Hydrothermal conversion of sewage sludge: Focusing on the characterization of liquid products and their methane yields

The conversion of dewatered sewage sludge (DSS) into bio-oil and hydro-char through hydrothermal conversion (HTC) has been well studied, while the treatment of wastewater (HTCWW) was overlooked. The present study aimed to investigate the organic compositions of HTCWW obtained under different temperatures and residence time (170-320 °C, 0.5-6.0 h) and their potentials use for methane production through anaerobic digestion (AD). Results showed that the methane yield achieved to 286 mL CH4/g COD when HTC temperature was 170 °C while it was decreased to only 136 mL CH4/g COD at 320 °C. The methane yield of HTCWW was also decreased with the increase of HTC residence time. Proteins and volatile fatty acids (VFAs) were the main products of HTCWW but more recalcitrant degradable compounds e.g. humic acid-like, melanoidins, nitrogen heterocycle, and phenols were formed through 3D-EEM and GC-MS analysis, which might result in the lower methane yield of HTCWW at increased temperatures. Furthermore, microbial analysis showed that different microbial community compositions for anaerobic degradation of the HTCWW samples were established due to the different organic compositions. Therefore, the integration of HTC and AD can be a potential complementary process for energy recovery from DSS.