THE LEACHING SYSTEM |
The most common standard leaching system design consists of a series of horizontal trenches (leachlines), 36 inches wide, the bases of which are generally three or four feet below ground surface. A leachbed is simply the joining of the leach line trenches into one large square bed that may serve as a "space saver" in small-constricted lots. Sometimes deep vertical circular holes known as "Seepage Pits" or "Drywells" are also used. A "Seepage Pit" is a six-foot diameter pit lined with pre-manufactured concrete rings that may extend from the ground surface to 40 feet, the depth and size depending on the porosity of the soil. Seepage pit construction is the preferred leaching system. In leachlines, a level perforated pipe is used to distribute the wastewater throughout a rock absorption system where it eventually soaks into the soil particles. Aerobic (bacterial) treatment of the wastewater is accomplished as it flows through the rock and the various soil textures. Leachlines and leachbeds operate on the principle of evapotranspiration as well as percolation due to their close proximity to the ground surface. Because of the principal of evapotranspiration, leach lines and leach beds may not be placed under pavement, driveways, or planted areas requiring frequent watering. Seepage pits rely solely on percolation and are generally favored by industry as these may be installed under paved surfaces. Although septic tanks improve the quality of raw sewage, the effluent is still rich in pollutants and harmful organisms, and may not be discharged directly onto the ground surface, waters or groundwater’s. Effluent penetration in the soil is directly related to its porosity. Effluent percolates at rates that vary with the texture of the soil. Very coarse (sandy) textured soils generally provide smaller size leaching fields and pits because of good porosity characteristics, but may not provide effective treatment due to this porosity. Very fine soil particles (clays) may be too tight to allow wastewater to pass through. Soils feasibility (percolation) tests are conducted to determine the proper porosity for proposed absorption systems and are accomplished by registered civil engineers, certified engineering geologists, or approved registered Environmental Health Specialists.
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