Modeling of experiments on biofilm penetration effects in a fluidized bed nitrification reactor.

A four-component, diffusion-reaction model with double Michaelis-Menten kinetics was used to describe the experimental data obtained from a laboratory biofilm, fluidized-bed nitrification reactor. Theory and experiment demonstrated that the stoichiometric ratio (3.5 mg O(2)/mg NH(4)(+)-N) can be employed as a criterion to determine whether the limiting substrate is oxygen or ammonia. For the present work, in the range of concentrations where limitation occurred, 4 mg/L NH(4)(+)-N and 14 mg/L O(2), the ratio of oxygen to ammonia in the bulk liquid determined which substrate was penetration-limiting-O(2) if <3.5 and NH(4)(+) if > 3.5. Half-order kinetics with respect to the limiting substrate described the apparent overall rates. Simulations provided biofilm concentration profiles which demonstrated the role of the oxygen-ammonia ratio. Experiments indicated that, generally, high NO(2)(-) concentrations can be expected. These depend on the residence time, biofilm area, and oxygen concentration. This dependency was investigated with the model, as was the parametric sensitivity with respect to the saturation constants. Particularly important for the NO(2)(-) levels were the ratios of the saturation constants for oxygen.