A MBR (membrane bioreactor) was performed to quantify the improvement in nitrogen removal terms when pure oxygen and air were used to supply the aerobic conditions at different HRT (hydraulic retention time) and mixed liquid suspended solids concentrations. The improvement in the efficiency was 8.0% and 13.5% when pure oxygen was used with 12 h and 18 h of HRT repectively. The ammonium utilization rates with HRT of 18 h with pure oxygen was 1.23 mgN(t)/(L h) and with air 0.66 mgN(t)/(L h) and with HRT of 12 h was 1.14 mgN(t)/(L h) and 1.18 mgN(t)/(L h) respectively. Model simulation (DESSAS software) confirmed the validity for nitrogen removal, hydrolysis constant was increased (106% and 184%), decay constant was decreased and Monod's constant was increased (190.4% to 324.1%), and a multivariate analysis with CANOCO v4.5 demonstrated that temperature and HRT have the higher influence in kinetics. (C) 2012 Elsevier Ltd. All rights reserved.