Effects of the PSB-to-OW Ratio and Light Intensity on Biogas Composition during Phototrophic-Assisted Anaerobic Fermentation of Organic Waste

Abstract

The integration of phototrophic processes into anaerobic fermentation offers a promising strategy for improving biogas quality; however, the combined effects of the photosynthetic bacteria-to-organic waste (PSB-to-OW) ratio and light intensity remain unclear. In this study, the photosynthetic bacteria (PSB)-assisted anaerobic fermentation of municipal organic waste under varying PSB-to-OW ratios and light intensities was evaluated. An optimal PSB-to-OW ratio of 1:2 resulted in significantly higher CH₄ concentrations and lower CO₂ and H₂S concentrations than the other tested ratios did (p < 0.05). At this ratio, moderate illumination (2000 lx) resulted in the highest CH4 concentration (68.2 ± 0.3%), compared with 1000 lx (61.5 ± 0.4%) and 4000 lx (63.1 ± 0.5%). Under illumination, the CO₂ and H₂S concentrations were lower than those under dark conditions, indicating a more favorable biogas composition. Kinetic analysis indicated that CO₂ and H₂S exhibited apparent first-order behavior under illumination, with the highest rates occurring at 2000 lx (kH₂S = 0.093 day^-1; kCO₂ = 0.059 day^-1), suggesting light-dependent transformation processes. Metagenomic analysis revealed a microbial community dominated by sulfide-oxidizing (Chlorobium sp.), photoheterotrophic (Rhodopseudomonas sp.), and syntrophic bacteria, supporting coupled carbon conversion and sulfur transformation. Overall, optimization of the PSB-to-OW ratio and light intensity improved the biogas composition by increasing the CH₄ concentration while reducing the CO₂ and H₂S concentrations. These findings demonstrate a coupled mechanism of light-driven sulfide oxidation and carbon conversion, providing a basis for integrating phototrophic processes into anaerobic systems for improved energy recovery and biogas purification.