Nutrient removal capability of the MBR process has attracted more attention than organics removal in the past few years. Apart from the conventional schemes for nitrogen removal in MBR process, simultaneous nitrification-denitrification (SNDN) requires the most attention for further research. In order to fully understand the fundemantals and mechanism of SNDN in MBRs, a pilot plant was set up. A mathematical model was adopted for investigation and calibration against the observed values. This paper reports a study focusing on evaluating major mechanisms that govern nitrogen removal from domestic wastewater in membrane bioreactors. Two items need to be emphasized in this evaluation: (i) an MBR is basically regarded as an activated sludge process-a suspended growth bioreactor with total biomass recycle and substantially higher biomass concentration; (ii) in this context an AS model, namely ASM1R modified for endogenous respiration, is used for dynamic modelling and calibration of experimental results. The impact of diffusion through biomass which obviously exerts a significant effect on system performance and denitrification is evaluated with success using the adopted model by means of switch functions that regulate nitrification-denitrification with respect to dissolved oxygen concentration in the bulk liquid.