Enhancing anaerobic digestion yield of agro-industrial residues using solid–liquid extraction: a microbiome assessment

Valorizing industrial agro-residues such as tomato pomace (TP) and potato peel (PP) within circular economy frameworks is crucial to reducing environmental burdens while recovering valuable compounds and bioenergy. This study investigates the potential of solid–liquid extraction (SLE) of TP and PP, applied before anaerobic digestion (AD), to enhance the associated microbial community and the biogas yield. Ethanol was employed as the extraction solvent to maximize the recovery of value-added compounds (e.g. phenolic molecules). The residual biomass post-extraction is significant (>90%) and was then used as substrate in batch AD tests to evaluate its biochemical methane potential (BMP). Microbiome analysis was also performed to elucidate the bacterial communities associated with the observed changes in AD performance following SLE. Chemical characterization of both extracts revealed a recovery of 1.1 and 2.3 mg gallic acid equivalents - GAE/g dried sample of PP and TP, respectively. BMP assays demonstrated that extracted biomass yielded higher methane production (>1.5x) relative to raw materials, suggesting that residual ethanol present in the extracted material positively influenced methane yield and biogas quality. Indeed, the microbiome revealed significantly different patterns, indicating that extracted materials promote the growth of specific microbial species. The AD of extracted materials containing residual ethanol exhibits lower microbial diversity, with the class Bacilli accounting for around 50% of the overall community. This result reflects significant microbiome adaptation to the altered substrate composition. The results support the implementation of valorization pathways that simultaneously generate value-added compounds and optimize biogas production, advancing the transition toward sustainable, circular bio-based systems.