Electro-assisted, magnetite-enhanced EGSB system for high-rate anaerobic treatment of municipal wastewater at low temperatures

Abstract

This study investigated an expanded granular sludge bed (EGSB) system combining magnetite supplementation with electrochemical stimulation for high-rate anaerobic treatment of low-strength municipal wastewater (250 mg chemical oxygen demand (COD)/L) over a temperature range of 20-5 degrees C. Three EGSB reactors were operated in parallel at a hydraulic retention time of 8 h: a magnetite-supplemented reactor (M), a magnetite-supplemented reactor with electro-assistance (EM), and a control reactor without magnetite or voltage application (C). Self-embedding of submicron magnetite formed dense and compact magnetite-embedded granular sludge (MEG), enhancing sludge settleability, biomass retention, granule stability, and electron transfer activity. Reactor M exhibited COD removal and methane yield comparable to or greater than Reactor C across all tested temperatures, while Reactor EM consistently outperformed both, reflecting additional enhancement from electrochemical stimulation. At the optimal voltage of 0.6 V, Reactor EM demonstrated markedly superior performance over the other reactors, maintaining around 80% COD removal and 1.6-fold higher methane yield at 15 degrees C and 10 degrees C. Even at 5 degrees C, Reactor EM remained superior despite overall performance decline. Temperature changes induced shifts in microbial community structure, particularly among bacterial communities, with Lactococcus (presumably involved in electric syntrophy), Paraburkholderia, and Sphingomonas identified as key genera associated with multiple process-related variables. Net energy recovery varied notably with temperature, with the highest values observed in Reactor EM at 15 degrees C and Reactor M at 20 degrees C. Overall, these findings highlight the promising potential of electro-assisted MEG-EGSB system for high-rate anaerobic treatment of municipal wastewater at low temperatures and the need to consider site-specific operational conditions.