ases. It's very frustrating to be ordered to dig up the disposal system for inspection after it has been finished and landscaped. THE TREATMENT PROCESS We have discovered what goes on inside molecules and how big the universe is, but there is still major disagreement about what goes on inside a septic tank. Most experts agree, however, that the main reason to have a septic tank is so solid materials can be removed before the sewage reaches a drain field. Experience has shown that small particles and floating grease, if not removed, will cause the soil under the drain field to become clogged SO that the sewage effluent has no place to go but to the surface of the ground. The shortest amount of time the sewage should stay in the septic REGULATIONS AND PROCESSES Figure 2.1 Typical septic tanks. tank is about a day and a half. This is a common design parameter calculated as septic tank volume divided by inflow design rate. In this time, the liquid flows very slowly to the outlet and most solid materials settle to the tank bottom or rise to the liquid surface. The liquid coming out the tank's outlet should be relatively free of solids, including grease. Specific designs to assure adequate removal will be presented later. Figure 2.1 shows typical septic tanks. Most people assume that a septic tank is anaerobic; that is, the liquid in the tank has no oxygen in it and the bacteria which live in the liquid don't breathe oxygen the way animals do. The bacteria which live in the septic tank use some of the solid material for food and SO decrease the buildup rate of solid material on the bottom. Although it doesn't really matter to the homeowner, "anaerobic" is not a totally accurate description. The oxygen that enters the tank with the inflowing liquids is sufficient to keep some air-breathing creatures quite alive. What does matter to the homeowner is whether or not there is any noxious smell coming from the septic tank. Sewage with no oxygen in it stinks. Sewage which is saturated with oxygen is much easier on the nose. T