A sequencing batch reactor (SBR) with aerobic granular sludge was operated to determine the effect of different DO concentrations on biological nitrogen removal process and to investigate the spatial profiles of DO, ORP and pH as online control parameters in such systems. The results showed that DO concentration had a significant effect on nitrification efficiencies and the profiles of DO. ORP and pH. The specific nitrification rate was decreased from 0.0595 mgNH(4)(+)-N/(gMLSS min) to 0.0251 mgNH(4)(+)-N/(gMLSS min) after DO concentration was dropped off from 4.5 mg/L to 1.0 mg/L High DO concentration improved the nitrification and increased the volumetric NH4+-N removal. Low DO concentration enhanced TIN removal, while prolonged the nitrification duration. Also there existed a good correlation between online control parameters (ORP, pH) and nutrient (COD, NH4+-N, NO2--N, NO3--N) variations in aerobic granular sludge reactor when DO was 2.5 mg/L, 3.5 mg/L and 4.5 mg/L However it was difficult to identify the end of nitrification and denitrification when DO was 1.0 mg/L, due to no apparent bending points on ORP and pH curves. In conclusion, the optimal DO concentration was suggested at 2.5 mg/L. as it not only achieved high nitrogen removal efficiency and decreased the reaction duration, but also saved operation cost by aeration and mixing. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.