LABORATORY EXPERIMENTS OF BUOYANCY-INFLUENCED FLOW IN CLARIFIERS

A hydraulic laboratory experiment is described that was designed to ve rify numerical simulations of buoyancy-affected flow in clarifiers. Th rough variations of the flow rate, the water depth and the inflow susp ension concentration, a typical range of density influences prevailing in secondary clarifiers is covered. Since glass spheres serve as sett ling 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-d efined test cases for numerical simulations, i.e. to introduce boundar y 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 f low. The density effect turns out to stabilise the flow pattern and to improve the removal efficiency of the tank. When the density effect t hrough high suspended-solids concentrations becomes a dominating facto r, the overflow-rate concept known from settling tanks with discrete-t ype settling process will not apply any more, and the depth of the tan k becomes a determining parameter with regard to the performance of th e tank.