Computational fluid dynamics modelling of dissolved oxygen in oxidation ditches.

Abstract

This research aims to reveal new knowledge about the factors that affect the hydrodynamics, dissolved oxygen (DO) and aeration performance of a wastewater oxidation ditch. The literature is reviewed on the Computational Fluid Dynamics (CFD) modelling of wastewater aeration tanks. This study develops a CFD model of an aerated oxidation ditch, by taking into account two-phase gas-liquid flow, inter- phase oxygen mass transfer and dissolved oxygen. The main contributions to knowledge are the effect of bubble size distribution (BSD) and biochemical oxygen demand (BOD) distribution on the DO distribution. Species transport modelling predicts the BOD and DO distribution in the ditch. De-oxygenation of local dissolved oxygen by BOD is modelled by an oxygen sink that depends on the local BOD concentration. This is a novel approach to flow modelling for the prediction of the DO distribution. The local BOD concentration in the ditch may depend on either the local DO concentration or the local residence time. The numerical residence time distribution (RTD), heterogeneous flow pattern and DO distribution indicate that the flow behaviour in the ditch is non-ideal. Dissolved oxygen is affected by BOD distribution, bubble size, BSD, mechanical surface aeration and temperature. There is good agreement between the numerical simulation and both the observation of flow pattern and the measurement of mean DO. The BSD predicts a mean bubble size of around 2 mm, which is also the bubble size that best agrees with the measurements of DO. This study identifies that the BOD distribution and the BSD are key parameters that affect the DO distribution and improve the accuracy of the agreement with experimental data. In decreasing order of aeration performance are the air membrane diffuser, Fuch air jet aerator, Kessener brush surface aerator and Maguire hydro-jet aerator.