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In this article, the effects of chemical oxygen demand, nitrogen (COD/N) ratio (4.90, 9.59, and 14.44), and dissolved oxygen concentration (DO) (0.5, 1.0, 1.5, and 3.0 mg/L) on simultaneous nitrification and denitrification (SND) were investigated using an airlift intal circulation membrane bioreactor (AIC-MBR) with synthetic wastewater. The results showed that the COD efficiencies were consistently greater than 90% regardless of changes in the COD/N ratio. At the COD/N ratio of 4.90 and 9.59, the system nitrogen removal efficiency became higher than 70%. However, the nitrogen removal efficiency decreased to less than 50%, as the COD/N ratio shifted to 14.44. When the operating DO concentration was maintained at 1.0 mg/L in AIC-MBR, a satisfying SND was achieved. Either low or high DO concentration could restrain SND.