Mixer performance in two-phase aeration tanks. I. Full-scale flow and validation

Characterizing hydrodynamics in full-scale aeration tanks remains challenging due to limited in situ measurements under real operating conditions. As a result, most existing studies are restricted to laboratory-scale systems, which limits the direct applicability of their findings to actual treatment facilities. In this study, an integrated field-measurement and computational fluid dynamics (CFD) based investigation was conducted to analyze the hydrodynamic structure of a full-scale aeration tank operated with all mixers and diffusers active. Velocity measurements were collected at 120 spatial locations distributed over multiple depths and two symmetrical regions of the tank using an acoustic doppler current profiler (ADCP). The measured three-component velocity data (u, v, and w) were employed for rigorous component-wise validation of a three-dimensional two-phase CFD model. To improve model fidelity, the actual mixers installed in the plant were reverse-engineered through high-resolution three-dimensional scanning and explicitly incorporated into the CFD model using a sliding-mesh approach. The validated model revealed pronounced hydraulic deficiencies in the existing tank configuration. Only 13.2% of the total tank volume exceeded the critical velocity threshold of 0.30 m/s, while extensive stagnant regions dominated the central core of the reactor, indicating insufficient internal circulation and an increased risk of sludge deposition. Overall, the findings demonstrate that the current mixer-induced axial mixing is inadequate to ensure uniform circulation, highlighting the need for alternative operational scenarios to enhance the hydraulic efficiency of the system.