Laboratory experiments of buoyancy-influenced flow in clarifiers

A hydraulic laboratory experiment is described that was designed to verify numerical simulations of buoyancy-affected flow in clarifiers. Through variations of the flow rate, the water depth and the inflow suspension concentration, a typical range of density influences prevailing in secondary clarifiers is covered. Since glass spheres serve as settling suspension, the flocculation process is not considered. Therefore, the study concentrates on the investigation of the flow features and the interaction with the settling process. In order to provide well-defined test cases for numerical simulations, i.e. to introduce boundary conditions that can be formulated exactly in the numerical model, a new approach is introduced for setting the bottom boundary conditions, that is a perfect sediment removal while being a rough wall for the flow. The density effect turns out to stabilise the flow pattern and to improve the removal efficiency of the tank. When the density effect through high suspended-solids concentrations becomes a dominating factor, the overflow-rate concept known from settling tanks with discrete-type settling process will not apply any more, and the depth of the tank becomes a determining parameter with regard to the performance of the tank.