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A mound system is an alternative to the traditional rural septic system drain field. The mound system is an engineered drain field used in areas where septic systems are more prone to failure due to having extremely permeable or impermeable soils, soil with shallow cover over porous bedrock, and soils that have a high seasonal water table.
Contents
The primary waste liquids cleaning and purification action in a drain field is performed by a biofilm in the loose fill surrounding the perforated drain tile. If the soil permeability is too low, liquid is not absorbed fast enough, resulting in surface ponding of unsanitary liquids. If the soil permeability is too high, or is exposed to fractured bedrock, the wastewater quickly penetrates down to the water table before the biofilm has time to purify the water leading to contamination of the aquifer. In either situation, the mound system provides an ideal habitat for the biofilm and has the correct permeability to assure slow absorption of effluent into the mound before exiting as purified water into the surrounding environment.
History
The mound system was originally designed in the 1940s by the North Dakota College of Agriculture. It was known as the Nodak Disposal System. In 1976 the University of Wisconsin studied the mound systems under the university's Waste Management Project. This project published the first design manual indicating the appropriate site conditions and design criteria for mounds. In 2000 a new manual was released.
Design
The mound system includes a septic tank, a dosing chamber, and a mound. Waste from homes is sent to the septic tank where the solid fraction settles to the bottom of the tank. The effluent is sent to a second tank called a dosing chamber, from which the effluent is distributed to the mound at a metered rate (in doses). Wastewater is partially treated as it moves through the mound sand. Final treatment and disposal occurs in the soil beneath the mound. The mound system can also better handle the effluent since it doesn't all come into the mound at once allowing it to clean the effluent better which helps to keep the system from failing.
The absorption mound is built in layers at different depths. The depths of these layers are determined by the depth of the limiting layer of the soil, which may be a seasonal water table, bedrock, fragipan, or glacial till. Standards created by Ohio State University state that 24 inches of natural soil should be above the limiting layer in the soil. A layer of specifically sized sand is placed on top of the natural soil so that the natural soil and sand reach a depth of 4 feet. The distribution pipes that are fed by the dosing chamber are placed on top of the sand in layers of gravel. Then construction fabric and soil are placed on top of the gravel to help keep the pipes from freezing. The top layer of soil also allows the mound to be planted with grass or non-woody plants in order to control erosion
When installing a mound system, the soil in the area where the mound is to be placed will not be compacted or disturbed. Any trees that are on the area are cut away at ground level, and the roots and stumps left in place. The surface of the area for the mound is then roughened up with a chisel plough. This prepares the area for the sand. Work is done from upslope of the mound area so that the ground down slope of the mound does not get compacted. Tyler tables are used to help determine the area of a mound.
Time dosing is an important aspect in the functioning of the mound system. Research done by Darby on sand filters showed that short frequent doses of effluent onto sand filters with orifices that are closely spaced helps to improve effluent quality. By contrast, demand dosing releases large amounts of effluent at once, which passes through the sand very rapidly. This does not give the biota the proper amount of time to clean the effluent.