Three mechanisms are responsible for microbiological elimination processes in activated sludge: the survival of qualified organisms in the ecological selection process, the expression of specific enzymes and the absence of inhibitors limiting enzyme activity. A mathematical model with structured biomass has been formulated to improve the description of data from denitrification experiments. The model includes synthesis and decay of denitrification enzymes and is able to predict nitrate, nitrite and N2O concentrations. Kinetic parameters have been estimated and used to simulate the effect of cell saturation with enzymes in a waste water treatment process. Low dissolved oxygen concentrations in the anoxic reactor reduce the denitrification efficiency equally by inhibiting enzyme activity and enzyme synthesis: at 0.5 gm−3 O2 enzyme decay causes a cell saturation of below 40 %. Enzyme synthesis can take place in the sludge blanket of a secondary sedimentation tank and improve denitrification efficiency. The benefit of modelling with structured biomass is shown. The comprehension of experimental observations has been improved, and plant design and operation can be optimized. However, the multitude of unknown parameters still may restrict the validity of complex models.

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