Removal of volatile organic compounds like toluene from waste gases with a biofilter can result in clogging of the reactor due to the formation of an excessive amount of biomass. Excessive biomass formation changes the bed's pore structure and leads to the progressive obstruction of the bed that is accompanied with a build-up in pressure drop and flow channeling. While the existing biofilter models appear to capture adequately the. transport and reaction phenomena at the biofilm scale, they poorly address, or provide little insight about the connection between the aerodynamics, biological filtration (or clogging) and biokinetics at the bioreactor length scale. An attempt has been made with this contribution to fill in this gap by developing a unidirectional dynamic flow model based on the volume-average mass, momentum and species balance equations coupled with conventional diffusion/reaction equations describing apparent kinetics in the biofilm. Toluene biodegradation by biodegrading microbes immobilized on pelletized diatomaceous earth biological support media was chosen as a case study to illustrate the consequences of formation of excessive amounts of biomass. The simulation results were rationalized in terms of biofilm thickness, bed local porosity, gas-phase substrate residual concentration, and pressure drop rise in biological fixed-bed filters. (C) 2004 Elsevier Ltd. All rights reserved.