Distinguishing ethanol metabolism and electron transfer pathways in anaerobic sulfate-containing wastewater treatment systems: Impacts of reactor configuration and conductive materials.

Anaerobic systems are widely used to treat sulfate-rich industrial wastewater, where competition and cooperation between sulfate-reducing bacteria (SRB) and methanogens determine carbon flux and energy recovery. Reactor configuration and conductive materials (CMs) strongly shape microbial metabolism and electron-transfer pathways in such systems. In this study, eight anaerobic reactors, operated in sequencing batch reactor (SBR) and continuous-flow reactor (CFR) modes, were used to investigate ethanol-driven sulfate reduction and methanogenesis at a COD/sulfate ratio of 8:1, with Fe3O4 or powdered activated carbon as CMs. All reactors maintained stable long-term performance, showing nearly complete ethanol removal (99%), high sulfate reduction (up to 99%), and minimal volatile fatty acid accumulation. Reactor configuration was the primary driver of metabolic behavior. SBRs favored ethanol conversion to propionate and enriched incomplete-oxidizing SRB (IO-SRB), particularly Desulfobulbus. CFRs promoted ethanol oxidation to acetate and sustained efficient substrate conversion despite lower IO-SRB abundance. Without sulfate, IO-SRB in SBRs shifted from sulfate reduction to syntrophic propionate fermentation. In contrast, CFRs enhanced extracellular electron transfer (EET), reflected by higher sludge conductivity and the enrichment of Geobacter and Methanothrix, supporting direct electron transfer-mediated methanogenesis. Fe3O4 further strengthened EET in CFRs, while CMs in SBRs mainly stimulated syntrophic propionate degradation. Overall, sulfate availability, operational mode, and CM type jointly governed carbon-flux partitioning and electron-transfer pathways in sulfate-reducing anaerobic systems.