Separation and Containment
Compost toilets contain human waste in a vessel--one or more composting chambers--located either within a single box-type toilet unit, or in one larger composting chamber into which several toilet-seat units are attached via pipes. A larger composting chamber is usually located in a basement or other location at a lower elevation so that human excrement from a number of different toilet locations within a building will drop down into it. By containing the waste, a composting toilet prevents that waste from contaminating ground or water supplies until it is rendered inert by decomposition. Containment over time also ensures that human pathogens are rendered inert, as most die relatively quickly after they are removed from their human host.
A composting toilet uses gravity and the filtration effect of neutral, organic matter such as peat moss, leaves, or even popped popcorn, to separate human waste into liquid (urine) and solid (feces) components. While some urine and, in some models, a small amount of flush water, is added to the organic matter bulking agent to moisten it, most of the liquid drops to the bottom of the containment unit, where it then exits via a drain to a leach field, sand pit, or an alternative bio-filtration system such as a controlled filtration pond.
The solid fecal matter which remains in the composting containment unit, along with the organic bulking agent, decomposes much in the same way that yard waste decomposes in a compost pile: through controlled decomposition by the action of bacteria, fungi, and other micro-organisms. These organisms are either present in the organic matter added to start off the composting toilet, or are added by throwing in a small shovelful of forest soil or yard compost to get the process started.
In the presence of oxygen, supplied either by air pockets in the structure of the bulking agents or by the action of earthworms and other organisms in the composting material, the bulking material and sold human waste will break down into a rich soil-like material which makes a nutritive addition to dirt used in growing plants. In many places in the world this matter is used as a garden amendment; in the United States however, the laws of most states require at the least that this material not be used on crops grown for consumption; some states require that a licensed septage hauler remove the material.
Active System Components
While many composting toilets are simply chambers which work on principles of time, decomposition, and gravity, an increasing number of compost toilets include other active elements which hasten the composting process. These include handles to rotate the composting chamber within the toilet unit, much like the popular tumbling garden composters, aiding in aeration and even distribution of moisture. Other active elements might include venting fan systems to dry out the composting material and decrease any odor, or a separate chamber for storing liquid waste to be sprayed back over the solid material if it becomes too dry.