The Manual of Septic Tank Practice bases its sizing recommendations on the percolation rate of the soil. Table 5.2 is a simplified version of those recommendations. Refer to the section in Chapter 3 on Site Evaluation for a discussion of the percolation test. According to the CONVENTIONAL WASTE DISPOSAL SYSTEMS Table 5.1 Square Feet of Trench Bottom Area Required by the Uniform Plumbing Code No. of Bedrooms Soil Type Course sand or gravel Fine sand Sandy loam or sandy clay Clay with considerable sand or gravel Clay with small amount of sand or gravel Manual of Septic Tank Practice, these bottom areas are sufficient to allow for garbage disposals and automatic clotheswashers. Method 3 The Environmental Protection Agency's design manual, Onsite Wastewater Treatment and Disposal Systems, recommends application rates for various soil textures and their associated percolation rates. Table 5.3 was developed as a simplified version of those recommendations. The EPA design manual cautions that conventional trench or bed designs are not recommended for silty clay loam and clay loam, because of the difficulty of installation without smearing and com- pacting the soil. This damage could reduce by half the infiltration rate. Table 5.2 Square Feet of Bottom Area Recommended by the Manual of Septic Tank Practice Percolation Rate No. of Bedrooms (min/in.) 1 or less ONSITE WASTEWATER DISPOSAL Table 5.3 Square Feet of Bottom Area Recommended by the Environmental Protection Agency Percolation No. of Bedrooms Soil Type (min/in.) Gravel, coarse sand less than 1 unsuitable soil Coarse to medium sand Fine sand, loamy sand Sandy loam, loam 16-30 Loam, porous silt loam 31-60 Silty clay loam, clay 61-120 loam, clay It is important to note that the decrease in the percolation rate in loose soils resulting from the accumulation of the biological mat is not taken into account in Method 2. The reason that sewage dis- posal systems in loose soils are not failing like matches in a rain- storm is probably because of the great safety factor buil