HYDROTHERMAL PROCESSES AND ANAEROBIC DIGESTION: COUPLING THE PROCESSES FOR A BETTER SEWAGE SLUDGE MANAGEMENT

Abstract

Hydrothermal carbonization (HTC) technology is addressed in the framework of sewage digestate management. HTC converts digestate into a stabilized and sterilized solid (hydrochar) and a liquor (HTCL) rich in organic carbon. This study aims to optimize the HTC operating parameters, namely the treatment time, in terms of HTC slurry dewaterability and HTCL bio-methane yields in anaerobic digestion (AD); and to investigate the suitability of continuous hybrid fixed bed anaerobic filter reactor for treating sewage and agro-industrial digestate HTC products; hydrochar and HTCL. Initially, sewage digestate slurry was processed through HTC at different treatment times (0.5, 1, 2 and 3 h) at 190 °C, and the dewaterability of the treated slurries was addressed through capillary suction time and centrifuge lab-testing. In addition, biochemical methane potential (BMP) tests were conducted for HTCL under mesophilic conditions. HTC treatment time of 1 h at 190 °C was identified as the optimum trade-off for improved dewaterability and utilisation of HTCL for biogas production. 1 h HTCL bio-methane potential can cover around 25% of the HTC and AD thermal and electrical energy needs without considering the eventual use of the hydrochar as a biofuel. A continuous hybrid fixed bed anaerobic filter was operated for 300 days under mesophilic conditions at different organic loading rates (OLR); maximum OLRs of 7.4 and 10 gCOD/L/d were reached while treating HTC liquor and slurry, respectively. 15 g/L hydrochar were added to the reactor as a supplement while treating HTCL solely thus increasing the biogas production up to 153%. The reactor was fed with HTCL and hydrochar with an increasing mixing ratio, and the co-digestion impact was dependent on hydrochar concentrations. The results of the study indicate that the hybrid fixed bed anaerobic filter reactor is a promising AD configuration for treating HTCL and overcoming the HTC upscaling challenges, and the suitability of digestate hydrochar utilization as supplement material for AD.