An analytical pore-scale model is proposed for predicting the pressure drop of a biofilter by taking the effect of biofilm development into account. Besides the average particle radius and fluid parameters, the pressure drop is expressed in terms of the initial bed porosity, the biofilm affected porosity, particle sphericity, surface roughness coefficient, coordination number and biofilm thickness. The coordination number represents the number of particles in contact with a single one in the bed. The biofilm affected porosity is based on the assumption of biofilm overlap and particle contact. The sphericity for a cluster of particles in contact and biofilm overlap is quantified in terms of the biofilm thickness and coordination number and its effect on the pressure drop investigated. The model predictions are verified against experimental pressure drop data obtained from a biofilter in operation for 107 days. Expanded schist was used as packing material. The proposed model is also compared to a modified Ergun equation from the literature of which the empirical coefficients and porosity exponents were adjusted to be applicable to a biofilter. The model proposed in this study contains no empirical coefficients. An equation is also presented for predicting the biofilm affected specific surface area which requires the measured pressure drop and superficial velocity values as input parameters. The effect of the biofilm affected porosity on the pressure drop is also investigated in the case of no biofilm overlap and no particle contact. Lastly a sensitivity analysis is performed on the initial bed porosity and coordination number. (C) 2018 Elsevier B.V. All rights reserved.