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 O 2 concentrations. It was shown from theory and experiment that under low NH   4 +concentration conditions, if the O 2 /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 K m effect for oxygen. Theoretical support of these findings was provided by a differential diffusion‐reaction model which was used to simulate the experimental results.