A composting toilet system (sometimes called a biological toilet, dry toilet, or waterless toilet) contains and controls the composting of excrement, toilet paper, carbon additive, and, optionally, food wastes. Unlike a septic system, a composting toilet system relies on unsaturated conditions (material cannot be fully immersed in liquid), where aerobic bacteria and fungi break down wastes, just as they do in a yard waste composter.

Sized and operated properly, a composting toilet breaks down waste to 10 to 30 percent of its original volume. The resulting end product is a stable soil-like material called “humus,” which legally must be either buried or removed by a licensed septage hauler in accordance with state and local regulations in the United States. In other countries, humus is used as a soil conditioner on edible food crops.