Modelling a multiple reference frame approach in an oxidation ditch of activated sludge wastewater treatment

Abstract

Oxidation ditches (OD) are one of the most common aeration units or bioreactors in the activated sludge process. Flow and hydrodynamic characteristics play an important role in successful operation of an OD. It is difficult to study the rheology in a detailed way because it’s time-consuming and expensive. However, due to the development of computational fluid dynamics (CFD), such a study has become easier and less expensive. CFD is a powerful tool to simulate the hydrodynamics. Since in mixing operations, the generation of fluid flow is due to the input of mechanical energy in form of rotation of the impeller, the one of very important consideration is the choice of a model that describes impeller rotation. The flow features associated with multiple rotating parts can be analyzed using the multiple reference frame (MRF) capability. Based on previous studies on OD in the field of wastewater treatment (few numbers has been done) there is no direct mention of the size of rotating zone for the simulation so a fundamental analysis is required and can be helpful for further studies in this field. In this study the effect of rotating zone dimensions on the accuracy of oxidation ditch simulation has been analyzed and four model of moving frame have been set and simulated. Based on the analysis the bigger the radius of rotating zone is the higher the accuracy of modelling is. Moreover, the rotating zone in multiple reference approach cannot be semi-circle.