|The Use of Effluents from Biolatrines in Tanzania (ADF, 1996, 38 pages)|
Excreta, being an organic waste, is biodegradable. As it is digested, or biodegraded, by bacteria, part is converted into the end products of methane and carbon dioxide and part is converted into bacterial cells. Under anaerobic conditions, i.e. without air, the portion converted into bacterial cells has been estimated to be from 2 to 5 percent (Roscol F. Ward 1985).
In the bio-digestion process, biogas production results in methane and carbon dioxide. The remaining slurry loses most of the smell of excrete, and becomes more liquid than fresh excrete. These changes are caused by different bacteria working on the original matter input.
The most important chemical principle at work is the transformation of longer carbon chains such as cellulose, alcohols and organic acids (which form a good part of the fresh excrete) into short carbon molecules like methane (CH4) and carbon dioxide (CO2). See Figure 1.
Cellulose takes longer to break up and often stays for a considerable period. The smell of excreta is mainly caused by organic acids (carbon chains with a certain oxygen content) and aromatics (carbon chains in ring form).
The bio-digestion process works best under a restricted range of conditions. The minimum temperature is about 15°C. The maximum is 45°C and the optimum range is 30-35°C. Even more important than the temperature is temperature stability. Changes of more than 2°C per day are harmful to the process, since the bacteria adapt rapidly to prevailing conditions and must readapt when the temperature changes. Acidity is also a significant constraint, with the optimum ph range 6-8. Methane-producing bacteria are negatively affected by acidity outside these limits. Retention times are another significant variable, which will be discussed below.