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How Septic Systems Work

Septic systems provide a natural method of treatment and disposal of household wastes for those homeowners who are not part of a municipal sewage system. They are designed to treat wastewater for a specific site. Proper treatment of wastewater reduces health risks as it is unhealthy for humans, pets, and wildlife to drink or come in contact with surface or ground water contaminated with wastewater. Inadequate treatment of wastewater allows bacteria, viruses, and other disease-causing pathogens to enter groundwater and surface water. Hepatitis, dysentery, and other diseases may result from bacteria and viruses in drinking water. Disease-causing organisms may make lakes or streams unsafe for recreation. Flies and mosquitoes that are attracted to and breed in wet areas where wastewater reaches the surface may also spread disease.

Many cleaning products and chemicals used in the household can be toxic to humans, pets, and wildlife. If allowed to enter a failing septic system, these products may reach groundwater, nearby surface water, or the ground surface.

In the soil treatment portion of the system (drain field or mound), bacteria and viruses in the sewage are destroyed by the soil and naturally-occurring microscopic organisms. Nutrients are absorbed by soil particles or taken up by plants. However, these processes only work in soil that has air in it. The soil cannot be saturated with water. Near lakes, streams, and wetlands soil conditions may be saturated. When the soil is saturated, biological breakdown will be incomplete and nutrients will move much greater distances, sometimes hundreds of feet from the drainfield or mound, and possibly into surface water. Even systems that appear to be working well or that are in compliance with local design and installation codes may allow nutrients or bacteria to reach the ground or surface water.


Components of the Septic Tank

Septic Tank

The septic tank is the first step of the wastewater treatment process. The septic tank is a solid tank designed specifically to accept all wastewater from the home. Some installations may have two tanks in a row or one large tank with two compartments. Several tank designs are available but all tanks should have inlet and outlet baffles, inspection pipes, and a manhole for cleaning.

A septic system works by allowing waste water to separate into layers and begin the process of decomposition while being contained within the septic tank. Bacteria, which are naturally present in all septic systems, begin to digest the solids that have settled to the bottom of the tank, transforming up to 50 percent of these solids into liquids and gases. When liquids within the tank rise to the level of the outflow pipe, they enter the drainage system. This outflow, or effluent, is then distributed throughout the drain field through a series of subsurface pipes. Final treatment of the effluent occurs here as the soil absorbs and filters the liquid and microbes break down the rest of the waste into harmless material.

Septic systems cannot dispose of all the material that enters the system. Solids that are not broken down by bacteria begin to accumulate in the septic tank and eventually need to be removed. When the holding tank is not pumped out frequently enough, the solids can enter the pipes leading to and from the tank. This can cause sewage to back up into the house or cause the drain field to fail as the pipes and soil become congested. These problems are often costly to fix, pose a danger to public health, and are a significant source of water pollution. Seepage from inadequate or failing septic systems can contaminate both ground and surface waters. Malfunctioning septic systems are currently a leading cause of groundwater pollution.


All septic tanks should be checked on an annual basis to ensure they are working properly. Baffles, specially designed pipes which allow the sewage into and out of the tank, need to be checked to ensure they are not worn or damaged. The level of sludge (the material that accumulates at the bottom of the tank) needs to be measured to determine when the system should be pumped. Homeowners should routinely have their septic tanks pumped out every three to five years. However, the frequency with which your tank needs to be pumped may vary depending on the size of the tank and number of people in the household.

Use a Licensed Pumper and Hauler

Hire a licensed pumper and hauler to service your system. This is primarily for homeowner safety, since septic systems produce harmful fumes and can be dangerous to clean for those not adequately prepared. Using qualified professionals also reduces the chance of improper monitoring or possible damage to the system during pumping. Contact either your local health and sanitation department or check the Yellow Pages in your area phone book to find qualified pumpers. It is much less expensive to maintain a system than to repair or replace it later.

Use of Additives

The use of additives to help maintain the system is generally not necessary. They will not extend the amount of time required between pumping. The most effective method of breaking down the solid matter in the sewage is allowing the naturally present bacteria to digest it.

Other Ways to Prolong the Life of Your System

There are many ways to prolong the life of your septic system and lengthen the period between pumpings. Household water use directly controls how quickly waste travels through a conventional system. Wastewater that enters the tank requires time to allow the solids to settle to the bottom. The higher the volume of water that is introduced to the system, the less opportunity the wastewater has to settle in the holding tank and the less opportunity the bacteria have to break down the solids. Therefore, limiting the use of water in the home will go far in prolonging the life of the system.

