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✓'SECTION E - MOUNDS <br />A sewage treatment mound is a seepage bed elevated by soil fill to provide <br />an adequate separation distance between the rock layer in the mound and the <br />barrier layer such as saturated soil conditions or bedrock. The mound must <br />be carefully constructed to provide adequate sewage treatment. Mound <br />failures have usually been traced to improper construction practices. <br />Important factors in the design and successful operation of a sewage <br />treatment mound are location, size and shape, soil surface preparation, <br />construction procedures, distribution of effluent, and dosing quantity. <br />1 and 2 list conditions where a mound can successfully be used and <br />caution on wl.c use of large mounds with collector sewage systems. While <br />mounds sized to treat the effluent from one or two single-family residences <br />will be successful, large-scale mounds have and will continue to fail to <br />properly treat sewage because of incorrect design. With slowly permeable <br />soils, water table mounding will take place within the mound itself, <br />causing hydraulic failure. With rapidly permeable srils, the concentration <br />of nitrates moving downward may contaminate groundwater aquifers. <br />Properly designed and constructed sewage treatment mounds are an effective <br />method of onsite sewage treatment. Recent research at the Small-Scale <br />Waste Management Project indicates that residential mounds utilizing <br />pressure distribution will have *4*1 percent less nitrates percolating <br />downward than a standard subsurface trench system. Sufficient mounds have <br />been installed in Minnesota and elsewhere to prove that the mound treatment <br />system should be an accepted technology. Sewage treatment mounds should <br />not be considered alternative treatment systems, but rather preferred <br />treatment systems in many instances. <br />Pages 3 and *1 show a perspective view as well as a cross-section and plan <br />view of a mound. Mound construction begins with the layer of clean sand <br />which must be at least 1-foot thick. The top of the clean sand layer must <br />be level as must the rock layer which is placed upon the clean sand layer. <br />Distribution pipes, preferably for pressure distribution, are placed in the <br />rock layer which is covered with a layer of hay or straw plus a layer of <br />untreated building paper. These layers may be replaced by a single layer <br />of a permeable synthetic fabric. The purpose ot the layer over the clean <br />rock is to prevent the soil backfill from filtering dawn into the rock and <br />occupying the void spaces of the rock. A loamy sand cap, which is 6 inches <br />thick at the side and 12 inches thick at the center is placed over the rock <br />layer. The purpose of the loamy sand is to avoid undue soil compaction so <br />that the pore spaces are maintained, and soil air and moisture can move <br />freely. The entire mound area is covered with a 6-inch layer of top soil <br />upon which a grass cover should be established as soon as possibl . <br />The contractor is primarily responsible for proper mound construction. <br />There arc three layers, each of which if not tre'ted properly, can create <br />problems with the hydraulic performance of the mound. The first layer is <br />the natural or fill soil upon which the mound is to be constructed. If <br />this soil is wetter than the plastic limit, or if considerable construction <br />-i-