Anthraquinone-2-sulfonate immobilized on granular activated carbon inhibits methane production during the anaerobic digestion of swine wastewater

Granular activated carbon (GAC) and GAC modified with anthraquinone-2-sulfonate (AQS) were used as conductive materials during the anaerobic digestion of swine wastewater (SW). The electron transfer capacity (ETC) in the GAC-AQS was 2.1-fold higher than the unmodified GAC. Despite the improvement in the ETC, the GAC-AQS cultures showed an inhibitory effect, evidenced by the lowest methane productivity. Indeed, the cultures with unmodified GAC achieved 236 mL CH₄/g COD_i (chemical oxygen demand, initial), representing an increment of 1.14- and 2.05-fold compared with the control (without conductive materials) and GAC-AQS, respectively. In addition, the methane production rate (<i>R</i>_max) and yield were also improved with unmodified GAC, but they decreased with GAC-AQS. The role of solid-phase AQS (GAC-AQS) as a terminal electron acceptor during microbial respiration competes with methanogenesis for the electrons instead of serving as an electron conduit.