Septic systems are used to treat and dispose of relatively small volumes of wastewater, usually from houses and small businesses. Septic systems are also called onsite wastewater treatment systems, decentralized wastewater treatment systems, on-lot systems, individual sewage disposal systems, cluster systems, package plants, and private sewage systems.
According to the U.S. Census Bureau, about 26 million homes (one-fourth of all homes) in America are served by decentralized wastewater treatment systems. The Census Bureau reports that the distribution and density of septic systems vary widely by region and state, from a high of about 55 percent in Vermont to a low of around 10 percent in California. The New England states have the highest proportion of homes served by septic systems: New Hampshire and Maine both report that about one-half of all homes are served by individual systems. More than one-third of the homes in the southeastern states depend on these systems, including approximately 48 percent in North Carolina and about 40 percent in both Kentucky and South Carolina. More than 60 million people in the nation are served by septic systems. About one-third of all new development is served by septic or other decentralized treatment systems.
A typical septic system has two major components: a septic tank and a drain field. Wastewater flows from the house to the septic tank. The tank is designed to retain wastewater and allow heavy solids to settle to the bottom. These solids are partially decomposed by bacteria to form sewage sludge. Oil, grease and light particles float, forming a layer of scum on top of the wastewater. Baffles installed at the inlet and outlet of the tank to help prevent scum and solids from escaping (see Figure 1). Newer septic tanks can have a partial concrete dividing wall in the center, thus making two compartments. This helps ensure the sludge does not get forced out of the baffle into the drain field. Newer tanks can also have two manhole covers, one above each baffle.
A solid pipe then leads from the septic tank to a distribution box where the waste water is channeled into a drain field or leach field consisting of one or more perforated pipes set in trenches of gravel (see Figure 2). Here the water slowly infiltrates (seeps) into the underlying soil. Dissolved wastes and bacteria in the water are trapped or adsorbed to soil particles or decomposed by microorganisms. This process removes disease-causing organisms, organic matter, and most nutrients (except nitrogen and some salts). The purified wastewater then either moves to the ground water or evaporates from the soil. Trench systems are the most common type of system used in new home construction.
An alternative to the common drain field is the Seepage Pit (Dry Well). In this type, liquid flows to a pre-cast tank with sidewall holes, surrounded by gravel (see Figure 3). (Older versions usually consist of a pit with open-jointed brick or stone walls.) Liquid seeps through the holes or joints to the surrounding soil.
Alternative systems use pumps or gravity to help septic tank effluent trickle through sand, organic matter (e.g., peat, sawdust), constructed wetlands, or other media to remove or neutralize pollutants like disease-causing pathogens, nitrogen, phosphorus, and other contaminants. Some alternative systems are designed to evaporate wastewater or disinfect it before it is discharged to the soil or surface waters.
Septic systems that are properly planned, designed, sited, installed, operated and maintained can provide excellent wastewater treatment. However, systems that are sited in densities that exceed the treatment capacity of regional soils and systems that are poorly designed, installed, operated or maintained can cause problems. The most serious documented problems involve contamination of surface waters and groundwater with disease-causing pathogens and nitrates. Other problems include excessive nitrogen discharges to sensitive coastal waters and phosphorus pollution of inland surface waters, which increases algal growth and lowers dissolved oxygen levels. Contamination of important shellfish beds and swimming beaches by pathogens is also a concern in some coastal regions.
EPA has developed Voluntary National Guidelines for Management of Onsite and Clustered (Decentralized) Wastewater Treatment Systems (PDF) (62 pp, 1MB) to assist communities in establishing comprehensive management programs for septic wastewater systems to improve water quality and protect public health. There is more user-friendly information for homeowners who have septic systems on EPA's Septic Systems website, and in particular, on their new SepticSmart Home website.
Most septic system failures are related to inappropriate design and/or poor maintenance. Some soil-based systems (with a leach or drain field) have been installed at sites with inadequate or inappropriate soils, excessive slopes or high groundwater tables. These conditions can cause hydraulic failures and water resource contamination. Failure to perform routine maintenance, such as pumping the septic tank at least every 3 to 5 years, can cause solids to build up and restrict flow into the tank or to migrate into the drain field and clog the system.
Do not put the following items into sink drains or toilets: hair combings, coffee grounds, dental floss, disposable diapers, kitty litter, feminine hygiene products, cigarette butts, condoms, gauze bandages, fat, grease, oil, paper towels, paints, varnishes, thinners, waste oils, photographic solutions or pesticides. Another way of looking at this is that there are only three things that should go down your sink or toilet drains, the three Ps - pee, poop and (toilet) paper.
SepticSmart Home, USEPA
Septic Systems and Their Maintenance, University of Maryland Extension
The National Small Flows Clearinghouse has a Technical Assistance Hotline that can be accessed toll free at (800) 624-8301 or (304) 293-4191. You can also contact the Cooperative Extension Service Office nearest your home for information.