The compost present in the toilet harbors not only pathogenic, disease-causeing organisms, but also helpful ones that perform the tasks needed to convert human waste to a useful benign compost. Most of the organisms present in the compost are mesophiles that live at temperatures of 20-45ºC or 68-113ºF. This is the same temperature range that human pathogens operate in. The bacteria desired for a compost pile are hard to cultivate because of the temperatures in which they live. These organisms are called thermophiles and live at temperatures above 45ºC or 113ºF. These bacteria can turn waste to compost almost 6 months faster than the lower temperature bacteria. They also have the added benefit of heating the pile to a level that almost all pathogens cannot survive. This decreases the risk of contamination while handling the compost. When these temperatures are reached, these organisms can produce their own heat to maintain their environment. This can lead to a rapid loss of moisture and possible spontaneous combustion of the compost pile. Not to worry! This usually happens with very large piles of compost over 12 feet high. For composting toilets, it should be expected that the thermophilic bacteria will not be present in large numbers and because of this, composting will take a little longer to complete.
Moisture is very important for a composting toilet. Without moisture, no bacteria can perform the processes necessary for composting. In countries with hot climates, urine is diverted from the composting toilet and no moisture is added so that the feces dries out from the heat. This is NOT composting. Farmers than remove the dried waste from the toilet and apply it to their fields. When water is added, the waste springs alive with the no longer dormant composting bacteria and disease-causing pathogens increasing the risk to farmers of contracting these pathogens. The form of waste that is applied is equivalent to raw waste since no composting was allowed to take place. Maintaining the correct moisture content of between 45-70% is difficult because there is no way to determine the moisture present in the pile. If too much moisture is added, in the form of water or urine (explained in urine diversion page), then the compost will effectively drown with no air available. Anaerobic bacteria will begin their work of fermentation giving off methane and hydrogen sulfide much to the disgust of the users. If not enough is added, the composting process will slow or stop altogether. This will lead to a dramatic increase in the volume of solids present in the vault as the bacteria responsible for reducing the volume of the solid matter through composting are no longer active.
Aerobic conditions are important for proper composting and odor control. If anaerobic (no air present) conditions are allowed to exist, then foul and offensive odors like those explained above will be common. Providing proper ventilation by designing the composting toilet with a vent pipe similar to the VIP pit latrines installed in most developing countries and adding bulking agents like wood chips, straw and hay to increase the air spaces between carbon-rich material and human waste, are the best ways to increasethe oxygen level in the vault. If compost toilets are designed aerobically, there are usually little to no odor problems. More effective ways of improving the oxygen level require electricity to supply small fans that draw air throught the compost vault. This is typically not economically viable for most communities seeking to install composting toilets.
The proper balance of nutrients is vital to the composting process. A good carbon/nitrogen (C/N) ratio for a compost toilet is 20/1 to 35/1. This means far more carbon must be present in the vault to the nitrogen for good composting to proceed. Because of the composition of feces and urine, a lot of carbon in the form of hay, straw, wood chips, and sawdust have to be added to the compost toilet. A good practice is to have the carbon material present next to the toilet and after use, the person dumps a handful of the material into the vault. Also, after a vault has been emptied it should be filled approximately 2/3 of the way with loose material rich in carbon. The material will subside as people use the toilet and the weight of the feces packs it down. The pile will also reduce in size because of the actions of the microbes in the pile, which result in water loss. The pre loading of this loose material provides pore spaces of air to get trapped in the pile and also cavities for air to flow through, which is used by aerobic organisms during composting.
There is a table below describing the approximate percent nitrogen and C/N ratio present in certain organic materials considered for composting.
CARBON / NITROGEN RATIOS
|Sources: Gotaas, Harold B. (1956). Composting - Sanitary Disposal and Reclamation of Organic Wastes (p.44). World Health Organization, Monograph Series Number 31. Geneva. and Rynk, Robert, ed. (1992). On-Farm Composting Handbook. Northeast Regional Agricultural Engineering Service. Ph: (607) 255-7654. pp. 106-113. Some data from Biocycle, Journal of Composting and Recycling, July 1998, p.18, 61, 62; and January 1998, p.20.|
Source: The Humanure Handbook. Jenkins Publishing, PO Box 607, Grove
City, PA 16127.
To order, phone: 1-800-639-4099. www.jenkinspublishing.com