Previous efforts to describe biofiltration use continuum models with empirical relationships to describe air-flow in the biofilter, and conventional diffusion/reaction equations to describe processes in the biofilm. Biofilters are packed-bed reactors. They differ from conventional reactors because during their operation biomass is being created, which changes the bed's pore structure. This may eventually clog the reactor and prevent further flow of air. All current models ignore the importance of effects that occur at the pore network level. A new biofilter network model is presented here which aims to understand the importance of the pore network structure. The model accounts for biofilm growth. With this model the biofilter removal efficiency and the pressure drop can be related to the bed's pore network structure. Pressure drop changes resulting from the growing biofilm are described and explained. The effects of different initial pore size distributions on the efficiency of the biofilter are discussed. The model predicts that under certain conditions non-uniform biofilm growth will occur in the biofilter.