Ionic liquid pre-treatment of olive pomace for lignin recovery enhances microbial-driven methane production during co-digestion with pig slurry

Olive pomace is a by-product with potential for resource recovery in the context of a circular economy, where anaerobic digestion is regarded as a key technology to achieve zero-waste biorefineries. This study assessed the potential for biogas production from residual olive pomace following an ionic liquid (IL) pre-treatment for nanoscale lignin recovery. Mono-digestion and co-digestion of untreated and residual olive pomace with pig slurry were compared in terms of process performance and microbial communities. Olive pomace was pre-treated with [Et3NH][HSO4] (60% in water) using a 1:10 ratio (w/w) for 4 h at 120 °C. The pre-treatment reduced surface compactness by removing structural components and increased cellulose bioavailability. Mono-digestion of untreated pomace exhibited strong inhibition, low methane yield (15 mL CH4 ·g−1 COD), accumulation of volatile fatty acids (VFAs) and highly specialised microbial communities with strong enrichment in members of the VFA-accumulating family Prevotellaceae . Conversely, mono-digestion of residual pomace resulted in higher initial methane yield (222 mL CH4·g−1 COD), attributed to the higher cellulose bioavailability and enzymatic biodegradability. However, methane production drastically dropped after 42 days. Methanogenesis inhibition in mono-digestion systems was associated with the low alkalinity of the substrates. Indeed, the co-digestion of both untreated and residual pomace with pig slurry achieved stable performance, with methane yields of 80 and 166 mL·CH4 g−1 COD, respectively. Upon co-digestion, the microbial communities in pig slurry predominated, especially members of Bacillota . These findings highlight the potential of recovering both biopolymers and bioenergy from olive pomace (and pig slurries) within a biorefinery context.