Biological nitrogen removal was simulated for a Vacuum Rotating Membrane (VRM) type membrane bioreactor (MBR) operated in METU campus. In order to simulate the biological MBR plant, a dynamic model that describes the process is needed. In this thesis, the Activated Sludge Model No.1 (ASM1), which still is the most widely used model developed by the International Association on Water Quality (IAWQ), has been used to simulate the carbon oxidation, nitrification and denitrification processes occurring in the plant using AQUASIM software package. Once the model was established, sensitivities of the model parameters were analyzed. Then, parameter estimation was carried out for the optimization of the sensitive parameters. As we have several distinct data sets available two parallel modeling study was carried out for the calibration of the model. Finally, the calibrated model by different data sets was validated by using the remaining data sets. The model results were consistent with the measured data especially in terms of MLSS concentration in the system. However model results for the nitrogen removal were not extremely successful / the reason for this might be the inadequate available data on nitrification-denitrification process occurring in the system.