Speciation and chemical interactions in nitrifying biofilms. I: Model development

Steady-state models of nitrifying biofilms are developed taking into account the mass transfer of neutral and ionic species, electroneutrality, pH-dependent Monod kinetics, chemical equilibrium, and the presence of a boundary layer. Due to the assumption of bulk conditions, the rigorous biofilm model often predicts the washout of one of the biofilm species involved in a series of sequential biofilm transformations such as nitrification. Coupling the rigorous biofilm model with reactor mass balances yields the rigorous reactor model, which calculates the bulk conditions where both of the biofilm species remain and is thus more widely applicable. Substantial changes in the pH across the boundary layer and the biofilm are predicted; therefore, rates of nitrification may be improved by increasing bulk pH to high (alkaline) values, which results in optimum (neutral) pH within the biofilm. Varying influent conditions such as pH and buffer capacity can result in significant changes in reactor performance with respect to complete nitrification. The validity of the nitrification models are tested by comparison of profile predictions with that of experimental data, with encouraging results.