Composting toilets are often used as an alternative to central wastewater treatment plants (sewers) or septic systems. Typically they are chosen (1) to alleviate the need for water to flush toilets, (2) to avoid discharging nutrients and/or potential pathogens into environmentally sensitive areas, or (3) to capture nutrients in human excreta. Several manufactured composting toilet models are on the market, and construct-it-yourself systems are also popular.
These should not be confused with pit latrines, all of which are forms of less controlled decomposition, and may not protect ground water from nutrient or pathogen contamination or provide optimal nutrient recycling.
Manufactured composting toilet systems
Self-contained composting toilets are slightly larger than a flush toilet, but use roughly the same floor space. Some units use fans for aeration, and optionally, heating elements to maintain optimum temperatures to hasten the composting process and to evaporate urine and other moisture. Operators of composting toilets commonly add a small amount of absorbent carbon material (such as untreated sawdust, coconut coir, peat moss) after each use to create air pockets for better aerobic processing, to absorb liquid, and to create an odor barrier. This additive is sometimes referred to as bulking agent. Some owner-operators use microbial starter cultures to ensure composting bacteria are in the process, although this is not critical.
Remote, central, and under-floor models each feature a chamber below the toilet stool (such as in a basement or outside) where composting takes place. These are typically used for high-volume and year-round applications as well as to serve multiple toilet stools. Several systems are available as well as many build-it-yourself options. (See Youtube Video Below)
Build-it-yourself, site-built, and owner-built design
These are not to be confused with direct outdoor composting, which typically uses a collector bucket, where each deposit is covered with sawdust or other dry organic material, with the collector periodically being hand transported to an outdoor composting bin, where it may be added to yard waste or other organic material being composted.
Although there are many designs, the process factors at work are the same. Rapid aerobic composting will be thermophilic decomposition in which bacteria that thrive at high temperatures (104-140 °F) oxidizes (breaks down) the waste into its components, some of which are consumed in the process, reducing volume, and eliminating potential pathogens.
Drainage of excess liquid or leachate via a separate drain at the bottom of the composter is featured in some manufactured units, as the aerobic composting process requires moisture levels to be controlled (ideally 50%): too dry, and the mass decomposes slowly or not at all; too wet and anaerobic organisms thrive, creating undesirable odors. This separated liquid may be diverted to a graywater system or collected for other uses.
Urine can contain up to 90 percent of the nitrogen, up to 50 percent of the phosphorus and up to 70 percent of the potassium present in human excreta. In healthy individuals it is usually pathogen free, although undiluted it may contain levels of inorganic salts and organic compounds at levels toxic to plants.
The other requirement critical for microbial action (as well as drying) is oxygen. Commercial systems provide methods of ventilation that move air from the room, through the waste container, and out a vertical pipe, venting above the enclosure roof. This air movement (via convection or fan forced) will vent carbon dioxide and odors.
Most units require manual methods for periodic aeration of the solid mass such as rotating a drum inside the unit or working an "aerator rake" through the mass. Composting toilet brands have different provisions for emptying the finished product, and supply a range of capacities based on volume of use. Frequency of emptying will depend on the speed of the decomposition process and capacity, from a few months (active hot composting) to years (passive, cold composting). With a properly sized and managed unit, a very small volume (about 10% of inputs) of a humus-like material results, which can be suitable as soil amendment for agriculture, depending on local public health regulations.