1981-11-27 Septic System Design Report

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#615 Ringer Subdivision Page 7 installed in these limiting soils, a set of governing criteria have evolved over the last 50 years. Specifically, trench systems are not allowed: 1) where the percolation rate is slower than 60 minutes per inch of water level drop in a percolation test hole; and 2) where the highast known water table or indicated saturated soil layer is less than 3' below the bottom of the trench. In most cases, these same soils have a varying amount of topsoil which is usually "loamier" and will accept and treat a limited amount of effluent. The pressure -mound system is designed specifically to use this loamy topsoil layer for effluent treatment and disposal. In brief, a 12-24" layer of sand (placed over the natural roughened topsoil) is fed effluent from a pressure distribution system in an overlying rock bed. The biomat "valve" forms at the rock/ sand interface which is 12-24" above the natural soil. Because of the pressure distribution, an unsaturated air/effluent contact is maintained as treatment occurs in the sand bed. This treated effluent is then dispersed into the topsoil and allowed to move laterally through the topsoil,possibly even past the boundaries of the mound system (yet still below the surface) until it eventually seeps downward or evaporates over a wide area. It would be poor planning for Orono to allow installation of standard trench drainfields in soils which have high-water table characteristics, since many systems likely would eventually fail, as was the case in Medina-Morningside. Since Orono has shown a strict desire to limit the extension of sanitary sewers, it appears reasonable to allow (and, in fact, promote) the use of alternate on -site sewage treatment methods where development pressures persist. Mound systems have been installed over the last 10 to 15 years around the country as an alternative to trenches. In Orono, pressure distribution mounds were first installed in 1978. To date, the City'has 13 pressure mounds in service, with no recorded failures. In my opinion, under specified site conditions, mound systems are a safe, sanitary, reliable method of sewage treatment.