Synergistic suppression of nitrous oxide and residual nitrate in DNRA-enhanced MABR-PN/A hybrid systems

To resolve NO₃⁻-N production and accumulation in MABR coupled partial nitrification-Anammox (MABR-PN/A) for autotrophic nitrogen removal enhancement, an iron-carbon (IC) mediated NO₃⁻-N dissimilation system (IC-MABR-PN/A) was developed. Results indicate that when anaerobic ammonia oxidizing bacteria (AnAOB) are not added, IC-MABR-PN enhanced NO₃⁻-N dissimilation (reducing NO₃⁻-N by 11.86±0.68 mg/L vs. MABR-PN) due to nitrite dissimilation enzyme (Nrf) accumulation and stronger competition for NO₂⁻-N compared to nitrite reductase (Nir) under IC stimulation (2.23 log₂). Besides, IC concurrently strengthened denitrification in IC-MABR-PN (TN decreased by 6.48±0.28 mg/L but N₂O accumulation by 37.9 ± 5.8 μg/L vs. MABR-PN). Notably, when AnAOB was added to IC-MABR-PN, hydrazine synthase (Hzs, 1.83 log₂) in IC-MABR-PN/A exhibited stronger binding affinity than nitric oxide reductase (Nor, -1.96 log₂), favoring NO competition in IC-MABR-PN/A and suppressing N₂O pathways (reducing its emissions to one-ninth of that in IC-MABR-PN). Anammox further regulated biofilm hydrophobicity to mitigate interfacial repulsion and iron-carbon-induced compaction, enhancing nitrogen transfer. The study achieved synergistic NO₃⁻-N removal and N₂O reduction through IC-mediated NO₃⁻-N dissimilation and Anammox competition.