Biomass accumulation was studied in a biofilter packed with an organic/inorganic, hybrid packing material made of clay pellets plus clay pellets covered by a thin layer of compost and inoculated with activated sludge from a municipal WWTP. Operating conditions under high loads of toluene were selected to force plant failure by clogging. The biofilter achieved a remarkably maximum elimination capacity of 595 g toluene m(-3) h(-1) (1280 g toluene m(-3) h(-1) based on the first 25 cm of the biofilter). The evolution of biofilter performance and biomass growth over the packing material were studied until the reactor collapsed. Several variables related to biomass growth such as reactor weight, oxygen consumption, CO2 production, substrate removal and pressure drop were monitored. Alternated periods of substrate supply and starvation were tested to assess biomass growth and detachment. Moreover, pH and biomass content were periodically measured in the leachate to determine the washing efficiency of intermittent watering. Pressure drop measurements demonstrated that watering was an effective technique to wash the excess of biomass accumulated. Experimental data also permitted to determine important parameters for biofilter modeling as the biomass growth yield and the stoichiometric coefficients of toluene biological oxidation. Variables monitored are sensitive to biomass accumulation and provided enough description of the system towards development of a comprehensive model considering biomass growth to predict and prevent clogging in equivalent processes. (C) 2012 Elsevier B.V. All rights reserved.