1985-12-17 Septic System Design Report

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i615 Ringer Subdivinion .Page 7 installed in these limiting soils, a :yet of governing criteria have evolved ever the last 50 years. Specifically, trench system are not allowed: 1) where the percolation rate is slower than 60 minutes per inch of water level drop in a pe-colation test holes and 2) where the highest 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 "loamior" and will accept and treat a limited amou• t of effluent. The pressure -mound system is designed specifically to use this loamy topsoil layer for affluent 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. Tho 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 diapnrsed into the topsoil and allowed to move laterally through the topsoil,possibly even past the boundaries of the mound systcm (yat still below the surface) until it eventually seeps downward or evaporates over a wide area. 7t would be poor planning for Orono to allow installation of standard trench drainfields in soils which have high-water table characteristics, since many systc= likely would eventually fail, as was the case in Medina-tiorningside. Since Orono has shown a rtrict desire tc limit tho extension of sanitary sewers, it n,-piars reasonable to allow (and, in fact, pro-otc:) the use of r'.".-csrnato on -site sewage treatment mothod3 whore development pressures persist. Mound syntemp have boon installed over the last 10 to 15 years around tho co,intry no an alternative to trcnchea. In Orono, pros3ura distribution mounlo were first installcl in 1978. To date, the .Ci1_y*ha3 13 pressures rioiinds in cervic;,, with no recorded failures. In my opinion, under specified site conditions, mound systems are a safe, sanitary, reliable method of sewage treatment.