A mathematical model for a fixed-bed biofilm reactor is developed in this work, in order to investigate the roles of reaction, adsorption and mass transfer on substrate consumption. Mass transfer throughout the biofilm and support is described by simultaneous Fickian diffusion and convective Bow. Numerical solutions were obtained for phenol biodegradation by finite element collocation methods. In terms of substrate removal and production of a good effluent quality, the results show the beneficial role introduced by adsorption in a biodegradation process, as well as the beneficial effects of the convective flow through both the biofilm and support, particularly when a higher intraparticle convective velocity is assumed to occur through the biofilm.