Synergistic Effects of Subcritical Water Pretreatment and Anaerobic Digestion of Brewers’ Spent Grains for Biogas Production

The sustainable management of brewer’s spent grain (BSG) is critical for advancing circular bioeconomy strategies in the brewing industry; however, its efficient conversion to bioenergy remains limited by lignocellulosic recalcitrance. In this study, subcritical water hydrolysis (SWH) is systematically evaluated under mild conditions as an environmentally friendly pretreatment to simultaneously enhance the solubilization of carbohydrates and proteins and improve the anaerobic digestion (AD) performance of BSG. Under relatively low-severity conditions (130 °C, 15 MPa), SWH promoted extensive depolymerization of BSG, releasing up to 146 mg g−1 of total reducing sugars and 18 mg albumin g−1 of soluble proteins, while generating organic acids that influenced hydrolysate pH. Unlike previous studies that primarily focused on solid BSG digestion or high-severity pretreatments, this work directly compares the biomethane potential of SWH-derived hydrolysate and solid BSG under controlled BMP assays. The hydrolysate supported stable microbial activity and efficient degradation of volatile fatty acids, achieving a maximum methane yield of 712 L CH4 kg−1 TVS, significantly exceeding the yield obtained at 12.5% solid BSG loading (469 L CH4 kg−1 TVS). These results demonstrate that mild SWH substantially enhances BSG biodegradability and methane recovery while revealing critical trade-offs between organic loading, conversion efficiency, and process stability. Overall, this study provides new process-level insights into the integrated use of SWH and AD for BSG valorization, positioning SWH as a scalable, low-chemical, and sustainable pretreatment strategy for maximizing renewable biogas production from agro-industrial residues.