The three phase fluidized-bed biofilm reactor represents a recent innovation for improving the productivity of bioprocesses involved in the use of sparingly water-soluble gases, Knowledge of allowable liquid operating velocities and biomass concentration in the liquid phase is critical to the proper engineering of such reactors. A predictive model for the allowable liquid operating velocities and the biomass concentration in a three-phase fluidized bed biofilm reactor is developed. The model uses process parameters such as reactor size, support media properties, biofilm properties, gas and liquid flowrate as input variables, Based upon the developed method, a parametric study on the effects of various parameters on the range of operating liquid flowrate as well as the biomass concentration is carried out. The parameters considered include media size and density, biofilm thickness, reactor size, and gas and liquid velocities.