The soil treatment system is where the final treatment and disposal of the septic tank effluent takes place.
Common terms for the soil treatment system are: drain field, leach field, seepage bed, soil absorption field and mound. 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.
The two most commonly used types of soil treatment systems are trenches and mounds.
Trench: Drain field trenches effectively treat liquid flowing from the septic tank. A drain field trench is generally 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 with or raised slightly 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.
Final treatment of wastewater occurs in the soil. Uncompacted, unsaturated soil should surround the trenches of the drainfield. There are millions of naturally-occurring beneficial microscopic organisms in every square inch of soil. These organisms complete the wastewater treatment process by killing disease-causing organisms in the sewage and removing nutrients. The beneficial bacteria in the soil need air to live. A zone of unsaturated soil must be present below the drainfield for complete treatment.
The biomat is a black tar-like substance made up of organic material, microorganisms, and bacteria. As a drain field matures the biomat grows along the walls and bottom of the drain field trenches. The billions of living organisms naturally occurring in the biomat act as a filter for wastewater traveling through the drain field. While effluent passes slowly through the biomat the bacteria living there act as a filter digesting much of the organic matter before it passes into the surrounding soil. When drain field soils flood, or the organic balance is upset the biomat can grow into an impermeable barrier causing water to back up into the septic tank and eventually into the household plumbing.