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Black Oak Estates Page No. 2 Design An "average" shallow percolation rate of 80 min./in. was selected as conservatively representative of the surface clay soils throughout the development. The Required Absorption Area chart from The Manual of Septic Tank Practice terminates at a percolation rate of 60 minutes per inch. However, by requiring"raking"of trench and pit walls and construction inspection to eliminate the potential for soil smear, it is reasonable to extrapolate the chart to the design rate of 80 min./in. This produces a required absorption area of 360 square feet per bedroom. A 24 inch wide leach line with 12 inches of gravel below the pipe has and average of 3.0 square feet of absorption area per lineal foot of trench. Therefore the following is the computed length of leach line required for a"shallow"only system 360/ 3 = 120 ft. of leach line required per bedroom. It is proposed, however, to utilize both shallow leach lines and dry wells to dispose of effluent. Therefore, shallow leach lines will be used to dispose of approximately 40% of effluent. This would require 50 feet of leach line per bedroom. By assuming a sewage generation of 100 gal/day/bedroom, this would leave 60 gal/day/bedroom to be disposed of in the dry wells. Part of the concern of using dry wells is the minimal de-nitrification which they provide. Therefore it is proposed to utilize a 5.0 foot long by 2.0 foot wide pea gravel filter at the end of each leach line in order to provide some additional de-nitrification of the effluent and to also help consume some of the suspended solids. By using the filter, it is felt that the"sewage factor" can effectively be doubled to 8%. Also, 42 inch diameter dry wells are proposed to make it easier to scarify the sides of the pits. The following shows the low, average, and high sewage disposal rates for 42 inch diameter by 25 foot deep dry wells based on the results of our deep pit percolation testing: Low = (446 gal/day) x(0.08) x (1.17) = 42 gal/day High = (17,928 gal/day) x (0.08) x (1.17) = 1680 gal/day Average = (3375 gal/day) x (0.08) x (1.17) = 316 gal/day Since it is desired to dispose of 60 gal/day/bedroom in the dry wells, then by proposing at least 0.6 dry wells per bedroom, each dry well would be required to dispose of approximately 100 gal/day, which is 3 times below the average percolation test reading. rpE RSO � 1 > w j W O � r �SSOG♦