Anaerobic treatment of low-strength municipal wastewater with electroactive magnetite-embedded granules under mainstream conditions

This study examined the potential for the anaerobic treatment of low-strength wastewater (250 mg chemical oxygen demand/L) using expanded granular sludge bed (EGSB) with conductive magnetite-embedded granules (MEGs) under mainstream conditions (25℃). The inclusion of magnetite, with high specific gravity and conductivity, enhanced the settleability, structural stability, and electron transfer capability of MEGs compared to granules without it. Within the hydraulic retention time (HRT) range of 4–12 h, the MEG-EGSB reactors maintained superior treatment performance, along with comparable or higher methane production, compared to the control EGSB reactors lacking MEGs. This performance enhancement was more pronounced under conditions of higher hydraulic and organic loading. The MEG-EGSB reactors achieved approximately 90% organic removal efficiency at 8–12-h HRTs, a range comparable to that of the aerobic activated sludge process, with minimal loss of magnetite. The results from RNA-based microbial community analysis, alone with the consistently higher methane content in the biogas from MEG-EGSB reactors, supported the potential development of electric syntrophy between exoelectrogenic bacteria and electrotrophic methanogens in electroactive MEGs. This capability of MEGs to promote electro-syntrophic methanogenesis could potentially enhance methanogenic degradation of organic matter. The overall findings suggest that the MEG-EGSB process is a promising candidate for anaerobic municipal wastewater treatment.