High-rate anaerobic treatment of domestic sewage in EGSB reactors using amended granular sludge down to 10°C

Abstract

With growing demand for sustainable wastewater solutions, anaerobic treatment of domestic sewage has drawn increasing attention. However, under mainstream conditions, it faces kinetic and operational limitations due to the dilute organic content and large flow volume of domestic sewage. This study presents the development of electroactive magnetite-embedded granular sludge (MEG) through the in situ self-embedding of conductive submicron magnetite particles into methanogenic granules as a strategy to enhance the performance and stability of anaerobic domestic sewage treatment under low-temperature mainstream conditions (25-10 degrees C, 8-h HRT). Expanded granular sludge bed (EGSB) reactors supplemented with magnetite (EM) achieved superior chemical oxygen demand (COD) removal (77-90% at 250 mg COD/L influent) and operational stability across all temperature conditions, compared to non-supplemented control reactors (EC; 65-76%). Notably, EM reactors sustained 77% COD removal even at 10 degrees C with a short HRT of 8 h, demonstrating the functional robustness of MEG under low-temperature mainstream conditions. Total methane yield in EM reactors also remained comparable to or higher than that in EC reactors (0.30-0.33 vs. 0.28-0.31 L/g COD fed), with approximately half or more present in dissolved form. This performance enhancement is attributable to the improved settleability, structural stability, and electron transfer capability of MEG, facilitating biomass retention and fostering electric syntrophy. Enrichment of exoelectrogenic bacteria and electrotrophic methanogens, alongside elevated expression of extracellular electron transfer-related genes, further supported the development of electric syntrophy. These findings highlight the potential of the MEG-EGSB system as a promising anaerobic alternative for mainstream domestic sewage treatment.