Hydrovoltaic Energy Harvesting from Sewage Sludge Induces Its Efficient Anaerobic Digestion

The hydrovoltaic effect originates from leveraging water-material interactions to generate electricity. Sewage sludge inherently possesses an abundant porous structure and water-solid interfaces favorable for hydrovoltaic power generation. Herein, we explored the influence of promoting the hydrovoltaic effect of sludge on its subsequent methanogenesis during anaerobic digestion (AD). It was observed that a maximum open-circuit voltage of 0.62 V was achieved from sludge, and in its subsequent AD, the methane production and proportion of methane in biogas increased by 82% and 24.6%, respectively, indicating that the hydrovoltaic effect of sludge enables the direct recovery of electricity and significantly enhances its subsequent methanogenesis. The stable isotope-labeled AD experiments demonstrate that the hydrovoltaic effect enhanced water participation in CO2-reduction methanogenesis. Statistical analyses of variations in physicochemical properties of sludge, key enzymes closely related to electron/proton transfer, and microbial community in AD reveal that the hydrovoltaic effect induced significant enhancement of water-mediated proton-coupled electron transfer-associated methanogenesis, providing a thermodynamic advantage for methanogenic reactions. It was further verified by metagenomic and metatranscriptomic analyses, which showed that the expression levels of key genes associated with the classical and RuBisCO-mediated CO2 reduction methanogenic pathways were almost all significantly upregulated. This study provides a reference for directly recovering electricity from sludge by utilizing inherent properties while inducing efficient AD.