Anaerobic Valorization of Non-Detoxified Pentose Liquor from Sugarcane Bagasse Hydrothermal Pretreatment for Sequential Hydrogen and Methane Recovery

This study evaluated the anaerobic valorization of real, non-detoxified sugarcane bagasse-derived pentose liquor. Direct fermentation without nutrients, co-substrate, or buffering was limited, reaching low-to-moderate (37–54%) carbohydrate conversion efficiency at low organic loading rate (OLR < 5.6 kg COD m−3 d−1) levels. Hydrogen yield (HY) was also low (0.63–0.89 mol H2 mol−1 carbohydrateconverted) under these conditions. Increasing pentose liquor concentration up to 0.8 L L−1 and OLR of 30.2 kg COD m−3 d−1 suppressed hydrogen production, coinciding with lactate accumulation and the occurrence of homoacetogenesis. Enhanced fermentation with sucrose, nutrients, buffering, shorter hydraulic retention time (12 h) and 37 °C improved stability and carbohydrate conversion (50–61%) under higher OLR, although HY considerably decreased (0.14–0.35 mol H2 mol−1). Methanogenesis was effective in both single- and two-stage systems, with carbohydrate conversion usually above 95%, COD removal up to 83.5%, methane fractions above 60%, and methane yields reaching 299.8–301 NmL CH4 g−1 CODremoved. Furfural and 5-HMF showed phase-dependent transformation, indicating partial in situ detoxification during anaerobic conversion. Overall, pentose liquor is better valorized through integrated hydrogen–methane recovery than by fermentation alone. Future studies should focus on both applying more effective strategies to buffer the fermentative stage and adopting more conservative approaches (lower OLR) to start up the methanogenic systems.