Anaerobic co-digestion of coffee husks and microalgal biomass after thermal hydrolysis

•Thermal hydrolysis pretreatment improved anaerobic digestion of substrates at 120 °C.•Biodegradability of coffee husks increased after co-digestion with microalgae.•Co-digestion showed a synergistic effect, with 87% increase in terms of methane yield.•Energy balance of coffee inverted from negative to positive values with co-digestion. Residual coffee husks after seed processing may be better profited if bioconverted into energy through anaerobic digestion. This process may be improved by implementing a pretreatment step and by co-digesting the coffee husks with a more liquid biomass. In this context, this study aimed at evaluating the anaerobic co-digestion of coffee husks with microalgal biomass. For this, both substrates were pretreated separately and in a mixture for attaining 15% of total solids (TS), which was demonstrated to be the minimum solid content for pretreatment of coffee husks. The results showed that the anaerobic co-digestion presented a synergistic effect, leading to 17% higher methane yield compared to the theoretical value of both substrates biodegraded separately. Furthermore, thermal hydrolysis pretreatment increased coffee husks anaerobic biodegradability. For co-digestion trials, the highest values were reached for pretreatment at 120 °C for 60 min, which led to 196 mLCH4/gVS and maximum methane production rate of 0.38 d−1.