m information provided by M.G. Teske, Illinois Department of Public Health; D.K. Sauer, University of Wisconsin, and the Manual of Septic Tank Practice. perforated PVC. The top half of the joints in the drainage tile should be protected (Figure 5.12), and if perforated PVC is used, the pipe should be placed holes down. The underdrain pipe should slope one-eighth inch per foot toward a dosing chamber or other disposal system. There should be one drainage pipe for each distribution pipe, and the high end of the drainage pipe should be vented to the surface. Dosing of both the uncollected and collected subsurface filters is recommended. A dosing frequency of three or four times a day is best. Elevated sand filters, both intermittent and recirculating, will accept much higher loading rates than the subsurface filters. This is not because a very coarse sand is used - the sand is the same as used for subsurface filters-but because a strict maintenance schedule allows periodic correction of the effects of overloading. Table 6.8 shows the approximate loading rates to be used for the intermittent and recir- culating sand filters. The loading rates shown for the recirculating filter are for flow from the septic tank; the recirculating flow is not counted in sizing the filter surface. The intermittent sand filter has two filter units, each sized to accom- modate the full flow from the septic tank (Figure 6.26). If you chose to use an aerobic treatment plant to reduce the potential for odors, then you can double the loading rates shown in Table 6.8. The filter surface should allow for the accumulation of around 2 in. of sewage effluent each time the filter is dosed. A dosing frequency of two times per day is suitable for medium (0.25 mm) sand, while a dosing CONVENTIONAL SYSTEM MODIFICATIONS Inlet 18" min. Splash Plate 24-30" Pea Gravel Underdrain Gravel Figure 6.26 Intermittent sand filter. frequency of about seven times per day is recommended if 1-mm diameter sand is used. When one filter section is clogged to the degree t