Hydrothermal carbonization of dry toilet residues as an added-value strategy - investigation of process parameters

Human faeces from a dry toilet are converted via hydrothermal carbonization to obtain a sterilized carbonaceous material. During this process the original material undergoes consecutively hydrolysis, water elimination and polymerization reactions. Consequently, the oxygen content is reduced, leading to a material with a better dewaterability and an attractive higher heating value (HHV = 22-28 MJ kg −1). The influence of pH-value, set by the addition of citric acid, the reaction time and the reaction temperature are investigated. By thermo-gravimetric analysis it is shown that especially higher acid concentration as well as higher reaction temperatures and longer reaction times are necessary to fully convert the feedstock into a stable carbon-rich material. As pathogens are destroyed by hydrothermal carbonization, nutrient recovery becomes a relevant aspect. The analysis shows that alkali salts such as sodium and potassium are dissolved in the aqueous phase, but an important proportion of the phosphorus and nitrogen remain in the hydrochar. This finding is the basis for phosphorus recycling or to produce an organic fertilizer.