Abstract In 2005, a Task Group on biofilm reactor modeling was established by the German Water Association (DWA). The main motivation was to evaluate the potential of mathematical biofilm models to describe full-scale biofilm reactors applied in waste water treatment. An IWA Task Group on biofilm modeling recently has published a scientific and technical report evaluating the characteristics of different modeling approaches (Wanner et al., 2006). The focus of the current Task Group is on the processes specific to the reactor, on problems related to upscaling, and on the comparison of the various biofilm models applied to full-scale treatment systems. Initial results from the DWA task group will be presented. As a basis for model comparison, results from a full-scale tertiary biofilter that was operated for more than 4 years were used. Influent to the biofilter consisted of pretreated waste water with very low COD concentrations. The main parameters of the biofilter are shown in Table 1.