Coupling Ion Exchange with Partial Nitritation and Anammox (IX-PN/A) for Efficient Mainstream Nitrogen Removal

This study investigates the performance and microbial community dynamics of a novel single-stage ion exchange partial nitritation and anammox (IX-PN/A) reactor for sustainable nitrogen removal from low-strength wastewater. The IX-PN/A process addresses key challenges in mainstream anammox application, including low ammonia concentrations, slow bacterial growth rates, and competition from heterotrophic bacteria. The reactor integrates ion exchange using natural zeolite carriers with biological nitrogen removal processes (partial nitritation and anammox) in a fixed-bed configuration, enabling in situ biological regeneration without chemical regenerants.The reactor was operated continuously for over 250 days under varying operational conditions to evaluate system stability and microbial community adaptation. The system achieved stable nitrogen removal performance with ammonia removal efficiencies ranging from 63.5 to 87.0 percent and total nitrogen removal efficiencies of 55.9 to 64.3 percent across all operational stages.High-throughput 16S rRNA gene amplicon sequencing was performed to characterize bacterial and archaeal community structure and spatial distribution within the reactor. Samples were collected from multiple locations including different carrier materials (zeolite and lava rock), suspended sludge fractions, and upper and lower reactor zones. Microbial community analysis revealed distinct spatial organization with functional groups occupying substrate-defined niches. Ammonia oxidizing bacteria were predominantly found in outer biofilm layers with higher oxygen exposure, while anammox bacteria were closely associated with the zeolite surface in oxygen-limited zones. The presence of comammox Nitrospira was detected and likely contributed significantly to ammonia oxidation under low-oxygen conditions.