Nitrogen removal efficiency and pathways of the simultaneous nitrification and denitrification process under...

To explore strategies for further reducing aeration energy consumption in the simultaneous nitrification and denitrification (SND) process, an SND reactor was constructed to treat low carbon-to-nitrogen (C/N) ratio domestic wastewater under ultra-low dissolved oxygen (DO) conditions (DO < 0.05 mg·L⁻¹). The effects of hydraulic retention time (HRT) and C/N ratio on nitrogen removal performance were systematically evaluated, and batch experiments were conducted to determine nitrification and denitrification rates. Under influent condition of 50.0 mg·L⁻¹ ammonia and a C/N ratio of 2.2, the reactor maintained stable ultra-low DO levels, achieved effluent ammonia concentrations below 5.0 mg·L⁻¹, and attained optimal SND efficiencies exceeding 80.0%. Although a shortened HRT destabilised performance, increasing the C/N ratio successfully restored nitrogen removal stability. Nitrification and denitrification rates ranged from 4.20-30.89 mg·L⁻¹·h⁻¹ and 1.57-21.92 mg·L⁻¹·h⁻¹, respectively, under C/N ratios from 2.2-8.0. High-throughput sequencing and metagenomic analysis revealed that heterotrophic nitrification and aerobic denitrification dominated nitrogen transformation, with microorganisms utilising low-energy nap and nirK enzymes to adapt to the ultra-low DO environment. These findings provide novel insights into energy-efficient nitrogen removal pathways under ultra-low oxygen conditions.