Enhancing biomethane production from food waste using olive pomace hydrochar: An optimization study

•Hydrochar improved the stability and performance of the AD of FW.•The addition of hydrochar increased both the biodegradability and methane yield.•The highest experimental YCH4 is 477±7 Nml/g VS obtained for 20 g/L of hydrochar.•Two types of RSM- CCD are compared and the highest R2 is visualized for CCCD.•Based on the RSM results, the optimal value of YCH4 is 428.8 for 20 g/L of hydrochar.