Composting toilet

Inexpensive do-it-yourself compost toilet at Dial House, Essex, England, utilizing an old desk as the toilet unit.

A composting toilet is an aerobic processing system that treats excreta, typically with no water or small volumes of flush water, via composting or managed aerobic decomposition. This is usually a faster process than the anaerobic decomposition at work in most wastewater systems, such as septic systems.

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 (see latrine, pit latrine, and arborloo or tree bog), 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.

Types

Manufactured composting toilet systems

Self contained composting toilet system

"Self-contained" composting toilets complete or begin the composting in a container within the receiving fixture. "Remote," "central," or "underfloor" units collect excreta via a toilet stool, either waterless or micro-flush, from which it drains to a composter. "Vacuum-flush systems" can flush horizontally or upward with a small amount of water to the composter. "Micro-flush toilets" use a small amount of water usually 1 pint (.5 liter) per use.

"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.

In contrast, "desiccating toilets" dry the excreta to destroy pathogens, though one study suggested that drying can result in rehydration of pathogens when in contact with moisture later.

The performance testing standard for composting toilets in the United States is American National Standard/NSF International Standard ANSI/NSF 41-1998: Non-Liquid Saturated Treatment Systems. Systems might also be listed with CSA, cETL-US, and other standards programs.

Build-it-yourself, site-built, and owner-built design

Further information: How to build a waterless toilet

Henry Moule's earth closet, patented in 1873. Example from around 1875. Rear chamber for dispensing cover material

Site-built indoor composting toilet designs vary, ranging from rollaway containers fitted with aerators to large concrete sloped-bottom tanks.

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.

Public use

Public composting toilet facility on E6 highway in Sweden

Increasingly, composting toilet systems are commonly used in water closets in public facilities. One example is the three-storey C.K. Choi Building at the University of British Columbia (Canada), which features five composting toilet systems with 12 toilet stools that serve 300 employees. They may also be found in various places around Europe, like many of the roadside facilities in Sweden (see image).

Composting toilets greatly reduce the volume of excreta on site through psychrophilic, thermophilic or mesophilic composting and yield a soil amendment that can be used in horticultural or agricultural applications as local regulations allow.

Operating process

File:Remote compost toilet.jpg

Remote type composting toilet system

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 (40-60 °C / 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% +/- 10): too dry, and the mass decomposes slowly or not at all; too wet and anaerobic organisms thrive, creating undesirable odors (cf. Anaerobic digestion). This separated liquid may be diverted to a graywater system or collected for other uses.

A urine-diverting-dehydration toilet

An approach that is becoming more common is the "dry" toilet, or urine-separating (also: urine-diverting) toilet. Where solar heat is used, this might be called a "solar" toilet. These systems depend on desiccation to achieve sanitation safety goals features systems that make use of the separated liquid fraction for immediate area fertilization.

Urine can contain up to 90 percent of the N (nitrogen), up to 50 percent of the P (phosphorus) and up to 70 percent of the K (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.

See also

• How to build a waterless toilet
• Chemical toilet
• Clivus multrum
• Ecological sanitation
• Humanure
• Incinerating toilet
• Latrine
• Pail closet
• Outhouse
• Toilet
• Vermicomposting

External links

• Alternative Sanitation
• Sustainable Sanitation Alliance (susana.org)
• Humanure Handbook online (PDF also available)
• Practical Action's guide on building DIY composting-toilets
• "What is a Composting Toilet System and How Does it Compost?"
• The Composting Toilet System Book (PDF also available)
• Converting to a Composting Toilet on a Boat