Dynamic simulation models of the activated sludge process are a planning and designing tool solving and answering problems that cannot be dealt with by static design models or procedures. In dynamic modelling, however, the settling and thickening processes have to be taken into account which have been neglected until now.

Therefore, a dynamic model simulating the processes of settling and of thickening in the secondary clarifier is developed and finally integrated into a dynamic biochemical reaction model. The validity of the model approach is evaluated by simulation runs showing the velocities of hindered settling, the concentration profiles of MLSS over the tank depth, and the effluent concentrations of MLSS. Basic variables for the validation are the influent MLSS concentrations and the sludge volume index quantifying the settling characteristics of the sludge.

The consequences for the MLSS content in the aeration tank, as well as for the plant effluent quality, of neglecting and of including the clarifier model into the integrated model are shown at variable conditions of wastewater inflow, especially for periods of higher storm water flows to the treatment plant

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