Characteristics of digested sludge-derived biochar for promoting methane production during anaerobic digestion of waste activated sludge

•Digested sludge was used to produce biochar (DSBC) for solid waste recycling.•Response surface methodology was used to optimize DSBC synthesis.•Optimized DSBC was produced at 516 °C held for 192 min (increase rate: 13.23 °C/min).•DSBC increased the methane production from sludge anaerobic digestion by 48%.•Surface groups such as phenols boosted the biochar’s electron exchange capacity.