Role of Microoxic Control in Alternating Anoxic/Oxic Systems for Selective Enrichment of N ₂ O-Reducing Bacteria and N ₂ O Mitigation in Wastewater Treatment

The biological reduction of N₂O to N₂ relies on N₂O-reducing bacteria (N₂ORB), which involves enhancement of the N₂O-reducing enzyme (N₂OR) and selection of N₂ORB. This study investigates the short-term effects of microoxic pretreatment on N₂OR and long-term operational strategies for wastewater treatment aimed at achieving N₂O mitigation. In denitrification batch experiments, microoxic pretreatment (with dissolved oxygen (DO) at 0.2 mg/L for 2 h) enhanced N₂OR levels to 1.27 times that of the control group, resulting in a 35% to 65% reduction in N₂O emission flux. Through long-term reactor operation (294 days), controlling the microoxic environment during the oxic stage revealed that the second anoxic stages of the anoxic/oxic/anoxic (AOA) or anoxic/oxic/anoxic/oxic (AOAO) processes emitted no N₂O. Isotope analysis confirmed a high N₂O reduction ratio of 90.46%. The coexpression of <i>nos</i>Z with cytochrome c oxidase and CRP/FNR transcriptional regulators in clade I N₂ORB reflects the metabolic versatility of denitrifiers, driven by the integrated environmental selection of long-term microoxic conditions in oxic stages coupled with step-feeding strategies in anoxic stages. These findings highlight the AOA and AOAO processes under such controls as effective strategies for achieving both efficient nitrogen removal and N₂O mitigation.