Enhancement of biological slow filtration by micro-electric field with granular electrodes: Influence of anode position and microbial characteristics

To enhance the efficiency of decentralized rural drinking water treatment, this study investigates the application of a weak micro-electric field with granular electrodes in biological slow filtration (BSF) systems, with a focus on the influence of anode position and microbial responses. Four reactor configurations were evaluated over 78 days: a non-electrified control (BSF-0), a surface-anode system (BSF-S), a mid-layer anode system (BSF-M), and a short-filter system with surface anode (BSF-SS). The bio-electrochemical system with electric field of 0.67 V/cm (BSF-S) demonstrated superior and more balanced contaminant removal performance compared to both non-electrified and stronger electric field (1.0 V/cm, the other two BSFs) systems. Specifically, BSF-S achieved the optimum performance, with turbidity, COD, and NH 3 -N removal rates of 98.06 ± 2.08 %, 70.16 ± 9.76 %, and 84.53 ± 12.40 %, respectively, during stable operation, representing notable enhancements over BSF-0. Microbial analysis revealed that the surface anode promoted the formation of an electroactive biofilm with high extracellular polymeric substance (EPS) content and microbial diversity (685 unique OTUs), and established a distinct functional zonation, enriching nitrifying Nitrospirota (10.74 %) in the bottom layer and anaerobic degraders like Fimbriimonadaceae (11.47 %) in the middle. In contrast, the mid-layer anode led to microbial heterogeneity and selected for specialized electroactive taxa (e.g., Cupriavidus at 21.11 %) capable of heterotrophic nitrification–aerobic denitrification, but with lower overall diversity, while the short-filter system suffered from inhibited nitrification due to insufficient hydraulic retention. The results demonstrate that optimizing the synergy between a weak electric field and filter bed architecture is crucial for fostering a stable, stratified, and functionally diverse microbial ecosystem, which significantly enhances pollutant removal and system stability, providing a promising strategy for upgrading rural drinking water purification.