Nearly 20 million homes, which include almost 30% of the population of the United States, dispose of their waste-water through an on-site disposal system. The most commonly used type of system is the septic tank, which is an individual treatment system that uses the soil to treat small wastewater flows. The system is usually used in rural or large lot settings where centralized wastewater treatment is impractical. Septic tank systems are designed specifically for each site, using standardized design principles that are usually state-regulated. Septic tank systems commonly contain three components: the septic tank, a distribution box, and a drainfield, all of which are connected by conveyance lines.
The septic tank serves to separate solids from the liquids in the wastewater. All sources of wastewater, including those from sinks, baths, showers, washing machines, dishwashers, and toilets , are directed into the septic tank, since any of these waters can contain disease-causing microorganisms or environmental pollutants. The size of the septic tank varies depending on the number of bedrooms in the home, but an average tank holds 1,000 gal (3,790 l) of liquid. Wastewater in a septic tank is treated by anaerobic bacteria that digest organic materials, while encouraging the separation of solid materials from the wastewater. The solids accumulate and remain in the septic tank in the form of sludge , which collects at the bottom of the tank, and also in the form of scum, which floats on the top of the wastewater. Periodically (for example, every two or three years) the indigestible sludge and scum (referred to as septage) are removed from the tank by pumping and are disposed of in a septage disposal system (like a municipal sewage treatment system). Unfortunately, many homeowners do not properly maintain septic systems by pumping them out as frequently as every two to three years and often wait until there is a back up in the system or some other type of problem. Periodic pumping is designed to prevent the solids from leaking out of the septic tank in the wastewater effluent . The effluent from the septic tank is a cloudy liquid that still contains many pollutants (including nitrogen compounds, suspended solids, and organic and inorganic materials) and microorganisms (including bacteria and viruses, some of which that may be potentially pathogenic), which require further treatment.
Treatment of the wastewater effluent from the septic tank is continued by transporting the wastewater by gravity to a soil absorption field through a connecting pipe. The absorption field is also referred to as the soil drainfield or the nitrification field. The absorption field consists of a series of underground perforated pipes covered with soil and turf, which may be connected in a closed loop system. The wastewater enters a constructed gravel bed (the trench) through perforations in the pipe, where it is stored before entering the underlying unsaturated soil. The wastewater is treated as it trickles into and through the soil by filtration and adsorption processes as well as by aerobic degradation processes before the wastewater enters the ground water. Filtration removes most of the suspended solids and may also remove microorganisms. Adsorption is the process by which pollutants and microorganisms are attracted to and held on the surfaces of soil particles, thus immobilizing them. Adsorption attracts such nutrients such as phosphorus and some forms of nitrogen (mostly ammonium [NH4]) and is most effective when fine-textured soil is used as the adsorption medium. However, soils with a very fine texture, such as soils high in clay, may have too low permeability to allow much wastewater to pass through the soil. Microbial degradation results in the removal of many remaining nutrients and organic materials. If the volume and type of soil underlying a soil absorption system are adequate, most pollutants (with the exception of nitrate nitrogen) should be removed before the wastewater reaches the groundwater .
Estuaries (bays, harbors, etc.) often experience nitrogen loading from septic systems along the shore (within 656 mi [200 m] of the shore or greater in sandy soil environments). Typically on-site disposal systems such as septic tanks cannot remove more than 50–60% of the nitrogen (mostly in the form of nitrate) and it ends up in the nearby coastal waterway, which is a problem because many of the estuaries around the world are experiencing problems with eutrophication as a direct result of overloading of nitrogen.
Although some difficulties can arise with septic systems, there are some simple practices that can prevent common problems. For example, groundwater pollution and surfacing of untreated or poorly filtered effluent from a septic tank system can be prevented by ensuring that excessive amounts of water are not allowed to enter or flood the drainfield. Reduced production of wastewater, or water conservation , is recommended to prevent system overload. Water from roof drains, basement sump pump drains, and other rain water or surface water drainage systems should be directed away from the absorption field. The functioning of the system can also be damaged by the addition of such materials as coffee grounds, wet-strength towels, disposable diapers , facial tissues, cigarette butts, and excessive amounts of grease, which can clog the inlet to the septic tank, or if carried out of the septic tank, may impede drainage of wastewater in the soil absorption field. The septic tank should be pumped more frequently if a garbage disposal is used.
Groundwater pollution can also be caused by the addition of hazardous chemicals to the septic tank system, which may be transported through the system to the ground water without removal or treatment in the system. Hazardous chemicals may be found in such commonly used products as pesticides, solvents, latex paint, oven cleaners, dry cleaning fluids, motor oils, or degreasers.
Siting requirements for a soil absorption system depend on the amount of daily sewage flow and site conditions that affect the ability of the soil to absorb, treat, and dispose of septic tank effluent without creating a public health hazard or contamination of ground or surface waters. If a proposed site is located on a gently sloping surface that is not susceptible to flooding , and has at least 6 ft (1.8 m) of well-drained, permeable soil, with a low content of coarse fragments, only a minimum area is required for installation of the absorption drainfield. However, area requirements increase as slope increases and soil permeability, depth of suitable soil, and depth to groundwater decrease. However, at some sites, the soil type, depth, or site topography may not be suitable for the use of the conventional soil absorption drainfield, and modifications or additions to the conventional system may be required.
Siting requirements usually also include that sufficient area be reserved for installation of a repair system if the original system fails. This additional area should meet all requirements of the original soil absorption system and should be kept free of development and traffic. The disposal site should also be located at safe distances from ground water supply sources, wetlands , lakes, streams, drain tile, and escarpments where seepage may occur, as well as set back from buildings and roads that may interfere with the proper operation of the system.
