Transforming Anaerobic Granular Sludge Systems for Mainstream Municipal Wastewater Treatment through Magnetite-Embedded Granules and Electrochemical Stimulation

Abstract

The global shift toward sustainable and energy-efficient wastewater treatment calls for alternatives to conventional aerobic activated sludge processes, which are highly energy-intensive due to aeration and dissipate the intrinsic chemical energy of wastewater. Anaerobic treatment offers a promising solution by converting organic matter into methane-rich biogas while substantially reducing energy input. However, its application to low-strength municipal and domestic wastewater under mainstream conditions remains limited by low substrate concentrations, biomass washout, and suppressed microbial activity at ambient and low temperatures. This doctoral research addresses these challenges by developing magnetite-embedded granular sludge (MEG) and integrating electrochemical stimulation to promote direct interspecies electron transfer (DIET), thereby improving methanogenic activity, granule stability, and process resilience under mainstream conditions. Study I examined the feasibility of applying MEG to the anaerobic treatment of low-strength municipal wastewater at 25℃ using expanded granular sludge bed (EGSB) reactors. Duplicate reactors, with and without submicron magnetite supplementation, were compared in terms of granule morphology, physicochemical characteristics, and microbial communities. Magnetite was successfully self-embedded within granules, improving density, settling, and structural stability. The formation of conductive MEG promoted DIET between electroactive bacteria and methanogens, improving COD removal and operational stability without major design modifications. These results established MEG as a simple and cost-effective approach for improving mainstream anaerobic treatment efficiency. Study II extended the concept to low-temperature conditions (25–10℃), which addressed one of the key barriers to anaerobic treatment in temperate and cold regions. Despite reduced enzymatic activity, the MEG maintained higher COD removal, methane productivity, and biomass retention than the control. Physicochemical and microbial analyses confirmed that magnetite embedding facilitated DIET and stabilized methanogenesis under kinetic limitations. Higher electron transport system (ETS) activity, greater granule conductivity, and higher levels of DIET-related genes (pilA and omcS) provided potential evidence of promoting extracellular electron transfer. These findings demonstrate that MEG can effectively mitigate temperature-induced performance deterioration, thus extending the applicability of anaerobic technologies to colder climates. Study III advanced the concept by integrating electrochemical stimulation with MEG and developing an electro-assisted EGSB system for low-strength wastewater treatment at low temperatures (20–5℃). Optimal voltage application (0.6 V) increased methane production and organic removal, while excessive voltage (0.9 V) inhibited performance due to electrochemical stress. The combined effects of conductive magnetite and external voltage promoted electroactive biofilm development, increased ETS activity, and improved process stability at low temperatures. Microbial community analysis revealed enrichment of electrotrophic Methanothrix and hydrogenotrophic Methanobacterium, indicating stimulation of DIET-mediated and syntrophic acetate oxidation pathways. Energy balance analysis showed a net positive energy gain at 15℃, which confirmed the potential of the electro-assisted MEG-EGSB system for energy-efficient operation in mainstream conditions. Collectively, this thesis demonstrates that magnetite-embedded granule formation and electrochemical stimulation are effective and complementary strategies for overcoming kinetic and stability constraints in mainstream anaerobic treatment. Study I established the feasibility of MEG for improving granule structure and methanogenesis at ambient temperature. Study II confirmed the robustness and electroactivity of MEG at low temperatures. Study III introduced electro-assisted MEG- EGSB technology, which achieved higher methane yields and energy recovery at subambient temperatures. Together, these studies provide mechanistic insights and engineering guidance for developing next-generation, energy-positive anaerobic wastewater treatment processes that harmonize with global sustainability goals.