Biochar and granular activated carbon mitigate inhibitory stress and enhance methane production in brewery spent grain anaerobic digestion

Anaerobic digestion (AD) of brewery spent grain (BSG) aligns with lignin valorization through biogas recovery and the generation of value-added carbon materials. In lignocellulosic biorefineries, pretreatments increase BSG solubility but can produce byproducts that challenge AD stability. This study evaluated the effects of biochar (BC) and granular activated carbon (GAC) at 10–30 g/L during the co-digestion of raw BSG and hydrolysates produced by thermo-alkaline (ALK) and steam explosion (EXP) pretreatments, focusing on methane production, process stability, and the microbial community. In the steam explosion hydrolysate, 10 g/L BC increased methane yield by 133%, shortened the lag phase from 5.2 to 1.4 days, and p-cresol decreased by 72%. In thermo-alkaline systems, cumulative yields were comparable to the blank. However, 30 g/L GAC increased the methane production rate by up to 10.5%, reduced volatile fatty acids (VFAs) accumulation by 82%, and increased p-cresol removal to 74%. The performance gains arose from selective adsorption and buffering together with redox-active surfaces that enable Direct Interspecies Electron Transfer (DIET), accelerate acetate and propionate conversion, and promote microbial aggregation. Effects varied with the substrate and conditions, with the strongest response in the steam-exploded hydrolysate characterized by elevated free ammonia, VFAs, and p-cresol. Community profiling showed enrichment of DIET partners, notably Thauera , Syntrophobacter , and Methanothrix . These findings demonstrate that carbonaceous materials mitigate metabolic stress and reinforce syntrophic interactions, offering a practical strategy to enhance methane recovery from lignocellulosic biorefineries. • Biochar and GAC enhanced methane recovery from pretreated BSG. • g/L biochar boosted the methane yield by 133% in the steam explosion hydrolysate. • p-cresol inhibition was alleviated by 72–74% with carbon mediators. • Carbon surfaces enable DIET, adsorption, buffering, and microbial aggregation. • Thauera degraded aromatics, supplying intermediates for methanogens.