Enhanced biological nitrogen removal driven by iron dosing in a membrane bioreactor wastewater treatment plant

Ferric iron (Fe 3+ ) dosing has been a potential strategy to enhance biological nitrogen removal in wastewater treatment. This study investigated a membrane bioreactor (MBR)-based wastewater treatment plant (WWTP) with Fe 3+ addition at the primary settling stage, integrating long-term operational data, controlled batch experiments under varied dosing regimens, and metagenomic sequencing. Operationally, the Fe-enhanced MBR achieved ultra-low effluent ammonia concentration (NH 4 + -N) of 0.14 mg N/L and total nitrogen (TN) of 4.32 mg N/L. Batch tests showed that Fe 3+ dosing reduced net NH 4 + accumulation by 39% relative to the control, with nitrate as the dominant product at 9.0 mg N/L and Fe 2+ formation at 6.8 mg/L. This indicated that nitrate was generated by the Fe 3+ -dependent anaerobic ammonium oxidation (Feammox) pathway. Metagenomic analysis revealed a diverse Fe N cycling consortium composed of iron-reducing bacteria, denitrifiers, nitrate-dependent Fe 2+ oxidizers, anammox bacteria, ammonia oxidizers, comammox and Feammox-associated Acidimicrobiaceae sp. (0.02%). KEGG-based annotation further identified the enrichment of iron transport and redox-associated genes, including the Fe 2+ transport proteins feoAB (64 RPKM), the Fe 3+ uptake proteins afuBC (90 RPKM), and electron-transferring flavoproteins ETFa (85.99 RPKM), ETFb (93.69 RPKM), and ETFd (54.55 RPKM), indicating enhanced cellular electron transfer potential under iron availability. These findings suggest that Fe 3+ dosing may promote coupled iron‑nitrogen transformations and represent a practical strategy for enhancing nitrogen removal in MBR systems under anaerobic conditions. • Fe 3+ -dosed MBR achieved high nitrogen removal, outperforming 95% of WWTPs. • Fe 3+ dosing enhanced anaerobic NH 4 + -N removal by 39%. • First detection of Feammox bacteria (Acidimicrobiaceae sp.) in full-scale WWTP. • Metagenomics revealed Fe N cycling bacteria and enriched electron transfer genes.