Hydrothermal pretreatment of brewer's spent grain: A pathway to sustainable biogas production and waste valorization

Brewer's spent grain (BSG), a by-product of the brewing industry, faces significant waste management challenges but holds potential as a substrate for biogas production via anaerobic digestion (AD). However, its lignocellulosic complexity limits microbial degradation and methane yield. In previous studies by our research group, various treatment strategies for BSG were explored, including approaches aimed at recovering high-value compounds. Building upon this foundation, the present work evaluates the potential of hydrothermal pretreatment to enhance methane production from BSG via AD. The novelty lies in the combination of a continuous hydrothermal pretreatment reactor with meta-transcriptomic analysis of the anaerobic digestion process, enabling a direct correlation between operational performance and microbial functional shifts. Hydrothermal pretreatment was applied to partially hydrolyze cellulose and hemicellulose into fermentable sugars, improving biodegradability while minimizing the formation of inhibitory compounds. Results showed that hydrothermally pretreated BSG supported methane productions over 1500 mL CH 4 /L_reactor·day, markedly higher than the 100–500 mL CH 4 /L_reactor·day observed for untreated BSG. Volatile solids degradation efficiency improved by 30 %, while methane content in biogas increased from 30 % to 65 %. Genomic analysis of the microbial consortium revealed enhanced activity of methanogenic archaea and fermentative bacteria associated with the increased methane production. This integrated approach not only disrupts lignocellulosic barriers more effectively but also provides deeper insights into microbial functionality, reinforcing hydrothermal pretreatment as a viable strategy for boosting biogas yield and advancing sustainable waste-to-energy solutions. • Hydrothermal pretreatment increased methane production by over 200 %. • Volatile solids degradation efficiency improved by 30 % compared to untreated BSG. • Stable operational conditions (pH 7.0–7.5, alkalinity ∼3500 mg CaCO 3 /L) were maintained throughout the process. • Genomic analysis identified a higher abundance of Bacteroidota and Methanobacteriales in pretreated BSG. • Hydrothermal pretreatment offers a scalable solution for brewing waste-to-energy conversion.