t tank (Figure 4.1) is recommended by the Uniform Plumbing Code and required by some states. The first, or inlet, compartment should be at least two-thirds of the total volume WASTE PROCESS Figure 4.1 Typical septic tank design. of the tank. It is in this first compartment where most of the solids are deposited and where the biological decomposition is vigorous. The second compartment acts as a final settling chamber where most of the remaining solid material, and the smaller and lighter particles, settle to the bottom. The settling efficiency is related to the area (length and width) of the tank, and the storage capacity is proportional to the depth. Recommended tank dimensions are compromises between these two functions for any given volume. Table 4.2 provides dimensions which satisfy the Uniform Plumbing Code. The floors and walls, if made of concrete and poured in place, should be at least 5 in. thick, and the top, 4 in. thick. Any wall sections over 6 ft long should contain steel reinforcing rods. The top should contain reinforcing rods spaced 6 to 9 in. apart. Concrete septic tanks should be coated on the inside with a bitumi- Table 4.2 Septic Tank Inside Dimensions Tank Volume (gal) Length Width Depth 8 ft 0 in. 3 ft 6 in. 5 ft 0 in. 10 ft 0 in. 3 ft 6 in. 6 ft 0 in. 12 ft 0 in. 3 ft 6 in. 5 ft 0 in. 46 ONSITE WASTEWATER DISPOSAL nous coating. Sewage is a very corrosive mixture and will decompose unprotected concrete. If the tank is to be positioned in ground water, the outside should be coated as well. If ground water is very high, the septic tank may float and have to be filled with water in order to get it to sink into its hole. Inspection ports should be provided at both the inlet and outlet ends of the tank. Two manholes, one over the inlet device and one over the outlet device, are recommended. These holes should be a minimum of 20 in. in diameter. If the tank is to be very deep in the ground, extensions of the manholes to within 8 in. of the ground surface will make location and opening easier. Baff