t into the recommendations Research carried out at the University of Wiscon- sin shows that, to be safe, you should allow a maximum application rate of 1.2 gal per day per ft2 of trench bottom or 125 ft2 of trench bottom per bedroom. So which method should you use? If you are subject to the Uni- form Plumbing Code as administered by local or state building author- ities, using Method 1 as a minimum might save you a hassle. The same goes for any other locally required method. If a sizing require- ment does not exist in your area, use the generalization that the larger the trench absorption area, the longer the system will last without repair or replacement. If you will need the system for only one or two years prior to the availability of a sewer, find the smallest of the three sizes presented and cut that in half (after convincing the appropriate regulatory agency). If you want the system to last a lifetime, choose the largest area recommended. If the system will be used seasonally, keep in mind that during the time it is in use, it will have to handle the full daily load. Example As an example of drain field design, let us say that a new three- bedroom house is being built in an area governed by the Uniform Plumbing Code. The soil is mostly sandy loam. A percolation rate of CONVENTIONAL WASTE DISPOSAL SYSTEMS 20 min per in. confirms this. Table 5.1 shows that 300 ft2 of bottom area is required. Since there is plenty of land available for the sys- tem, we choose a 2-ft deep trench. The backhoe to be used has only an 18-in. bucket, so we decide to make the trench two buckets, or 3 ft wide. The length of the trench will be 300 ft divided by 3 ft, or 100 ft long. This, by the way, is the maximum length for a single trench because of problems of leveling and equalizing effluent distribution by gravity. Gravel must be added to support the sides of the trench and to provide space to hold the slugs of wastewater that come from the septic tank. Minimum depth of fill with gravel is 6 in. below the distribution pipe accord