Human feces contain an abundance of bacteria and viruses. One gram of feces can contain 10 million viruses, 1,000,000 bacteria, 1,000 parasite cysts and 100 parasite eggs. These microscopic organisms cause a variety of diseases that are responsible for over 13 million deaths a year. A key concern in using human feces as fertilizer is therefore the elimination of pathogens. The length of the composting process must be long enough to reduce pathogens to a “safe” level. The compost residence time will vary depending on the type (link) of latrine used, as well as the pH, moisture and temperature of the humus.
The United States Environmental Protection Agency (EPA) has set guidelines for acceptable pathogen levels in biosolids that are to be used for land application. Biosolids applied to agricultural lands must meet Class A standards of:
- Less than 1000 fecal coliforms/ 1g biosolids
- Less than 1 helminth egg/ 4g biosolids
- Less than 1 PFU viruses/ 4g biosolids
The compost is considered safe for land application when it meets these standards.
The methods of pathogen reduction in feces are related to temperature, moisture content, and aerobic decomposition of the organic matter present. High temperature will inactivate most pathogens, especially if maintained over extended periods of time. It is important to consider the average local temperature when deciding on operation procedures for a composting latrine. Latrines in tropical climates will require less time to complete the composting process. The presence of moisture in the compost decreases the rate of pathogen destruction. This is the idea behind the separation of urine and feces in some latrine designs. The elimination of urine from the waste will lead to a faster composting time. The urine itself is generally not harmful and can be safely applied to crops (in dilute form) after a couple days of storage.
U.S. Wastewater Treatment Guidelines (EPA)
The World Health Organization
Water Policy International - Exploring health risks from excreta for linear vs. cyclical sanitation systems.