Energy and nutrient recovery by spent mushroom substrate-assisted hydrothermal carbonization of sewage sludge

Hydrothermal carbonization (HTC) has been recognized as a promising sewage sludge (SS) treatment technology for effective pathogen elimination, bioenergy recovery, organic contaminant destruction and volume reduction. However, the solid product (hydrochar) of SS after HTC as fuel has the problems of high ash content, high nitrogen content and low calorific value. The aqueous phase (AP) produced is still considered a burden and has become a bottleneck in the development of HTC. In this study, co-HTC of SS with spent mushroom substrate (SMS) is conducted, and the fuel properties of hydrochar and the quality of the AP as a liquid fertilizer are investigated. In comparison with hydrochar of single SS, the energy yield and higher heating value of the hydrochar from co-HTC were significantly increased by 12.1–44.8 % and 33.2–137.8 %, respectively, reaching their maximum of 72.75 % and14.98 MJ/kg, respectively. Co-HTC can improve safe handling, storage and transportation, and combustion performance of hydrochar. Furthermore, the AP of co-HTC could significantly increase the biomass of pakchoi, which was 140.9 % and 90.7 % of AP from single SS and Hoagland nutrition solution (represents commercial fertilizer), respectively. The AP of co-HTC as fertilizer can recover 62.03–64.65 % nitrogen from SS and SMS. These findings suggest that co-HTC of SMS with SS is a promising technology for the conversion SS into renewable resource products for fuels and N-rich liquid fertilizer while also significantly improving fuel and fertilizer quality.