BACKGROUND: Several models have been developed to simulate the decay of pollutants concentration along the biofilter and to predict its performance. Despite the evidence, it is common that most models ignore the effect of variable biomass along the biofilter. An equation that represents the variable amount of active biomass along the column was included in the modelling of a biotrickling filter; it was obtained by measuring the active biomass at different heights. Validation of the model was carried out using experimental data obtained at different H2S loads. RESULTS: The simulation considering the expression for variable active biomass along the column shows better correlation with experimental results. With the diffusion coefficient that shows the best fit with the experimental results; 1.35 x 10(-9) m(2) s(-1), the value of the Thiele module is 2 x 10(-3), indicating that biooxidation of H2S is controlled by mass transfer. CONCLUSIONS: A better correlation between experimental results and model prediction is obtained when the expression for variable active biomass along the column is considered in the modelling. (C) 2010 Society of Chemical Industry