pection ports or backing up into the bath- tub drain. The saturated level in the bed should never be above about 6 in. below the sand surface. If you plan to install an overflow into an absorption trench, be sure that the bottom surface (invert) of the overflow pipe is below the invert of the inlet pipe; but just an inch or two lower, since you want to use all the evapotranspiration you can get before the overflow is used. On level sites, this 6-in. maximum inlet depth requirement presents some difficulties. Normally, the house sewer stub-out will be at least a foot below the ground surface. This, plus the slope down to the septic tank, the drop to the septic tank outlet, and the final slope to the ET bed, usually puts the inlet pipe near or below the bottom of the bed. (Mobile homes are not faced with this diffi- culty.) When this is the case, there is no choice but to catch the effluent in a dosing chamber and pump it up into the bed. See the LOW-IMPACT SYSTEMS Figure 7.5 Polyethylene sheeting joints. section in Chapter 4 on dosing chambers for details on design and installation. A single sheet of liner is best, but sheets over 200 ft wide are expensive. So use two layers of 6-mil black polyethylene. Use lock joints 1 ft wide (Figure 7.5) to join the sheets together, and stagger these joints SO one doesn't lie exactly on top of the other. Don't glue the sheets together, because increasing strain on the liner as the bed settles could pull the glued joints apart. The weight of the sand and water on the lock joint should keep it watertight while allowing some movement of the liner. There must be a tight seal at the point where the inlet pipe pene- trates the liner. The pipe may be inserted through a circular hole cut in the liner, and then the liner gathered around the pipe and pressed to it with a stainless steel flexible hose clamp (Figure 7.6). The same technique will work for the overflow pipe if one is installed. Place a 2-in. layer of sand on the iner, and put about 6 in. of gravel on this sand pad (Figure 7