Mathematical models of activated sludge process are well recognised and widely implemented by researchers since 1980's. There is also numerous software available for modelling and simulation of activated sludge plants, but practical application of those tools is rather limited. One of the main reasons for such a situation is a difficult process of model calibration the requires extended data sets collected at investigated plant. Those data are usually not included in a standard plant monitoring plan. In the paper the problem of model calibration with the data sets derived from standard monitoring plan is discussed with a special regard to simulation objectives and data availability. The research was conducted with operational data from Białystok Wastewater Treatment Plant. The model of the plant was based on Activated Sludge Model No.3 developed by IWA Task Group and implemented in ASIM simulator. Calibration and validation of the model gave promising results, but further applications should be carefully considered, mainly due to uncertainties underlying input data.