In our approach the organic matter of the feces sludge is completely mineralized to carbon dioxide, water and minerals such as phosphate salts. The remaining streams are off-gas and a mixture of water and minerals. The off-gas contains mainly nitrogen, carbon dioxide and oxygen and can be safely vented to the atmosphere. The aqueous stream may be utilized as a fertilizer.
Feces mineralization in detail
When mixed with air and heated above ca. 400°C under high pressure, the feces sludge decomposes and is oxidized completely to carbon dioxide and water. This process is called “hydrothermal oxidation” or HTO. The water in the sludge does not evaporate but mixes with the air and provides a reaction environment for an efficient conversion of the organic matter within a few minutes.
We are developing a comprehensive computer model of the HTO reactor to be built for the Autarky toilet. This computer model is fed with data from experiments carried out in small autoclaves with real fecal sludge. We determined that the oxidation reaction is rapid above ca. 300°C and runs to completion within a few minutes at 400°C. The next step is to build a small laboratory-scale prototype to perform experiments under more realistic conditions and to test the model predictions. With the validated model a full-scale prototype will be designed and built in 2016.