n per in. gives a requirement of 563 ft2 of absorption area. The effective absorption area of a seepage pit is the area of the sidewall below the inlet pipe. The bottom area is normally not included, probably because it tends to clog up with fine particles and slime. The sidewall area can be calculated by multiplying the depth below the inlet pipe by the circumference of the pit (see the Appendix for the formula for the area of a cylinder), but it would be easier to use Table 5.4. If you want to know the effective absorption area of a pit 8 ft in diameter and with an effective depth of 15 ft, just add the areas for two 8-ft diameter pits, one with a 10-ft depth and one with a 5-ft depth. In our previous example, we needed 563 ft2 of absorption area. To make it easy on the backhoe operator, we could choose three pits, each 10 ft in diameter and 6 ft deep. Or we could provide a real challenge and order one hole 3 ft in diameter and 60 ft deep. Either way, we may need special permission from the regulatory agency. According to the Uniform Plumbing Code, special permission is required for pits of less than 4 ft in diameter and for pits greater than 6 ft in diameter. Also, special permission is required for seepage pits with an effective depth of less than 10 ft. Another method of sizing seepage pits is sometimes used. After the pit is excavated and lined, it is filled with water and allowed to drain three times. The third time, the rate of drop of the water in the pit is measured, and that rate is used to size any additional pits necessary. In other words, the seepage pit itself is used as a percola- CONVENTIONAL WASTE DISPOSAL SYSTEMS 89 tion test hole. The percolation rate produced by this method proba- bly will not correlate well with standard percolation test results and may significantly overestimate the rate. Multiple pits should be connected in series, and should be sepa- rated by 6 ft of undisturbed soil or three times the diameter of the pits, whichever is greater. Most seepage pits are circular pits with li