tion systems as a guide. If subsurface sand filters are properly designed and use proper materials, dosing is not necessary, although it is recommended. Experience has shown that for whatever reason, gravity-fed subsur- face sand filters tend to provide unsatisfactory performance after a short period of time. Underdrains may or may not be required, depending on the application. Dosing to elevated sand filters is required, and underdrains are almost always necessary, as the sur- face area under the filters is normally very small. Design The uncollected sand filter trench (Figure 6.22) is a bit wider and deeper than a conventional absorption trench. Normally, about 2 ft of filter sand is placed in the trench, followed by 8 in. of gravel embedding the distribution pipe, and the trench is backfilled with a foot or SO of top soil. Therefore, 3.5- to 4-ft deep trenches are stan- CONVENTIONAL SYSTEM MODIFICATIONS Gravel Figure 6.22 Uncollected sand filter trench. dard. The trenches are wide since the horizontal sand surface is the only absorptive surface. There is no sidewall absorption area avail- able. To maximize the sand surface area, the trenches are dug 30 to 60 in. wide. Sand filters wider than 60 in. which have a single distribution pipe may present distribution problems. The edges of the bed may receive too little effluent. Multiple distribution pipes may be placed in a sand filter bed to even out the distribution. If the uncollected sand filter trench is to be used to overcome the lack of cleansing capacity of coarse sand and gravel, then a liner (18- mil PVC or bentonite clay) along the sides is required to prevent the effluent from running across the sand surface and out the side of the trench (Figure 6.23). Another method, less reliable, is to place the distribution pipe in a trough in the sand. If, from Table 3.3 or a percolation test, it appears that the native soil has a percolation rate significantly slower than the filter sand will have after the slime layer has formed (about 6 min per in.), then a sid