Microelectric field-enhanced air-lift A/O process: Mechanisms for advanced nutrient removal in low C/N rural domestic...

The predominant challenge in treating rural domestic sewage (RDS) stems from its low carbon/nitrogen ratio (C/N), which fundamentally restricts the direct applicability of conventional municipal sewage treatment processes. To address this challenge, this study developed an innovative microelectric field-enhanced air-lift A/O process. The results demonstrated that this process achieved average removal efficiencies of 78.91% for total nitrogen (TN) and 93.80% for total phosphorus (TP), representing a significant improvement over conventional process. Mechanistic investigations revealed that the enhanced nitrogen removal primarily originated from the microelectric field-driven efficient reduction of NO₃^--N in the anoxic zone, while phosphorus elimination resulted from synergistic interactions among chemical (sweep-flocculation by iron oxides/ions), physical, and biological processes. Microelectric field can stimulate microorganisms to produce more extracellular polymeric substances (EPS) and increase the ratio of protein (PN) and polysaccharide (PS). Metagenomic-based taxonomic comparison illustrated significant enrichment of nitrifying and denitrifying bacteria in the microelectric field-enhanced system, especially promoting the growth of autotrophic denitrifying bacteria, thereby providing an alternative electron source for nitrogen removal. Furthermore, the increased abundance of nitrogen/phosphorus metabolism-related functional genes and key enzymes provided molecular-level evidence for the enhanced metabolic pathways. Through parameter optimization, the optimal operating conditions were determined as follows: hydraulic retention time (HRT) of 12 h, internal recirculation ratio of 150-170%, and voltage of 1 V. This study demonstrated that microelectric field-enhanced technology offered an efficient and feasible strategy for addressing the challenges of nitrogen and phosphorus removal in RDS with low C/N.