Abstract

To elucidate how dissolved oxygen (DO) regulates nitrogen removal in the completely autotrophic nitrogen removal over nitrite (CANON) process, three continuous-flow reactors were operated under micro-aerobic conditions. Results revealed that minor DO variations (0.36–0.51 mg/L) triggered dramatic bifurcation in performance and microbial ecology, demonstrating chaotic effects characterized by nonlinear dynamics and sensitive dependence on initial conditions. A superior total nitrogen removal rate of 0.38 kg/m 3 /d and a NH 4 + -N removal efficiency of 86.7% were achieved at 0.36 mg/L DO. However, a slight increase to 0.51 mg/L DO significantly enhanced nitrite-oxidizing bacteria (NOB) activity and nitrate accumulation. Lower DO favored anammox bacteria and their essential genes ( hzs / hdh ), while elevated DO promoted NOB competition and oxidative stress responses, evidenced by Fe-Mn SOD gene upregulation and altered extracellular polymers composition. Our findings establish a direct link between minor DO fluctuations and macro-scale functional outcomes, providing a mechanistic framework for predicting and controlling CANON process.