Combined acid pretreatment and co-hydrothermal carbonization to enhance energy recovery from food waste digestate

•A combination of acid pretreatment and co-hydrothermal process was proposed.•Combination strategy significantly increased the energy potential of hydrochar.•The hydrochar had a maximum HHV of 25.8 MJ kg−1.•The minimum E was 102.0 and 90.4 kJ mol−1 by FWO and KAS methods, respectively.•Combustion performance of hydrochar was also improved by combination strategy.