Practical identifiability of biokinetic parameters of a model describing two-step nitrification in biofilms

Parameter estimation and model calibration are key problems in the application of biofilm models in engineering practice, where a large number of model parameters need to be determined usually based on experimental data with only limited information content. In this article, identifiability of biokinetic parameters of a biofilm model describing two-step nitrification was evaluated based solely on bulk phase measurements of ammonium, nitrite, and nitrate. In addition to evaluating the impact of experimental conditions and available measurements, the influence of mass transport limitation within the biofilm and the initial parameter values on identifiability of biokinetic parameters was evaluated. Selection of parameters for identifiability analysis was based on global mean sensitivities while parameter identifiability was analyzed using local sensitivity functions. At most, four of the six most sensitive biokinetic parameters were identifiable from results of batch experiments at bulk phase dissolved oxygen concentrations of 0.8 or 5 Mg O-2/L. High linear dependences between the parameters of the subsets (K-O2,K-AOB; mu(AOB)) and (K-O2,K-NOB, mu(NOB)) resulted in reduced identifiability. Mass transport limitation within the biofilm did not influence the number of identifiable parameters but, in fact, decreased collinearity between parameters, especially for parameters that are otherwise correlated (e.g., mu(AOB) and K-O2,K-AOB, or mu(NOB) and K-O2,K-NOB). The choice of the initial parameter values had a significant impact on the identifiability of two parameter subsets, both including the parameters mu(AOB) and K-O2,K-AOB. Parameter subsets that did not include the subsets mu(AOB) and K-O2,K-AOB or mu(NOB) and K-O2,K-NOB were clearly identifiable independently of the choice of the initial parameter values.