Mound designers base sizing calculations on native ground slope, volume of waste water to be treated and height of the saturated water condition in the soil. Landscapes with high water tables require higher mounds. Soils that require increased amounts of time for a given volume of water to percolate into the soil require mounds to cover a greater area. Drain fields, by most state codes, require 3 feet of unsaturated soil for effective treatment. Mound septic systems produce high quality effluent but are prone to several problems.
Mound septic systems include pressurized lateral pipes for discharge of the effluent. To reduce pump size to pressurize the pipes, small perforations are drilled into the pipes. These perforations are susceptible to plugging by debris washed through the septic tank. Effluent pump screens reduce the possibility of plugging by filtering debris. Recent systems are designed with end line clean-outs that can be opened to pressure jet the lines and remove any debris or plugging.
Freezing has been a problem in mound systems that are underused or that were not covered with enough cover. Since mounds are raised above the ground level, frost can penetrate the soil at multiple angles. Proper sloping of pipes back to the pump tank, weep holes in pressure lines, insulated pipe and at least 12 inches of cover soil reduce potential freezing problems. Establishing grass and insulating the mound with straw in the winter can further reduce freezing problems.
Mounds must be designed with system longevity and maintenance in mind. When liquid is loaded on a specific area at a higher rate than the soil can absorb, the liquid will leak out of the mound. Mounds designed on slopes must be installed so the mound's length distributes liquid across the slope. Liquid from mounds installed incorrectly on a slope will stack on top of each other and eventually overload the soil and fail.
Building mound septic systems occur above the native layer of the soil. Installation of these systems cannot be done by removing sod or material from the site. If material is removed, surface water from rain can collect in this area and overwhelm the percolation rates of the soil. Sod must be broken up prior to placing a sand base over the percolation area. If the sod is not broken up properly, the organic material will rot and potentially seal off the interface between the drain field and the soil, not allowing liquid to move into the subsoil.