compartment floors, but the drain pipe (or manifold, if one is used) should slope at a grade of about one-eighth inch per foot toward the disposal system after it leaves the filter housing. The upstream end of the underdrain pipe should be vented to the surface (but within the sand filter housing). Support the upstream end of the underdrain pipe and the vent pipe before adding the 8 in. of coarse gravel. If the distribution pipes are to be internal to the filter housing and rise through the sand, install and support them before adding the gravel. After placing the coarse gravel, add a 3-in. layer of pea gravel or a layer of filter fabric. Add 24 to 30 in. of filter sand and level its surface. Normally, there should be at least 18 in. of housing well above the sand surface to allow for ponding without back- siphoning of the effluent. The distribution pipes should have an elbow to direct the flow of effluent down onto the splashplates as shown in Figure 6.26. There should be about 12 in. between the bottom of the elbow and the splashplate. Attach the valves or other mechanism for diverting the flow from one compartment to the other and lay the filter supply lines and effluent lines. Use a sand bed for these lines if the soil is rocky. If you will be using a high water alarm, the switch should be set to activate the alarm when the effluent ponds to a depth just under the distribution pipe elbow. Finally, build and install a tight-fitting wood cover for the filter and add insulation if the unit will be located in a cold climate. Make the cover attractive enough SO that you can use it for a deck. Put a lawn chair on it and watch the sunset to the sound of rushing water. The recirculating sand filter construction techniques are the same as for the intermittent filter with respect to the filter housing and mate- rials. There is, of course only one chamber in the filter housing of the recirculating filter. Be careful when installing the septic tank, recirculation tank, and sand filter housing, SO that both the septic tan