Light stimulated H₂O₂ inhibition on methanogenesis during anaerobic digestion towards enhanced VFAs production

Producing volatile fatty acids (VFAs) from anaerobic digestion (AD) of sewage sludge is of strong interest and can be realized via arresting methanogens. Herein, in situ hydrogen peroxide (H₂O₂) dosage has been demonstrated to be an effective approach to suppress methanogenesis and promote VFAs accumulation in the presence of illumination. Two AD reactors operating under different conditions produced average VFAs concentrations of 11,456.5 mg COD L^-1 (R_Light, 80 mg L^-1 H₂O₂) and 11,896.5 mg COD L^-1 (R_Dark, 380 mg L^-1 H₂O₂). The comparable VFAs concentrations indicated that methanogenic activity was effectively inhibited under both conditions, thereby preventing further conversion of VFAs to methane. However, visible light significantly reduced the H₂O₂ dosage level from 380 to 80 mg L^-1 to achieve complete methane inhibition, likely due to light-activated H₂O₂ producing ·OH. The light-exposed H₂O₂ elevated intracellular reactive oxygen species (ROS) levels, leading to a sharp decline in methanogen abundance (only 5.02 % remained compared to the inoculum). Firmicutes became dominant, increasing from 14.03 % (inoculum) to 52.35 % under high oxidative stress. Metagenomic functional predictions inferred microbial communities that tended to mitigate oxidative stress by reducing ROS generation, as shown by the relative gene abundance reduced from 0.0577 to 0.0472 %. A side-effect of H₂O₂ inhibition was the enrichment of NH₄⁺ and PO₄^3-, increased by 1.44 and 5.26 times, respectively; those nutrient compounds could be potentially recovered.