it is recommended to evaluate the fluxes of COD and nitrogen, before the model calibration is started. By means of mass balances only the state of equilibrium can be described. Therefore, as regards model calibration, only parameters responsible for long-term behaviour can be estimated, i.e. the heterotrophic yield YH, the fraction of biomass yielding particulate organics fp, the heterotrophic decay rate bH and the concentration of particulate inert organics XI in the influent. A correct estimate of these parameters is a prerequisite for the validity of the simulation results, not only in regard to excess sludge production and SRT, but also with respect to the oxygen consumption for carbon removal and, therefore, to denitrification capacity.