Molecular transformation and metabolic insights of microbial electrolysis treatment and valorization of post-hydrothermal liquefaction wastewater

Hydrothermal liquefaction presents a promising approach for the conversion of wet waste into biocrude and biofuels. However, the post-hydrothermal liquefaction wastewater poses significant challenges for treatment and valorization due to its high concentration and complex nature. In this study, we investigated the conversion pathways of major organic contaminants within the microbial electrolytic treatment of wastewater from food waste hydrothermal liquefaction. To achieve this, we employed high-performance liquid chromatography and 2D nuclear magnetic resonance. Our findings demonstrate volatile fatty acids and monohydric and polyhydric alcohols were effectively transformed through the synergistic metabolism of fermentative and electroactive bacteria, which led to over 70% chemical oxygen demand removal of the recalcitrant compounds and a record high H₂ production rate (1.62 L L⁻¹ d⁻¹). We also employed the liquid-state ¹⁵N nuclear magnetic resonance on wastewater samples for the first time and revealed that the nitrogen-containing heteroaromatics were persistent to microbial electrochemical treatment. By integrating the chemical profiles with bioanode community profiles, we constructed a metabolic network that provides insights for enhancing treatment efficiency and facilitating resource recovery.