perly designed, installed, used, and maintained system should last a lifetime. As with municipal systems, the water coming out of the pipe at the end of the treatment process (septic tank) has to go somewhere. The typical recipients of effluent from septic tanks are soil, and often the ground water. The purposes of designing and installing a proper liquid waste disposal system are to dispose of household wastewater while eliminating the health hazard of sewage on the ground, and to protect the ground water from contamination, SO that it may continue to serve as a water supply for individuals and communities. By far the most common disposal system employed in onsite sew- age disposal is the drain field. It consists of excavations-usually trenches-filled with gravel through which distribution pipes run (Figure 2.3). Effluent comes out of holes, slots, or joints in the distri- bution pipe and runs over and through the gravel and into the soil. Some portion of the effluent may evaporate or be taken up by plant roots and released to the atmosphere (transpired), depending upon the depth of the excavation, type of soil, and plant cover. If the disposal system is designed and installed properly, all of the water entering the drain field excavations should be able to enter the soil and not accumulate in the excavation. Also, there should be suffi- cient clearance between the excavation bottom and the ground water SO that the effluent is sufficiently cleansed before it enters the ground water. Cleansing by the soil works in several different ways. The bacte- rial slime-layer, the "biological mat" that gradually forms on the trench bottom, is responsible for more than 95% of the cleansing that occurs. An obvious cleansing mechanism is simple filtration. Particles in the sewage effluent which are bigger than the spaces between the soil particles get stuck between the soil particles and cannot proceed to the water table. These sewage particles can then act as filters themselves. When too many solids from the septic tank REGUL