Distinct effects of conductive materials on anaerobic sulfate-rich wastewater treatment under varying operational modes

Conductive materials (CMs) have shown great potential in enhancing anaerobic digestion of sulfate-rich wastewater by improving electron transfer. However, how the effectiveness of CMs varies under different reactor operational modes that shape microbial communities and influence system performance remains poorly understood. This study systematically compared sequencing batch reactors (SBRs) and continuous-flow reactors (CFRs) amended with magnetite (Fe₃O₄) or powdered activated carbon (PAC) for treating sulfate-rich wastewater. CM amendments significantly accelerated sulfate reduction, volatile fatty acids degradation, and methane production, especially with the addition of Fe₃O₄. Maximum methanogenesis rates in CFRs increased from 31.2 mg COD/(g VSS·h) without CMs to 51.0 and 39.7 mg COD/(g VSS·h) with the addition of Fe₃O₄ and PAC, respectively. Methanogenesis in SBRs was severely inhibited by elevated hydrogen sulfide concentrations, and supplementation with 1 g/L CMs failed to alleviate this inhibition. CFRs favored direct ethanol-to-acetate conversion, whereas SBRs activated ethanol-to-propionate metabolic pathway mediated by Desulfobulbus. CM additions led to increased sludge conductivity and electron transport activity. Specifically, PAC strongly enhanced electron transfer in CFRs by promoting e-pili and cytochrome gene abundances, whereas Fe₃O₄ in SBRs predominantly acted as an external conductive conduit, partially substituting intrinsic microbial conductive structures. Key sulfate-reducing bacteria (SRB), including Unclassified_f_Desulfovibrionaceae, Desulfomicrobium, Desulfolutivibrio, and Desulfovibrio, dominated the expression of e-pili and cytochrome genes associated with the direct interspecies electron transfer, which was promoted by CFR operation through the enrichment of SRB. Microbial co-occurrence network analysis further highlighted Desulfovibrio, Methanothrix, and Geobacter as central keystone species mediating robust syntrophic electron transfer networks. These findings provide critical insights for optimizing sulfate-rich wastewater treatment through strategic selection of reactor modes and CMs.