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

Steady-state models of nitrifying biofilms are developed taking into accoun t the mass transfer of neutral and ionic species, electroneutrality, pH-dep endent Monod kinetics, chemical equilibrium, and the presence of a boundary layer. Due to the assumption of bulk conditions, the rigorous biofilm mode l often predicts the washout of one of the biofilm species involved in a se ries of sequential biofilm transformations such as nitrification. Coupling the rigorous biofilm model with reactor mass balances yields the rigorous r eactor model, which calculates the bulk conditions where both of the biofil m 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 (alka line) values, which results in optimum (neutral) pH within the biofilm. Var ying influent conditions such as pH and buffer capacity can result in signi ficant changes in reactor performance with respect to complete nitrificatio n. The validity of the nitrification models are tested by comparison of pro file predictions with that of experimental data, with encouraging results.