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0615 Ringer Subdivision <br />Page 7 <br />installed in these limiting soils, a set of governing criteria <br />have evolved over the last 50 years. Specifically, trench systems <br />are not allowed: 1) where the percolation rate is slower than 60 <br />minutes per inch of water level drop in a percolation test hole; <br />and 2) where the highest known water table or indicated saturated <br />soil layer is less than 3' below the bottom of the trench. <br />In most cases, these same soils have a varying amount of topsoil <br />which is usually "loamier" and will accept and treat a limited <br />amount of effluent. <br />The pressure -mound system is designed specifically to use this <br />loamy topsoil layer for effluent treatment anal disposal. In <br />brief, a 12-24" layer of sand (placed over the natural roughene3 <br />topsoil) is fed effluent from a pressure distribution system in <br />an overlying rock bed. The biomat "valve" forms at the rock/ <br />sand interface which is 12-24" above the natural soil. Because <br />of the pressure distribution, an unsaturated air/effluent contact <br />is maintained as treatment occurs in the sand bed. This treated <br />effluent is then dispersed into the topsoil and allowed to move <br />laterally through the topsoil,possibly even past the boundaries <br />of the mound system (yet still below the surface) until it <br />eventually seeps, downward or evaporate:; over a wide area. <br />It would be poor planning for Orono to allow installation of <br />standard trench drainfields in ::oils which have high-water table <br />characteristics, since many systems likely would eventually fail, <br />as was the case in 1•iedina-'.orninrjside. Since Orono has shown <br />a strict desire to limit- the c�:t-ension of sanitary sewers, it <br />appears reasonable to allow (and, in fact, prornote) the use of <br />alternate on -sit,- sewage I:reatmcnt methods whore devclopncnt <br />pressures persisc. ldound have b;:en installed over the <br />la:;t 1.0 to IS year:;around the country as an alternative to <br />trenches. In Orono, prc:;:;ure distribution mounds were first <br />installed in 1970. To date, the City'has 13 pr,--usure mounds <br />in service, with no recorded failures. <br />In fay opinion, under shecificd :.ite conditions, mound sy::tems <br />are a safe, sanitary, reliable method of sewage treatment. <br />