Sustainable valorization of coffee pulp: Evaluation of the pretreatment effect on the phenolic compounds recovery and anaerobic digestion of the residual biomass

Coffee pulp is the main solid waste derived from wet processing, posing environmental risks if not properly handled. However, its valorization can benefit both the environment and the regional economy. This study examines the holistic approach of phenolic compounds extraction, coupled with the further valorization of the residual biomass through anaerobic digestion, while an economic assessment indicates the most favored pathway. Several pretreatment methods were considered (squeezing, grinding, ultrasounds (US), microwaves, and supercritical CO 2 (SC CO 2 )), followed by the evaluation of the extract content in total phenolics and flavonoids, while the anaerobic digestion process for CH 4 production followed. The US and SC CO 2 methods showed maximum phenolics extraction efficiency up to 58.3 % and 35.2 % respectively, compared to the reference. The biochemical methane potential tests also indicated higher methane yields for the cases of SC CO 2 (up to 31 %) and US (up to 22 %), although the rest of the pretreatment methods were not very promising. Finally, an economic assessment was conducted using the zero net benefit approach to investigate the minimum phenolic mixture selling price and the anaerobic digestion contribution on the profitability. Accordingly, the US pretreatment method was found to be the most efficient, with mixed phenolics selling price of 15.3 €/kg, namely 66 % less than a typical selling price, marking the US as the most economically feasible option. Overall, the study showed the importance of combining pretreatment and phenolic compounds extraction along with anaerobic digestion of the residual biomass for a sustainable valorization of coffee pulp to improve the environment and economic feasibility.