 Don't load the sand too heavily, even on a seasonal basis. Over the filter sand in the trench is placed 6 to 8 in. of 0.75 to 2.5- in. gravel. The 4-in. perforated distribution pipe for gravity-feed or CONVENTIONAL SYSTEM MODIFICATIONS Filter Material Distribution Pipe 6-10" Gravel Liner 20-30" Peg Gravel Gravel Collection Pipe Figure 6.25 Collected subsurface sand filter. the 1- to 2-in. distribution pipe for pressure dosing is embedded in this gravel about 2 in. from its top surface. The slope of the distribu- tion pipe should be about one-sixteenth inch per foot where a dos- ing chamber is not used. If a dosing chamber is used, then the distribution pipe should be level. Over the gravel is placed the filter fabric, hay, or untreated building paper to prevent the backfill from moving into the filter. If you will use more than one filter trench, keep them more than about 10 ft apart. A collected subsurface sand filter (Figure 6.25) is sized the same as the uncollected sand filter trench except that the sizing is unaffected by the percolation rate of the surrounding soil. The collected filter is a bit deeper than the uncollected filter because gravel and underdrain pipes are placed below the filter sand. Both the sides and bottom of the collected filter must be lined with bentonite clay, 18 mil PVC or other suitably impermeable material. In soils containing sufficient clay, compaction may serve as well as a liner. The grade of sand required is the same for the collected subsurface filter as for the uncollected filter. The underdrain system is embedded near the bot- tom of 6 to 8 in. of 0.75 to 2.5 in. crushed stone or gravel with 3 in. of fine (pea) gravel above the coarse gravel to retard the downward migration of the filter sand. The fine gravel could be replaced by a layer of filter fabric. The underdrain pipe can be drainage tile or 4-in. ONSITE WASTEWATER DISPOSAL Table 6.8 Loading Rates for Elevated Sand Filters (gpd/ft2 Effective Sand Size (mm) Intermittent Recirculating NOTE: Loading rates calculated fro