Coupling Hydrodynamic and Biokinetic Growth Models in Aerated Wastewater Treatment Processes

In this thesis, a coupled hydrodynamic and wastewater biokinetic finite volume based CFD model for an aeration tank in OpenFOAM has been created to understand the effect of the hydrodynamics on the biological processes. A pilot-scale aeration tank that is aerated using fine membrane diffusers along the base has been designed and manufactured. A procedure for conducting lab experiments using an acoustic Doppler velocimeter to record velocity measurements was outlined. A series of aeration tank experiments with flow rates ranging from 18 – 108 L/min through membrane diffuser setups that involved 1 or 3 diffusers were conducted in which ADV velocity measurements were taken and have been used to validate a CFD model. Additionally, it was found that certain diffuser configurations showed pseudo - 2D behaviour such that the recorded data could be used to validate 2D simulations of the aeration tank. A CFD model using the Eulerian-Eulerian multiphase formulation in OpenFOAM was created to replicate the bubble driven fluid flow and free surface effects in the pilot-scale aeration tank. The influence of the inlet conditions, bubble diameter size and bubble dynamic models on the generated results were investigated and compared with the experimental data to validate the modelling choices. As a result, a 2D and 3D CFD model of the aeration tank was defined and validated against the experimental ADV data.

Using the results, a procedure for coupling the biokinetics into the hydrodynamics was described in OpenFOAM. The difficulties that arose from transferring a two-phase solution with a free surface to a single-phase solver was outlined and solutions to the issues were defined and assessed. The mass transfer of oxygen into the fluid was modelled and compared with experimental results from the membrane diffuser manufactures to confirm the accuracy of the model. The oxygen mass transfer model was used to assess how the membrane diffuser setup and flow rate impacts the oxygenation of the reactor. It was found that increasing the number of aerating diffusers while keeping the total air flow rate the same significantly increased the oxygenation of the tank in comparison to just increasing the air flow rate which was found to only slightly increase the oxygenation. Additionally, a curve fitting procedure was described to derive a global oxygen transfer rate coefficient and saturation value from the CFD simulation for specific aeration tank setups and assessment of the values found they could give insight to the hydrodynamic behaviour in the reactor. The simulations were further extended to include the biokinetics to describe the biological interactions. A simple biokinetic aeration model was proposed to assess the impact of the hydrodynamics, inlet and outlet locations, and flow rate on the biological processes in tank. It was found that inadequate mixing in the 2D simulation resulted in twice the required amount of time to reach the maximum biomass concentrations compared with the equivalent perfectly mixed reactor. It was shown that the location of the inlet and outlet with the same hydrodynamic flow fields could influence the biological processes. It was found that there was no difference in the biological performance of the 3D reactor with an aerating flow rate of 0.3 and 0.6 L/s such that it would be inefficient to aerate the tank at 0.6 L/s. Finally, the full ASM1 was implemented into the coupled model and compared with the conventional ASM1 model to assess the performance of the aeration tank at producing and removing nitrates and ammonium. It was found that inadequate mixing resulted in reduced efficiency of the reactor at producing and removing nitrates and ammonium, respectively, which would further impact the performance of the sequential rectors.