TEMPERATURE AND TIME
There are two primary factors leading to the death of pathogens in humanure. The first is temperature. A compost pile that is properly managed will destroy pathogens with the heat it generates.
The second factor is time. The lower the temperature of the compost, the longer the subsequent retention time needed for the destruction of pathogens. Given enough time, the wide biodiversity of microorganisms in the compost will destroy pathogens by the antagonism, competition, consumption, and antibiotic inhibitors provided by the beneficial microorganisms. Feachem et al. state that three months retention time will kill all of the pathogens in a low-temperature composting toilet except worm eggs, although Table 7.14 (also from Feachem) indicates that some additional pathogen survival may occur.
A thermophilic compost pile will destroy pathogens, including worm eggs, quickly, possibly in a matter of minutes. Lower temperatures require longer periods of time, possibly hours, days, weeks, or months, to effectively destroy pathogens. One need not strive for extremely high temperatures such as 65°C (150°F) in a compost pile to feel confident about the destruction of pathogens. It may be more realistic to maintain lower temperatures in a compost pile for longer periods of time, such as 50°C (122°F) for 24 hours, or 46°C (115°F) for a week. According to one source, "All fecal microorganisms, including enteric viruses and roundworm eggs, will die if the temperature exceeds 46°C (114.8°F) for one week." 42 Other researchers have drawn similar conclusions, demonstrating pathogen destruction at 50°C (122°F), which produced compost "completely acceptable from the general hygienic point of view." 43
A sound approach to pathogen destruction when composting humanure is to thermophilically compost the organic refuse, then allow the compost to sit, undisturbed, for a lengthy period of time after the thermophilic heating stage has ended. The biodiversity of the compost will aid in the destruction of pathogens as the compost ages. If one wants to be particularly cautious, one may allow the compost to age for two years after the pile has been built, instead of the one year that is normally recommended.
In the words of Feachem et al., "The effectiveness of excreta treatment methods depends very much on their time-temperature characteristics. The effective processes are those that either make the excreta warm (55°C/131°F), hold it for a long time (one year), or feature some effective combination of time and temperature." The time/temperature factor of pathogen destruction is illustrated in Figure 7.7.
In short, the combined factors of temperature and time will do the job of "turning turds into tomatoes."
Source: The Humanure Handbook. Jenkins Publishing, PO Box 607, Grove City, PA 16127. To order, phone: 1-800-639-4099.