Composting toilets, according to the Washington State Department of Health, are defined as:

“A system designed to store and compost (primarily by unsaturated, aerobic microbial digestion) human excrement (human urine and feces), ideally to a stable soil-like material called “humus.” These systems are commonly designed to accommodate fecal and urinary wastes (human excrement), toilet paper and small amounts of organic carbonaceous material added to assist their function.”

The key variables determining the degree to which (and efficiency with which) human wastes are transformed into compost (referred to as a “stabilization” in the state health code) are temperature, oxygen content, and carbon to nitrogen ratio.    The ideal ranges for these variables are as follows:

Temperature:  
68F to 112F – This is the typical temperature range for most composting toilets.   Mesophilic bacteria dominate in this range.
113F to 160F – Thermophilic bacteria dominate in this range.  

Pathogens:

Pathogens are deactivated or destroyed in composting toilets in four ways:

1) Because many pathogens cannot/do not survive for long periods outside of a human host, containment in the toilet for sufficient residence time will deactivate or kill them. 

2) Beneficial soil bacteria and other microbes are better adapted to the composter environment than pathogens and will out-compete them for these resources such as oxygen and food.

3) Some soil organisms naturally produce chemicals that destroy pathogens (such as antibiotic-like substances or toxins that prevent reproduction)

4) Most human pathogens present in feces will be killed if subjected to temperatures above 131F for a sufficient amount of time.  It is important to note, however, that most toilets will require external heating to achieve this temperature.

Additionally, two pathogens of concern are more resistant to environmental stresses.  These are: Ascaris lumbricoides – the common round worm – and protozoan oocyst-producing species such as Cryptosporidium parvum – a type of parasite that causes diarrhea.   Due to concern about these pathogens, care must be taken in how the finished compost is applied.   This is discussed further in the “application” section of this article. 

Oxygen Content:
The aerobic bacteria that are most efficient at converting human waste to compost require oxygen to live and function.   The ideal moisture content in the composting chamber is 45-70%.    At a higher moisture content, anaerobic bacteria will dominate and slow the composting process.

Carbon:Nitrogen Ratio:
Composting microbes use carbon as an energy source for growth and nitrogen as a building block for proteins.   They require both to live, function, and reproduce.   The ideal C:N ratio is 25:1.   Adding carbon (wood chips, for example) to the composting chamber can help maintain this ratio and can also help maintain air pockets (sufficient oxygen).