Watch What Goes Down the Drain

Controlling what goes into the water that enters the system is just as important as reducing the amount of water that flows into the system. Never dispose of toxic or hazardous chemicals by dumping them down the drain as they have the potential to contaminate groundwater. Refrain from putting any plastic, cloth, or unnecessary paper products into the sewage system. Avoid using garbage disposals as they accelerate the accumulation of solids in the holding tank. Especially avoid putting any grease or oil in the disposal or drain. These can clog pipes and drain field soil and damage your system.

Soil Treatment

All septic systems include the same basic plumbing and septic tank components. Final treatment of wastewater occurs in the soil. Uncompacted, unsaturated, undisturbed soil must surround the soil treatment system. This system may be a series of trenches or a mound. Soil treatment kills disease-causing organisms in the sewage and removes nutrients. There are millions of naturally-occurring beneficial microscopic organisms in every tablespoon of soil. These complete the sewage treatment process.

The beneficial bacteria in the soil need air to live. Therefore, a zone of unsaturated soil must be present below the drainfield for complete treatment. In many areas a minimum of three feet of unsaturated soil below the drainfield is the recognized standard. Some local units of government have established more strict requirements, such as four feet of separation from saturated soil.

The biomat is a thin layer of fine solids, dead bacteria, and soil bacteria that forms where the sewage meets the soil. This biomat layer regulates how fast liquid passes out of the trench or bed into the soil so the soil beneath the trench remains unsaturated. Once the wastewater is through the biomat layer and three feet of unsaturated soil, harmful pathogens have been destroyed.

Site conditions and local requirements determine the soil treatment system for each site. If there is three feet of separation from the bottom of the drainfield trench to saturated soil, the least expensive distribution and soil treatment system is gravity flow to a simple trench system. If there is not the required separation for a trench, a mound is required. A mound system is an elevated drainfield built with clean sand. There are many small variations in design, but all trenches and mounds accomplish the same treatment function.

Soil Treatment System (Drain Field)

Septic tank and drain field
Typical Drain Field Design
Common terms for the soil treatment system are: drainfield, mound, seepage bed, leach bed, and soil absorption field. The soil treatment unit is where the final treatment and disposal of the septic tank effluent takes place.

A properly designed and installed soil treatment system will destroy all disease-causing pathogens and filter out the fine solids contained in the septic tank effluent. Phosphorus will be adsorbed by (attached to) soil particles, and nitrate-nitrogen may move through the soil with the water.

In the summer, a shallow drain field trench supplies water and nutrients to grass and trees. The nutrients that remain in the downward percolating water will be either changed to gas by soil bacteria or diluted. Nitrates in drinking water are rarely a problem with a soil treatment system when the nearby well is deeper than 50 feet and has a sealed casing.

The two most commonly used types of soil treatment (drain field) units are trenches and mounds.

Trench: Drain field trenches effectively treat liquid flowing from the septic tank. They are the most economical to install and are preferred when possible. A drainfield trench is a level excavation 18 to 36 inches wide and up to 100 feet long. The trench contains a perforated pipe in a bed of 3/4-inch to 2-inch diameter rock covered by natural or synthetic permeable fibers. Some soil treatment systems use large plastic tubing or some other chamber wrapped with fabric in the trench in place of rock. A 6- to 12-inch deep layer of topsoil covers the trench. Sewage flows through the holes in the distribution pipe, to the rock (or tube), through the biomat, and into the soil. Bacteria and fine sewage solids are removed or destroyed in this process.

The trench system may be laid out in one of many configurations to allow for the necessary square feet of surface. There are often inspection pipes on one or both ends of the pipes. These can be cut off at ground level and capped for easier lawn maintenance. The ground surface level of the soil treatment area should always be level or slightly raised above the surrounding ground to avoid excess rainfall flooding the system.

Mound (Elevated Seepage Bed): A sewage treatment mound is a seepage bed raised with clean sand to provide adequate separation between the wastewater in the mound and the saturated soil or an impermeable hardpan of soil or bedrock beneath. The mound is carefully constructed to provide adequate treatment of sewage. It is equally as effective in treating sewage as a trench system as long as it is properly constructed and operated, and the septic tank is maintained correctly. The mound system, as illustrated in Figure 3, has a pressurized distribution system of 1-1/2 or 2-inch perforated pipe in a layer of small rock. A layer of sand covers the rock. The mound is covered with topsoil and planted to grass. The grass should be mowed regularly.

Distribution System
Each site has a unique shape and slope. The soil type, percolation rate, water volume to be treated, and other factors determine how large an area is needed to properly treat sewage. To provide the necessary area, the design may be a series of many pipes, or "stepped" down a slope.

The distribution of effluent into the soil treatment system is accomplished using drop boxes and distribution boxes. The covers of either kind of box can be removed for inspection and cleaning. All pipes to the drainfield trench or seepage bed are solid with sealed connections.