A common problem encountered in drainfields is excessive development of a clogging mat at the interface between the gravel bed in the absorption trench and the underlying soil due to the accumulation of organic materials and the growth of microorganisms. The development of a clogging mat is a natural process and at some sites, such as those with sandy soils, may be desirable to slow the movement of the water through the sandy materials to allow for treatment of the wastewater to occur. However, excessive development of a clogging mat may result in formation of anaerobic soil conditions, which are less conducive to degradation of the organic waste materials, as well as in surfacing of the effluent or backing up of the wastewaters into the residence.
At sites with limiting features or where problems with the excessive development of clogging mats occur, landowners may modify or enhance the conventional soil absorption drainfield to increase its performance. Examples of such alterations include:
- Alternating drainfields: The wastewater effluent from the septic tank is directed to two or more separate drainfields; a section of the drainfield receives effluent for six to 12 months and then is allowed to rest for a similar period of time. The clogging mat that forms at the soil/trench interface dries and is oxidized during this dormant period, thus increasing the expected life span of the section and restoring aerobic soil conditions.
- Pressure distribution: A pressure head is created within the distribution pipe system in the drainfield. This is usually achieved by using a dosing tank and a pump or a siphon and yields uniform distribution of the wastewater throughout the system. The pressure distribution system differs from a conventional system because approximately the same amount of effluent flows out of each hole in the distribution pipes, rather than a concentrated amount of effluent flowing by gravity into a few localized areas. The effluent is discharged periodically to the drainfield so that a dose/rest cycle is maintained, and in turn allows for the wastewater to be absorbed into the underlying soil before additional effluent is added to the drainfield. The dose/rest cycle may also slow down the formation rate of the clogging mat that naturally occurs over time. Most commonly, landowners use these types of pressure distribution systems (which also employ vegetation to help evaporate liquids): (a) low-pressure subsurface pipe distribution system, which consists of a network of small-diameter perforated plastic pipes buried in narrow, shallow trenches; (b) mound system, in which wastewater is pumped to perforated plastic pipe that is placed in a vegetated sand mound constructed above the natural surface of the ground; and (c) evapotranspiration bed, in which a vegetated sand bed is lined with plastic or other waterproof material.
A wide variety of onsite septic systems exist from which to select the most appropriate for a specific site. The primary criterion for selection of an appropriate system is the ability of the system to protect public health and to prevent environmental degradation at the specific site.
Experimental septic treatment systems now run in private solar-powered greenhouses that house complete ecosystems of plants and fish to create a natural cleansing action that can process wastewater into clean drinking water in as little as 12 days. Systems were cropping up sparingly by the end of the 1990s, often in milder climates where the solar source is plentiful. One self-contained eco-system proved capable of servicing an entire small town at a development cost as low as $300,000. The potential for third world implementation is under consideration, and prototype systems developed in the early part of the decade were implemented even in the harsh New England climate , where water pollution is particularly severe.
Serious ecological threats in the northeast led to a series of comprehensive changes to the Massachusetts State Environmental Code (Title V) beginning in 1995. The state set restrictions on the siting and maintenance of septic tanks and systems and imposed requirements for state-certified testing of systems whenever a property title transfers. The code changes were prompted by dangerous levels of water pollution making it unsafe to swim and fish in many of the state's lakes and bays and contributing to toxins in the drinking water. The legislation requires that detailed engineering plans be included as part of construction permits. Initial inspection of existing systems is required combined with regular inspections of certain private and public systems. Follow-up inspections are mandated coincident with any changes to building occupancy. Permissible septic system additives are clearly defined.
Some developers hope to eliminate the septic tank altogether with high-tech waste systems allowing an entire sewer system to be housed inside a single-family dwelling, including a 91-gal (344-l) wastewater tank that operates on 240 volts of electricity. The system, with an ability to pump wastewater vertically or as far as 2 mi (3.2 km) horizontally, can easily follow the contour of the landscape. The system is easier and cheaper to build because it eliminates the deep digging associated with septic tank system installation and offers the advantages of a completely contained sewer system.
[Judith Sims ]
Design Manual: Onsite Wastewater Treatment and Disposal Systems. Cincinnati: U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, 1980.
National Small Flows Clearinghouse. So...Now You Own a Septic Tank. Morgantown, WV: West Virginia University, 1990.
Northern Virginia Planning District Commission. A Reference Guide for Homeowners: Your Septic System. Morgantown, WV: West Virginia University, National Small Flows Clearinghouse, 1990.
Small Wastewater Systems: Alternative Systems for Small Communities and Rural Areas. Washington, DC: U.S. Environmental Protection Agency, Office of Water Program Operations, 1980.
septic tank, underground sedimentation tank in which sewage is retained for a short period while it is decomposed and purified by bacterial action. The organic matter in the sewage settles to the bottom of the tank, a film forms excluding atmospheric oxygen, and anaerobic bacteria attack the solid matter, causing it to disintegrate, liquefy, and give off gases. The gases are discharged from a vent and the liquids overflow through an outlet into a disposal field where they can leach into the soil. Here aerobic bacteria purify the liquid. The Imhoff septic tank, an improvement over the ordinary septic tank, is still used in the United States; it is a two-story structure with the upper compartment used for settling the sewage, the lower one for the anaerobic disintegration of sludge. A sloping floor enables solid material to slide to the lower compartment, where, since the sludge is separated from the material in the sedimentation compartment, the action is more rapid. A cesspool is a simpler underground structure that allows the liquids to leach directly into the soil while retaining the solids. The solids are not as efficiently decomposed as in a septic tank and more frequent cleaning is necessary. Also, as the effluent is likely to contain more coliform bacteria than that of a septic tank, cesspools pose a greater threat to water supplies. Septic tanks and cesspools are usually used in rural areas. For urban sewage-disposal systems, see sewerage.