An Upflow Anaerobic Fixed Bed (UAFB) reactor packed with activated carbon was used to remove the azo dye Reactive red 272. The biomass grown on the activated carbon surface was composed of an adapted consortium of microorganisms. Residence time distribution test indicated that the reactor was a plug flow behavior. A dynamic mathematical model is presented for dye flux along the reactor and within the bioparticles composed of two regions: activated carbon core and biofilm. The model considers that the reaction is performed in the biofilm and in the liquid phase and includes dye transport by dispersion and diffusion. The concentration profile within the bioparticles changes with reactor height and time as the equilibrium is achieved. Changes in dye concentrations affect the concentration profile in the reactor and reduce the removal efficiency. The effectiveness factor depends on the reactor height and on the dye concentration at the inlet. (C) 2009 Elsevier Ltd. All rights reserved.