12-12-1987 Planning Packet - Laserfiche WebLink

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K-l SEWAGE TRE/VIMENT MOUNDS FOR PROBLEM SOILS Suitable soil provides excellent treatment of sewajjc tank effluent. The natural top soil should be utilized for treatment wherever possible. The top layer of a clay soil usually has a more rapid percolation rate than the underlying subsoil. Sandy soils have more organic matter and a finer texture in the top soil layer chan in deeper layers. Soils with adequate organic matter and calcium carbonate (limestone) are good adsorbers of the nutrient phosphorus found in sewage tank effluent. Some soils do not have a percolation rate in the range of 0.1 to 60 minutes per inch which is necessary for the adequate treatment of sewage and for the proper operation of the soil treatment system. In other soils, there is a seasonal saturation at depths closer than 3 feet to the ground surface such that adequate vertical separation of the soil treatment unit is not possible under "natural" conditions. Soils with a "pan" layer that restricts downward movement of liquid, or with bedrock, cither fractured or impermeable, have problems for adequate treatment and proper operation of the soil treat ment unit. One of the first solutions that should be explored for problem sites is to consider pumping sewage tank effluent to an area where the soil is suitable for the installation of a properly designed soil treatment system. Where 3 1/2 to 4 feet of soil cover exists above saturated conditions or "pan" conditions, trenches can be excavated into the natural soil. The trenc!>es must be constructed so that a 3-foot vertical separation initially exists between the trench bottom and the restricting layer. Then aerobic condi tions will exist in the soil under the trench rock and the sewage will be effectively treated. Thus, the excavation depth can be no greater than 6 to 12 inches into the natural soil (see page D-5). After the trench rock and distribution pipe are installed, soil fill is moved into the area to jirotect the trenches from physical damage. There should be at least 6 inches of soil fill above the top of the trench rock. Thus, 12 to 18 inches of fill above natural ground level mas be required to finisli tlac soil treat ment unit. It is recommended that the fill soil be a loamy sand to avoid undue compaction during placement. The area should be graded to provide for surface drainage so that runoff water docs not pond in the area of the trenches. A grass cover should be established as soon as possible. Such an installation will blend into the landscape and it usually will not be readily apparent that the "mounded" area contains the soil treatment unit. However, if the soil percolation rate is either too fast or too slow or barrier layers such as seasonally saturated soil or a "pan" exists closer than 3 feet, hauling fill and constructing a mound or "berm" may be a reasonable and cost-effective alternative to a standard soil treatment unit. A properly designed elevated bed can function properly and adequately treat sewage tank effluent on a long-term basis. A properly constructed mound should be placed on ;it least 24 inches of naturally-occurring or fill soil which is not seasonally saturated and whicli has a percolation rate no slower than 120 minutes per inch. The limit on the slowness of the percolation rate is to provide for adequate dov/nward and lateral movement of liquid. If tlie underlying, natur.il soil is extremely coarse textured, there must be a soil layer in the 24-inclt depth that is at least 12 inclu*s thick and has a pc>rcolation rate lower