Hydrothermal carbonisation of sewage sludge using recirculated process water

The aim of this study is to investigate the influence of process water recirculation on the Hydrothermal Carbonisation (HTC) of sewage sludge. The composition of the resulting products hydrochar and process water obtained by different recycling rates facilitate the evaluation of the material and energy recovery in subsequent process engineering steps. For this purpose, HTC experiments were performed on a laboratory scale, in which the resulting process water was re-used as educt for a follow-up experiment with varying rates of recycling. Therefore, a dry sludge/process water mixture was treated in a pressure reactor at constant conditions (temperature, reaction time, mixing ratio between dry sludge and (process-) water) and then filtered. Samples of the process water and the dried hydrochar were examined with regard to their composition. Thus, calorific value, elemental composition, ash content, content of volatile and fixed components of the hydrochar as well as the chemical oxygen demand (COD) and pH of the process water were examined. In addition, the filter cake resistance of the hydrocarbon/process water mixture was determined in order to evaluate the dewaterability of the HTC products. The results were finally compared. Keywords.