Experimental data from a biofilm reactor, in which two groups of organisms (Nitrifiers and Heterotrophs) compete for dissolved oxygen, were analyzed by a Mixed Culture Biofilm Model. The objective was to investigate to what extent and how fast relative abundance and spatial distribution of microbial species in a mixed culture biofilm changed upon variations of the bulk fluid substrate composition, and what the consequences of these changes were for substrate removal. Experimental results showed that within nine days the nitrification rate in a biofilm of constant thickness could change by a factor of five. Model predictions indicated that these changes must be due to a significant shift of the biofilm population. The distribution of the autotrophic and heterotrophic species over the depth of the biofilm turned out to be an important aspect of mixed culture biofilm behavior. Since it is difficult to observe the microbial population and its spatial distribution experimentally, the Mixed Culture Biofilm Model has proved to be a valuable tool for the interpretation of the observed phenomena.

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