Finite element analysis of flows in secondary settling tanks
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University of Cape Town
Secondary settling tanks (SSTs) form a crucial part of wastewater treatment plants. Besides having to produce the separation of suspended solids and clarified effluent the secondary settling tank is used to concentrate and recycle the settled sludge to the biological reactor. The efficiency of the biological reactor in the waste water treatment system is determined by the efficiency of this final clarifying process. Hydrodynamic models have been developed for simulating secondary settling tanks in order to gain a better understanding of the complex flow patterns in these tanks, and to make design and optimization of the SST internal features possible. These models use mainly the finite volume method. This thesis is concerned with the development and implementation of a finite element approach to the simulation of flows in SSTs. Although it is nowadays also possible to realise an unstructured grid within the FVM, the power of the finite element method (FEM) lies in its higher flexibility in fitting irregular domains and in providing local grid refinement. Generally, unstructured mesh procedures with the FVM require essential, additional orthogonality corrections, which affect the accuracy of the solution, and these corrections increase the computational cost due to the additional computations and increased iteration requirements. Structured mesh discretization may offer significantly shorter computation time. The FEM is therefore convenient for handling arbitrarily shaped domains and adaptation of complex internal features of SSTs, such as inlet and outlet arrangements.
Bibliography: leaves 158-169.
Kleine, D. 2002. Finite element analysis of flows in secondary settling tanks. University of Cape Town.