Impact of Emerging and Traditional Solvents on Anaerobic Digestion in Biorefinery Processes

The use of solvents for extracting high-value products in biorefineries is common, but their effects on anaerobic digestion (AD) are not well understood. This study systematically evaluates the impact of both emerging and traditional solvents─d-limonene, ethanol, n-hexane, 2-methyloxolane (Meox), and cyrene─on AD performance using batch tests. Specifically, the study aims to (i) assess the potential toxicity of these solvents and (ii) investigate microbial consortium adaptation. Given the increasing utilization of AD for the management of residuals and the production of renewable energy, understanding the effects of solvent residues on AD is crucial for designing sustainable biorefinery processes. Microcrystalline cellulose treated with the selected solvents was used as a model substrate to measure biochemical methane potential (BMP) in Phase I. The digestate from successful BMP tests was reused in Phase II to examine the potential microbial adaptation to the solvent presence. In Phase I, d-limonene exhibited a strong inhibitory effect on AD (no methane production at 50% concentration), while a moderate adaptive response was observed in Phase II. Meox displayed complex behavior, with relatively high BMP values (408 ± 80 N mL/gVS) up to 50% concentration in Phase I but reduced production in Phase II. Cyrene led to increased methane production in Phase II (463 ± 28 N mL/gVS at 30% concentration), suggesting microbial adaptation and potential solvent degradation. Ethanol and n-hexane performed the best: ethanol produced 732 ± 184 N mL/gVS at a 50% concentration, improving biodegradability and reducing lag phases at lower concentrations. n-Hexane increased methane production by about 30% at low concentrations, although its petroleum-based origin warrants further investigation regarding potential issues in digestate use for agriculture.