A numerical model for predicting the flow in clarifiers is presented which accounts for density-affected and turbulent flow as well as for the settling of activated sludge. The reliability of the two-dimensional model is demonstrated through verification by means of a laboratory experiment which was specially designed for this purpose. The numerical model is then applied for an evaluation of inlet and outlet arrangements. The inlet is positioned at the bottom and the inlet aperture is varied. An outlet at the top of the end wall is compared with outlets longitudinally extended at the surface. The alternatives are assessed by analysis of streamline patterns, layer charactersitics and the entrainment into the bottom current, i.e. the increase of its flow rate. The study demonstrates how numerical modelling can be used for systematic evaluation of the effect of geometrical configurations on the flow in secondary clarifiers.

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