sion valves are available and diversion boxes are easily constructed. Materials 1. two or more drain fields (any type) 2. diversion box or valve Construction The diversion valve is simply installed using the appropriate cement to attach the influent and effluent lines. The valve should be CONVENTIONAL SYSTEM MODIFICATIONS Figure 6.2 Diversion box. accessible from the surface and insulation placed in the access hole to prevent freezing. If clay pipe or bituminized fiber pipe is used, a diversion box may be easier to install. The box can be cast in place or purchased. A box as simple as a plastic ice chest could be used. Whatever the material, be sure that the connections are strong and watertight. There are some very effective adhesives on the market. Ask your friendly hardware dealer for the most appropriate one. Operation and Maintenance Other than an occasional inspection to see that there is no sewage collecting in the access hole, and switching the flow every year, there is no operation or maintenance procedure. ONSITE WASTEWATER DISPOSAL Figure 6.3 Elevated drain field. ELEVATED DRAIN FIELDS Application Elevated drain fields are used where conventional subsurface drain fields cannot be installed due to limited soil depth above bedrock or ground water. An additional 6 to 12 in. of clearance can be gained by raising the whole disposal system to the level where the distribution pipe discharges the sewage effluent just below the level of the surrounding ground (Figure 6.3). This additional clear- ance may allow conformance with regulatory requirements and make more complex and expensive alternatives unnecessary. The elevated drain field can be of any type, but conventional style trenches are most commonly used. Process The elevated drain field functions just as it would if it were not elevated. As long as the distribution pipe discharges into gravel fill that is entirely below the surrounding ground surface, and as long as the absorption rate into the soil is adequate to handle the inflow, the elevated system wi