A straight interpretation of the solids flux theory for a three-layer sedimentation model

One-dimensional settling models based on the conservation law calculate the flux of particles via discrete horizontal layers. Commonly at least 10 of these spatial elements with constant volume are considered in order to achieve a satisfying fit with data. This paper deals with a conceptual approach of a simplified layer model, its theoretical confirmation and its applicability. The model differs three layers with variable volume--clarification-, hindered settling- and compression zone--which are not derived from numerical requirements but from the basic principles of solids flux theory for batch sedimentation. In accordance to the three types of propagation velocities of solids flux theory--hindered settling, signal speed and propagation of concentration discontinuities--three density layers of settling particles of a suspension have been defined. Model development has focused on a reduction of model complexity for on-line control purpose and for coupling with a biokinetic model. Another intention was the model's universal applicability to both batch and continuous settling in order to compare different types of activated sludge strategies. The model has been evaluated against the analytical solution of the flux function and against data from full-scale SBR experiments and from a secondary clarifier under hydraulic overload.