This study presents a computational fluid dynamics (CFD) model of an aerated wastewater treatment oxidation ditch, taking into account gas-liquid flow, oxygen mass transfer and dissolved oxygen. Especially, the effect of the bubble size distribution (BSD) and the biochemical oxygen demand (BOD) distribution on the dissolved oxygen (DO) distribution has been considered. Species transport modelling predicts the DO and BOD distribution. De-oxygenation of local dissolved oxygen by BOD is modelled by an oxygen sink that depends on the local BOD concentration. Bubble coalescence and breakup models predict the BSD. The behaviour of the ditch is non-ideal, which is indicated by the residence time distribution (RTD), heterogeneous flow pattern and DO distribution. The parameters with the greatest influence on the dissolved oxygen are the BOD and bubble size. There is good agreement between the numerical simulation and the observations of flow pattern and measurements of mean DO. This study identifies that the BOD distribution and the BSD are important parameters for accurately predicting the DO distribution and which have been mostly neglected in the public research. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.