Biofilm processes are widely used; the most effective of these use fluidized bed reactors, the design and modelling of which is still difficult. A fluidized sand bed denitrifying reactor was run to clarify contradictions in the applicability of the Richardson-Zaki model for the description of fluidization hydrodynamics and to determine fully the true biofilm parameters experimentally. Biofilm density was found to be 1.055 +/- 0.018 g/cm(3). A fundamental parameter of the Richardson-Zaki model, the terminal settling Reynolds number, was found to be not characteristic of the particles. Consequently a new approach was developed for the description of biofilm-coated particle fluidization. The model is based on two new parameters : the ’expansion coefficient’ and the ’specific occupied particle volume at zero flow’, which are readily determinable and characteristic parameters of the fluidized particles, being independent of reactor size and shape, liquid velocity or of the quantity of carrier particles. The model is suitable for modelling bed porosity or biomass concentration as a function of the biofilm thickness and upflow liquid velocity. We found that there can be an optimal biofilm thickness above which not only can the diffusion limitation increase, but the overall biomass concentration decreases at a given liquid velocity.