hes again and assure that the surface of the trenches remains convex. A slight mound over the trenches is necessary to promote rainfall runoff. The collected subsurface sand filter is constructed exactly the same as the uncollected filter, except that the trenches must be deeper, their bottom surface adjusted to accommodate the underdrain system, and a disposal system for the filtered effluent must be installed. The collected filter trenches must be lined with 18-mil plastic (lapped and cemented at the joints) or lined with bentonite clay or other impermeable material. Some soils can be compacted to near impermeability. If you want to try this, take a soil sample to a soils expert first to see if it would be suitable. The bottom of the trench can be level in cross-section and sloped away from the manifold or supply line at a grade of 6 in. per 100 ft. With this type of collection system, no underdrain pipe is used, and a container (normally a dosing chamber) is placed at the downslope ends of the filter ONSITE WASTEWATER DISPOSAL trenches to collect the filtered effluent. From there it can be pumped to or allowed to drain to a disposal system. The other type of under- drain system consists of a collection pipe at the low point along a "V"-shaped trench below the filter sand (Figure 6.25), which carries the filtered effluent to a dosing chamber or disposal system. The slope of this trench should be one-sixteenth to one-eighth inch per foot away from the supply pipe or manifold. If several trenches are used, a collection manifold will be necessary. The first step in digging an underdrained filter trench is to contour the trench bottoms to suit the selected underdrain system. Then compact the soil or add the liner. If a plastic liner is to be used over rocky soil, the liner should be protected with an intervening 2-in. layer of sand. In any case, it would be wise to add at least an inch of sand over the plastic liner as well, to protect it from the gravel to be added next. Allow for this extra sand when determining 