Enhancing Methane Production from Food Waste via Anaerobic Digestion Using Waste-Derived Hydrogels: Improved VFA Conversion and Microbial Niche Formation

Anaerobic digestion (AD) offers a sustainable approach to food waste valorization through biogas production. Hydrogels, known for high water retention, porosity, and microbial compatibility, are increasingly explored as AD additives to enhance substrate diffusion, pH buffering, and microbial colonization. This study investigates waste-derived hydrogel materials, i.e., pure hydrogel (PH), biochar-hydrogel (BH), and LECA-hydrogel (LH), as multifunctional additives to enhance methane yield under varying organic loading rates (OLRs). At low OLR (0.3 g VS/L/d), BH40 (40 wt % biochar-hydrogel) achieved the highest methane yield (3.71 ± 0.21 L/g VS), producing 27.9% more methane than PH40 (40 wt % hydrogel) due to its buffering and conductive properties that supported syntrophic microbial activity and volatile fatty acids (VFAs) conversion. Conversely, at high OLR (0.9 g VS/L/d), PH40 yielded the highest methane (4.07 ± 0.28 L/g VS), attributed to improved pH stability and VFA utilization. Microbial analysis revealed PH40 enriched key methanogenic taxa, including Bacilli, Synergistia, and Cloacimonadia. Principal component analysis revealed hydrogel additives shaped distinct microbial communities, with PH40 promoting a methanogen-enriched cluster. Overall, this study highlights the novel use of waste-derived hydrogels as dual-function AD enhancers, demonstrating their cost-effective potential to improve methane yield while contributing to circular bioeconomy and sustainable waste-to-energy solutions.