Valorization of biogas production through disintegration of waste activated sludge using visible light ZnO-ZnS/Ag2O-Ag2S photocatalyst

The key requirements for accelerating the anaerobic digestion process and clean energy production involve the disintegration of the recalcitrant structure of waste activated sludge and transforming it into soluble organic components for microbial degradation. Herein, the visible light-active ZnO-ZnS/Ag2O-Ag2S nanocatalyst was used for the photocatalytic solubilization of sludge to improve methane production through anaerobic digestion. The photocatalysis released the soluble substances in sludge slurry, in which soluble chemical oxygen demand was increased from 410 to 3892 mg L−1 after 6 h. The volatile solids content was only decreased by 13.1%, which indicated that a short period of pretreatment could avoid the mineralization of organic matter. Methane production was improved by 54% in photocatalytic pretreated sludge, where the cumulative methane production was up to 0.6985 mL g−1 of VS compared with 0.4533 mL g−1 of VS in untreated sludge. Similarly, a higher organic matter removal efficiency of 54% was achieved due to the increased bioavailability of organic compounds under the influence of the pre-solubilization of sludge during photocatalysis. Moreover, photocatalysis reduced the start-up time for methanogenesis from 23 to 13 days. Overall, this study determined the significance of visible light-active ZnO-ZnS/Ag2O-Ag2S photocatalysis for sludge treatment and cleaner energy production, while providing a useful reference for its industrial applications.