The present thesis deals with the evaluation and optimization of the settling behavior of the circular basin system which is used in the clarification plant at voestalpine Donawitz. For the evaluation the current state of the system is captured and the prevailing currents and related sedimentation processes will be illustrated by using a CFD - software. For this purpose the well-known and mass-conserving finite volume method is applied in three dimensions. The mass-conserving finite volume method with integral formulation in three-dimensional space calculation is used. Starting out from the general partial differential equations for the conservation of mass, momentum and energy for any medium, the partial differential equations are discretized on a computational grid and the obtained system of equations is solved by an iterative method for the required flux characteristics. The simulation is conducted in cooperation with the company ICE - Flow Research GmbH. The simulation results provide the basis for further calculations and the interpretation of the optimized clear-water skimmer-system. In further consequence a concrete proposal is drafted, which provides a conversion or new construction of the clear-water skimmer-system. The optimization proposal involves a change in geometry of the overflow edges, and a reduction of the skimmer number. Additionally, a possibility for better oil retention and oil separation is presented. A discussion of the possible implementation concludes the work.