Combining a solid-state submerged fermenter with bioelectrochemically enhanced anaerobic digestion (BEAD) process for enhanced methane (CH4) production from food waste: Effects of the organic loading rates and applied voltages

This study investigated the performance of a two-stage reactor system comprising a solid-state submerged fermenter (3SF) and a bioelectrochemically enhanced anaerobic digestion (BEAD) process. Initially, the 3SF performance was optimized by evaluating the impact of leachate recirculation rates (LRRs) of 34 L/day, 50 L/day, and 75 L/day. The maximum hydrolysis and acidification yields of 635 ± 10 gsCOD/kgVS added and 531 ± 8 gsCOD SCFA /kgVS added were observed at an LRR of 75 L/day, demonstrating the significance of operating at higher LRRs. Furthermore, the performance of the BEAD was assessed at progressing organic loading rates (OLRs) of 3–12 gsCOD/L/day, with a fixed applied voltage of 1.4 V using FW leachate. At higher OLRs of 7–12 gsCOD/L/day, BEAD outperformed AD-control, achieving better sCOD removal efficiencies and CH 4 yield ranging from 94 % to 88 % and 0.35–0.30 L/gsCOD removed observed compared to 91–81 % and 0.28–0.23 L/gsCOD removed in the AD-control. Although performance at low OLRs of 3–6 gsCOD/L/day was similar across systems, however, BEAD achieves steady-state conditions quickly, demonstrating improved operational stability across a wider OLR range of 3–12 gsCOD/L/day compared to the 3–6 gsCOD/L/day in the AD-control. The impact of the applied voltages of 1 V, 0.6 V, and 0 V was also investigated at OLRs of 3 and 12 gsCOD/L/day. Notably, applied voltage had a more pronounced impact at a higher OLR, enhancing energy gain from 0.09 to 0.27 Wh/gsCOD removed compared to 0.03–0.066 Wh/gsCOD removed at the lower OLR of 3 gsCOD/L/day. Microbial community analysis revealed a higher relative abundance of H₂-producing butyrate-oxidizing bacteria and exoelectrogens in the BEAD system, indicating the coexistence of both conventional and exoelectrogenic CH 4 production pathways. Overall, the proposed 3SF-BEAD system enhanced the CH 4 yield, shortened the operational cycle, and enhanced stability at higher OLRs. • 3SF performance was improved by 12 % at a higher LRR of 75 L/day. • BEAD performance was better than AD-control at higher OLRs of 7–12 gsCOD/L/day. • CH 4 yield in the BEAD was 30–63 % higher than AD-control. • Applied voltage led to 11-fold more energy gain at a higher OLR of 12 gsCOD/L/day. • The operational cycle of 3SF-BEAD was 2-fold shorter than conventional systems.