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7. There shall be a layer of at least six but no more than 24 inches of <br />filter' material in the bottom of the trenches and beds. <br /> <br /> 8. Where dispesal trenches are constructed within ten feet of trees six <br />inches or larger in diameter, or dense shrubbery, or where it can reasonably be <br />anticipated that such vegetation will be present during the expected life of the <br />system, at least 12 inches of filter material shall be placed beneath the distribution <br />pi pe. <br /> <br /> 9~ Distribution pipe used in trenches or beds for gravity flow distribution <br />shall be at least four inches in diameter and constructed of sound and durable <br />material not subject to corrosion or decay or to loss of strength under continuously <br />wet conditions. <br /> <br /> (a) Agricultural drain tile shall be in 12-inch lengths and laid with 1/4 <br />inch open joints on grade boards. All open joints shall be protected on top by <br />strips of asphalt treated building paper at least ten inches long and three to six <br />inches wide or by other acceptable means. <br /> <br /> (b) Perforated plastic pipe shall be laid with one row of perforatons on the <br />bottom. PerforatiGns shall be at least 1/2 inch in diameter and spaced no farther <br />than 36 inches apart. <br /> <br /> (c) Other devices may be used to distribute sewage tank effltient over the soil <br />treatment area upon approval of the administrator. <br /> <br /> 10. The distribution pipes shall be laid level or on a uniform slope away <br />from the distribution device of no more than four inches per 100 feet. <br /> <br /> Il. The distribution pipes in beds shall be unifo~lly spaced no more than <br />five feet apart and not more than 30 inches from the side walls of the bed. <br /> <br /> 12. l'he filter material shall completely er:case the disposal pipes to a depth <br />of at least two inches. <br /> <br /> 13. The filter material shall be covered with untreated building paper or a <br />two-inch layer of hay or straw or similar, approved permeable materials. <br /> <br /> 14. The trenches or beds shall be backfiiled with the excavated material and <br />slightly crowned above finished grade to allew for settlin§. The backfill shall not <br />be compacted to a density greater than that of the original soil. <br /> <br /> 15. The minimum depth of cover over' the distribution pipes shall be at <br />least eight inches. The maximum depth of cover over the distribution pipes shall <br />be no more than 36 inches and ~referably no more than 24 inches. <br /> <br />16 <br /> <br /> <br />