Exploring temporal variations of oxygen saturation constants of nitrifying bacteria

Activated sludge models (ASM) are generally accepted as state-of-the-art in modeling wastewater treatment plants. In this paper, we assess the temporal variability of an ASM parameter—the oxygen half-saturation constant of autotrophic bacteria (KO,AUT). A series of respirometric experiments is performed for conventional activated sludge and sludge from a membrane bioreactor. KO values are estimated for both ammonium-oxidizing and nitrite-oxidizing bacteria. For parameter estimation, the Monod kinetic model structure is extended by a sensor model. Still remaining systematic deviations between data and model are considered by a thorough residual analysis: (1) uncertainty estimates of KO are corrected to reflect model structure deficiencies and (2) the inter-experimental cross-correlation of residuals is taken into account to assess temporal changes. We conclude that KO,AUT is a time variable parameter and should be considered accordingly.