the final depth of the trenches. Dig the collection chamber or dosing chamber excavation and a trench to connect it to the filter trench underdrain system. Add the coarse gravel to keep the underdrain pipe 2 in. above the filter bottom and place the underdrain pipe. Also, prop up the vent pipes and check all of the pipes for proper slope. Add the remaining gravel to make a total gravel depth of 8 in. and add a layer of filter fabric or 3 in. of pea gravel (do not use straw or untreated building paper). Add the filter sand, and place 2 in. of coarse gravel on top of the filter sand. The gravel surface should be level if dosing is used, or should slope at a grade of one-sixteenth inch per foot away from the manifold or supply pipe if dosing is not used. Flood the sand, place the distribution pipes, and make necessary connections. Backfill as described for uncollected filters. The elevated intermittent sand filter can be installed completely aboveground, partially buried, or buried to its rim. The filter hous- ing can be made of concrete block, poured concrete, or other dura- ble material. If concrete block is used, the bottom of the housing is normally a concrete slab. A "V"-shaped compartment floor makes it easier to get the effluent into the underdrain pipe. The inside sur- face of the concrete block should be coated with a bituminous com- pound. The same construction techniques used for septic tank con- struction can be used for this housing, although the cover should be made of wood, not concrete. The aboveground portion of the walls and the housing cover should be insulated if freezing is a possibility. CONVENTIONAL SYSTEM MODIFICATIONS One hole in each chamber is required for the distribution pipe and one hole each for the underdrain pipe. If perforated pipe is to be used for the underdrains, be sure to switch to unperforated pipe at the exit from the housing. Other- wise, use unperforated pipe with half-inch holes drilled in the por- tion inside the housing. The underdrain pipe can be placed directly on the 