Enhancing anaerobic digestion via synergistic integration of bioelectrochemical stimulation and conductive nanoparticles-mediated electron transfer.

The integration of electrically conductive materials and bioelectrochemical systems (BES) offers a promising approach to enhance anaerobic digestion (AD) by promoting extracellular electron transfer and stabilizing syntrophic interactions involved in methanogenesis. This study evaluated the combined effect of electrode polarization and magnetite nanoparticles (1 g L-1) on sewage sludge AD using two identical bioelectrochemical reactors operated under either applied voltage (0.8 V) or open-circuit potential (OCP) conditions. Under polarized conditions, magnetite supplementation significantly enhanced methanogenesis, increasing the maximum methane production rate by up to 51% compared with the control. This improvement was associated with faster volatile fatty acid consumption, particularly acetate and propionate, indicating accelerated syntrophic oxidation pathways. In contrast, under OCP conditions, the performance difference between magnetite-amended and control reactors was negligible, suggesting that conductive nanoparticles alone were insufficient to improve digestion kinetics without an electrochemical driving force. Higher current intensities were observed in the magnetite reactor, with current peaks correlated to acetate accumulation, supporting enhanced extracellular electron transfer. Energy analysis showed that the additional methane energy recovered exceeded the electrical energy required for polarization. These results demonstrate that magnetite-assisted bioelectrochemical stimulation is an energy-efficient strategy for improving sewage sludge stabilization and methane recovery.