1-D and 3-D Computational Fluid Dynamics Model Comparison of the Treatment Performance of a Full-scale Oxidation Ditch

Modeling an oxidation ditch with computational fluid dynamics (CFD) can be computationally expensive. While, a one-dimensional (1-D) model can reduce the expense, there could be a significant tradeoff in solution accuracy. This research sought to address how well a 1-D model can approximate the treatment performance of a 3-D model of a full-scale oxidation ditch. Both models incorporated bio-kinetics via the Activated Sludge Model (ASM)-1, to predict the treatment performance of the ditch based on the concentration of the ASM-1 components. When comparing the time-series of the concentration of ASM-1 components averaged over the ditch for 40 days, both 1-D and 3-D models displayed similar trends with slight differences in steady state values, except for soluble nitrate nitrite nitrogen (S_NO). The steady state value of S_NO was greater by more than 150% for the 1-D model than the 3-D model. This difference was attributed to heterogeneities in dissolved oxygen concentration predicted in the 3D model that were not captured by the 1-D model, leading the latter to under-predict the denitrification process. Additionally, the spiraling flow around the aerators that play an important role in determining the spatial distribution of dissolved oxygen cannot be represented in the 1-D model.