BACKGROUND: Pressure drop is a key parameter controlling cost efficiency of air cleaning biofilters. Pressure drop tends to increase with time due to accumulation of dust and biomass (DBM) within the filter medium. Although this is well known no easily applied expression for accurately estimating pressure drop under such conditions is available. Thus, evaluation of filter operating costs is at present uncertain. RESULTS: In this study the relationship between filter pressure drop and accumulation of DBM in the filter was examined using a commercial available material: light expanded clay aggregates (Leca (R)) consisting of porous rounded aggregates with similar shape. Pressure drop as a function of operating time (for a 60 day period), filter depth and DBM accumulation was measured on a set of pilot scale biofilter units filtering exhaust air from a full scale pig stable (as opposed to using artificially contaminated air). Pressure drop was observed to increase with time at all filter depths in response to DBM accumulation. Both DBM accumulation and pressure drop increase was most pronounced near the filter inlet. CONCLUSION: Pressure drop and DBM accumulation were closely related and both linked to the fraction of air-filled flow conducting voids. Based on the data a simple expression for predicting pressure drop in porous biofilter media as a function of DBM accumulation was developed. This expression can serve as an easily applied tool in assessment of biofilter cost efficiency. Copyright (c) 2012 Society of Chemical Industry