Inhibition study of model compounds from sludge-derived hydrothermal liquefaction aqueous product on anaerobic digestion

This study systematically evaluated the inhibitory effects of model compounds from sludge-derived hydrothermal liquefaction aqueous phase (HTLaq) on anaerobic digestion (AD) at both mesophilic and thermophilic temperatures using a total of 1008 anaerobic toxicity assays (ATA). Twenty representative compounds of suspected inhibitors, including nitrogen-containing heterocyclics like pyridines, pyrrolidinones, and pyrazines, as well as phenols and ketones, were tested at varying dosages (25, 50, 100, 200, 400, and 800 mg/L) to assess their impact on volatile fatty acids (VFA) generation and consumption, methane production, substrate utilization, and inhibitory compound degradation. Results demonstrated that thermophilic AD is generally more susceptible to inhibition than mesophilic AD, both in terms of acute and chronic toxicity. Compounds such as 3-methylcyclopentanone, indole, pyridine, 2-ethylpyridine, 3-aminopyridine, phenol, and 2-aminophenol were identified as the most toxic to both mesophilic and thermophilic AD systems. Indole and 2-aminophenol uniquely inhibited both methanogenesis and acetogenesis, while the others only affected methanogenesis. Despite their partial degradation, several compounds, including pyrazines, pyridines, ketones, and indole, exhibited recalcitrant behaviour under both mesophilic and thermophilic conditions, with the exception of δ-valerolactam. These findings contribute to a deeper understanding of HTLaq toxicity in AD and inform strategies for optimizing biogas production from sludge-derived HTLaq waste streams.