Numerical Modelling and Measurement in a Test Secondary Settling Tank

A numerical model and measurements of flow and settling in activated sludge suspension is presented. The numerical model is an attempt to describe the complex and interrelated hydraulic and sedimentation phenomena by describing the turbulent flow field and the transport/dispersion of suspended sludge. Phenomena as free and hindered settling and the Bingham plastic characteristic of activated sludge suspensions are included in the numerical model.

Further characterisation and test tank experiments are described. The characterisation experiments were designed to measure calibration parameters for model description of settling and density differences. In the test tank experiments, flow velocities and suspended sludge concentrations were measured with different tank inlet geomotry and hydraulic and sludge loads. The test tank experiments provided results for the calibration of the numerical model and for comparing measured and calculated result.

The numerical model could, fairly accuratly, predict the measured results and both the measured and the calculated results showed a flow field pattern identical to flow fields in full-scale secondary setling tanks. A specific calibration of the Bingham plastic characteristic was needed and further knowledge of activated sludge suspension rheology will be necessary to improve the numerical model.