he toilet seat is located directly above, on the first floor. Waste material passes through a chute below the toilet to an inclined surface. There, over a period of time, solid material migrates down this surface and is slowly decomposed by organisms (bacteria, molds, and fungi) present in the vault. The totally composted waste material is reasonably free of harmful organisms, has no unpleasant odor, and can be buried or used in the garden as a soil conditioner in areas where aboveground crops will be grown. Liquids, mainly urine, also run down the inclined surface, through the various solids, and are biochemically changed during this passage. Excess liquids must be evaporated by air circulation into, through, and out of the composting chamber. Circulation is enhanced by an electric fan. Figure 7.10 shows a small volume composting toilet. The decom- position process in this chamber is just like that which occurs in the larger one. But the composting process is aided by an internal heat- ing element, and electrically-operated stirring paddles increase aera- tion of the composting mass. The large volume composting toilet is emptied perhaps once per year; the small one, perhaps once per month. There are many designs for homemade compost toilets. They range from the simple modification of a 55-gal drum to solar- assisted art forms. All should have some common design characteristics. 1. They should be reasonably large; 1 yd3 is about the minimum size for four people. 2. They should be constructed of materials which are durable and can be made watertight. Commonly used materials are concrete block (sealed with waterproofing compound) and wood (covered with an epoxy resin). 3. Adequate ventilation should be assured to maintain aerobic condi- tions. A tall 6- to 8-in. diameter vent stack is adequate, but an assist fan is sometimes needed to keep liquid levels down. Solar photo voltaic cells can operate a small fan, if your sensibilities revolt at using commercial electricity or if it is not available in your house. All v