A single specie biofilm model was used to describe aerobic glucose degradation at 45°C in a biofilm reactor operated at three different hydraulic retention times (HRT). The model was calibrated to experimental data using the Monte Carlo filtering method. Eleven parameter sets out of 500 properly described the biofilm reactor performance for the three HRTs studied. After calibration, the single specie biofilm model was extended to a model with 10 different heterotrophic bacteria for studying microbial diversity in bulk phase and biofilm, as well as the competition between suspended and attached biomass. Independent of the HRT, the dominating and only remaining bacterial group in steady state was the same for bulk phase and biofilm. The dominating bacterial group was determined by the highest specific substrate affinity (ratio of /KS)of all considered bacterial groups. At a short HRT of 20 minutes, the microbial community in the bulk liquid was considerably influenced by biomass detachment from the biofilm, whereas both biomass detachment from the biofilm and microbial growth in the bulk liquid influenced the microbial community at a long HRT of 8 hours.