Hydrothermal carbonization of sewage sludge: Hydrochar properties and processing water treatment by distillation and wet oxidation

Hydrothermal carbonization is an innovative method of sewage sludge disposal. The solid product hydrochar (c.a.10%), may be converted to a fuel or fertilizer. Gaseous products (less than 10%) are mainly CO2 and nitrogen, but c.a. 80% of post-processing liquid presents as a challenging issue due to its high toxicity. Therefore, the main aim of this study was to determine the potentiality of hydrochars derived from sewage sludge as a fuel, at two temperatures: 200 and 220 degrees C, and 2 h of residence time, and successfully utilize the filtrates after the implementation of an adequate purification process. The results of hydrochars depicted better energy properties and dewaterability than in the case of raw sewage sludge, including improved chemical composition, a higher fixed carbon content and fuel ratio, and it also gained more effective filtration. The comprehensive analysis of the Thermal Analysis data proved that hydrochar can be combusted, but based on X-ray Fluorescence data a high tendency to slagging and contamination is expected. The highly toxic processing liquids indicated a great need for an appropriate treatment. Accordingly, distillation and the preliminary study of wet oxidation processes were applied. In both methods the chemical oxygen demand, an indicator of organic pollution, was significantly reduced. (c) 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

Hydrothermal carbonization is an innovative method for the disposal of sewage sludge, with the solid product hydrochar having potential uses as fuel or fertilizer. The gaseous products mainly consist of CO2 and nitrogen, while the post-processing liquid, which accounts for about 80%, is highly toxic. This study aimed to investigate the potential of hydrochars derived from sewage sludge as fuel, and to utilize the filtrates after purification. The results showed that hydrochars exhibited improved energy properties and dewaterability compared to raw sewage sludge, with better chemical composition and filtration efficiency.