e liner is not needed. Trenches in a native soil more coarse than a sandy loam should be lined on the side, while trenches in loam need not be lined. Proper sand size is critical to the satisfactory operation of any sand ONSITE WASTEWATER DISPOSAL Distribution Pipe Coarse Sand Gravel Liner Fine Sand Figure 6.23 Sand filter in highly permeable soil. filter. This is doubly true for subsurface sand filters since system repair and sand replacement are expensive and time-consuming. Refer to Figure 6.24 to help visualize the following description of sand size and uniformity analysis. The filter sand should be clean, coarse sand, all passing a 4-mesh- per-in. screen (U.S. Standard Sieve No. 3 1/2). The effective size of the sand should be between 0.25 (Sieve No. 60) and about 1.0 mm (Sieve No. 20) and preferably be between 0.4 (Sieve No. 40) and 0.6 mm (Sieve No. 30) in diameter. Effective size means the screen size through which passes no more than 10% by weight of the sand sample. The effective size affects the allowable loading rate, the clogging rate, and the effluent quality. Another measure of sand quality is its uniformity coefficient. This is the ratio of the screen size which passes 60% by weight of the sand to the screen size which passes only 10% of the sand sample. For example, if 60% of the total weight of your samples passes through a 1.0-mm mesh screen but only 10% passes a 0.4-mm mesh, then the uniformity coefficient would be 1.0 divided by 0.4, or 2.5. Normally, the uniformity coeffi- cient for sand filter sand should be no greater than 4, and 3.5 is preferable. If you can find some graded sand samples at a university extension service, the Soil Conservation Service Office, or a sand CONVENTIONAL SYSTEM MODIFICATIONS No. 3 1/2 1.0 mm No. 20 0.6 mm No.30 0.4 mm No. 40 0.25 mm No. 60 Figure 6.24 Effective size and uniformity of sand. 160 ONSITE WASTEWATER DISPOSAL and gravel operation, look at and feel the sand grades that would be suitable filter sand. Then you can judge the adequacy of the sand you inte