CFD study on the effect of the baffles geometry in sedimentation efficiency in wastewater treatments through Large Eddy Simulations.

Abstract

Sedimentation tanks represent one of the most important components of any water and wastewater treatment plants. The lack of knowledge of hydraulics in sedimentation tank leads to unnecessary capital and operating costs as well as water pollution in the form of excessive sludge. Improper and inadequate design cause overloading of filters, and lead to frequent backwashing, which in turn waste a significant percentage of treated water. Sedimentation tanks require a uniform flow field to ensure the correct operating conditions. Unfortunately, the development of circulation zones causes deep deviation from the uniformity, bringing negative effects on the efficiency of the tank. The focus of this study is to apply Computational Fluid Dynamics (CFD) to study and to improve the sedimentation efficiency through the optimization of the hydrodynamics inside settling tanks. In the present analysis, Large Eddy Simulation (LES) is used to simulate three-dimensional turbulent flow and scalar tracer transport in different settlers using a structured finite-volume discretization. The results show how the baffles' geometry, as well as the inlet design, influences the retention time distribution of the tank, varying the sedimentation efficiency. In particular, the increase of baffles inside the tank reduces the free space and allows a more uniform distribution of velocity vectors. This reduces both short circuits and recirculation zones bringing the flow closer to the ideal condition. On the other hand, the inlet position influences the velocity on the tank's bottom, with possible particles re-suspension effects.