably silver nitrate even in very small quantities will kill an entire culture.
- Roots from trees and shrubbery growing above the tank or the drainfield may clog and/or rupture them.
- 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.
- The flushing of salted water into the septic system can lead to Sodium Binding in the drainfield. The clay and fine silt particles bind together and effectively water-proof the leach field, rendering it ineffective.
- Excessive water entering the system will overload it and cause it to fail. Checking for plumbing leaks and practicing water conservation will help the system's operation.
- Very high rainfall, rapid snow-melt, and flooding from rivers or the sea can all prevent a drain field from operating and can cause flow to back up and stop the normal operation of the tank. High winter water tables can also result in groundwater 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 tanks by themselves are ineffective at removing nitrogen compounds that have potential to cause algal blooms in receiving waters; 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.
Environmental issues 
Some pollutants, especially sulfates, under the anaerobic conditions of septic tanks, are reduced to hydrogen sulfide, a pungent and toxic gas. Likewise, methane, a potent greenhouse gas, is another by-product. Nitrates and organic nitrogen compounds are reduced to ammonia. Because of the anaerobic conditions, fermentation processes take place, which ultimately generate carbon dioxide and methane.
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. Because phosphates can be the limiting nutrient for plant growth in many ecosystems, the discharge from a septic tank into the environment can trigger prolific plant growth including algal blooms, which can also include blooms of potentially toxic cyanobacteria.
Soil capacity to retain phosphorus is large compared with the load through a normal residential septic tank. An exception occurs when septic drain fields are located in sandy or coarser soils on property adjoining 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.
In areas with high population density, groundwater pollution levels often exceed acceptable limits. Some small towns are facing the costs of building very expensive centralized wastewater treatment systems because of this problem, owing to the high cost of extended collection systems. To slow development, building moratoriums and limits 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.
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. In Washington State, USA, 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 nonpoint 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.
Trees in 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, but the trees themselves tend to grow extremely vigorously due to the continuous influx of nutrients into the septic system.
At present (May 2012) the Environment Agency in the United Kingdom is inviting property owners with small scale septic tanks to register them with the organisation such that an exemption certificate may be granted. However, it is likely that a number of users of such systems may find that their system falls outside of the tolerances applied in order to claim exemption and as such a bespoke permit may be required. It is likely that the requirement to register septic tanks to gain an exemption certificate will be implemented from 1 January 2013.
All package sewage treatment plants must have the EN 12566-3 2005 Certificate to qualify for an Exemption Certificate. In-tank conversion systems would require a permit from the environment agency for discharge direct to a watercourse as these systems are not covered by any of the EN12566 Certifications.
See also 
- "Septic Systems for Waste Water Disposal". American Ground Water Trust. Retrieved 2008-05-20.
- Section H2 Building Regulations
- "Septic Tanks: The Real Poop". University of California Extension. Retrieved 2006-07-11.
- Silver nitrate toxicity
- Residential nutrient reduction
- Craig G. Cogger. "eb1475 Septic System Waste Treatment in Soil". College of Agriculture and Home Economics, Pullman, Washington. Retrieved 2006-07-11.
- Homeowner's Guide to Septic Systems
- U.S. Environmental Protection Agency
- Massachusetts Department of Environmental Protection
- Rhode Island Department of Environmental Management