Influence of infrared light illumination on biogas production and microbial communities in anaerobic digestion of primary sludge and fruit and vegetable waste.

Anaerobic digestion (AD) is a widely adopted technology for treating various waste streams, offering environmental benefits and carbon-neutral energy production in wastewater treatment plants. While most previous studies have focused on optimizing methane production under dark conditions, the impact of infrared (IR) light on AD performance remains largely underexplored. This study investigates the effect of IR light on the AD of primary sludge (PS) and fruit and vegetable waste (FVW). IR illumination substantially improved reduction of total chemical oxygen demand and volatile solids in PS mono-digestion. The methane yield from illuminated PS mono-digestion was approximately 1.5 times higher than that from dark conditions. In the co-digestion of PS with FVW, the methane yield under IR light was similar to that under dark conditions, but IR light illumination significantly improved methane content in the biogas. Microbial community analysis revealed that Rhodopseudomonas, a phototrophic bacterium, was dominant in the illuminated digesters. Especially, Rhodopseudomonas was highly enriched in the co-digestion of PS with FVW, comprising 60.78% of the microbial community. Rhodopseudomonas is known for its metabolic versatility and likely contributed to photoenhanced biogas production by facilitating conversion of organic matters into volatile fatty acids and hydrogen and fixation of carbon dioxide and nitrogen. Among archaea, Methanobacterium was the dominant genus across all conditions. These findings underscore the potential of IR light as a promising strategy to improve AD performance but the effects may vary upon different organic solid wastes. Further investigation is warranted to elucidate the underlying mechanisms of IR illumination enhanced AD.