In this study a model is developed which describes the mass transfer process inside a fluidized bed reactor, considering the effects of dispersion, convection and reaction in the liquid phase, as well as diffusion and reaction within the biofilm. Applying the method of volume averaging for the different scales of heterogeneity, it is possible to obtain the one-equation model to describe the concentration in the reactor, when the local mass equilibrium condition is valid. The complete modeling of the mass transfer process inside the reactor is presented. The hypothesis assumed, the constraints to be satisfied, as well as the boundary conditions for the various scales involved, are discussed. The modeling is applied to the denitrification process in the fluidized bed bioreactor. Three cases are evaluated in which the constraint conditions are totally or partially satisfied. The results are compared with the experimental results given in the literature. On analyzing the results, it can be concluded that the model developed allows the prediction, with good precision, of the concentration profile inside the fluidized bed bioreactor, showing a good agreement with the experimental data. Thus, it is possible to determine the constraint conditions in order to guarantee the validity of the one-equation model, and to use the model developed to predict the performance of the bioreactor for different operating conditions. (c) 2007 Elsevier B.V. All rights reserved.