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Kinetics of biofilm nitrification

The reaction rates (r  NH 4+ and r  NO 2− ) in the two‐step nitrification reaction were measured in a fluidized‐sand‐bed biofilm reactor under a range of steady‐state conditions with respect to bulk NH  4+ , NO  2− , and O2 concentrations. It was shown from theory and experiment that under low NH  4+ concentration conditions, if the O2/NH  4+ concentration ratio in the bulk liquid is less than the stoichiometric coefficient (3.4 mg/mg), then oxygen will be rate limiting. In all experiments r  NO 2− decreased more than r  NH 4+ under low oxygen conditions. This resulted in high NO  2− effluent concentrations under low residence time conditions. The influence of the oxygen penetration effects on the relative values of r  NH 4+ and r  NO 2− was experimentally shown to be caused either by the Nitrobacter location in the inner biofilm regions or by a Km effect for oxygen. Theoretical support of these findings was provided by a differential diffusion‐reaction model which was used to simulate the experimental results.