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.
The Composting Toilet System as a Biologically Active Filter
You should consider the composting toilet system a biologically active filter that diverts matter out of the wastewater stream and reduces it by microbial decomposition. When combined with graywater filters, such as grease traps for the kitchen, lint filters for the clothes washing machine, and hair traps for the bath tub, the total integrated composting toilet system replaces the separating functions performed by a septic tank or a municipal treatment plant. It also removes more than 95 percent of the total solids and most of the biological oxygen demand (BOD5). Nitrogen is reduced by more than 80 percent (it’s in the urine), and disease-causing organisms (pathogens) are all but eliminated.
The primary objective of the composting toilet system is to contain, immobilize, or destroy organisms that cause human disease (pathogens), thereby reducing the risk of human infection to acceptable levels without contaminating the immediate or distant environment and harming its inhabitants.
- This primary objective should be accomplished in a manner that:
- Is consistent with good sanitation (minimizing both human contact with unprocessed excrement and exposure to disease vectors, such as flies).
- Produces an inoffensive and reasonably dry end product that can be handled with minimum risk.
- Minimizes odor.
- A secondary objective is to transform the nutrients in human excrement into fully oxidized, stable plant-available forms that can be used as a soil conditioner for plants.
- The main components of a composting toilet system are:
- A composter connected to one or more dry (waterless), microflush, or urine-diverting toilets
- A screened exhaust system (often fan forced) to remove odors, heat, carbon dioxide, and water vapor (the byproducts of aerobic decomposition)
- A means of ventilation to provide oxygen (aeration) for the aerobic organisms in the composter
- A means of draining and managing excess liquid and leachate
- Process controls, such as mixers, to optimize and manage the process
- An access door for removal of the end product.
The composter should be constructed to separate the solids from the liquids and product a stable, humus end product with less than 200 most probable number (MPN) per gram of fecal coliform bacteria.