Performance assessment of mixers in two-phase aeration tanks using computational fluid dynamics and field measurements. II. Assessment of alternative mixing scenarios for performance enhancement

Ensuring efficient hydraulic performance in aeration tanks is a critical challenge for modern wastewater treatment facilities, as ungoverned flow distribution and weak axial mixing can compromise the overall process's stability. To address this, the present study employs an integrated approach that combines extensive multi-point field measurements with validated computational fluid dynamics (CFD) modeling, enabling a robust evaluation of flow behavior under real operating conditions. Unlike most previous studies, the entirety of the mixer setup installed in the treatment plant was directly embedded into the CFD model, ensuring an accurate representation of in situ hydraulic effects. The baseline analysis (case 0) revealed that the circulation generated was insufficient to sustain target velocities across the channel, leading to localized recirculation and weak axial momentum. Therefore, several operational scenarios were simulated to enhance flow distribution and mixing uniformity while maintaining practical feasibility. Additionally, a species transport model was incorporated as a qualitative diagnostic tool to evaluate outlet tracer concentration and mixing performance across different cases. The results showed that increasing the mixer speed by approximately 24% expanded the target-velocity zone by 16%. Applying counter-rotation between mixer pairs (case II) further increased this zone by 24% and promoted more balanced axial circulation. The combined configuration (case III) yielded the greatest enhancement, enlarging the target-velocity region by 32% and reducing outlet tracer concentration by 68%, effectively mitigating short-circuiting. Overall, the findings demonstrate that CFD-based scenario evaluation, reinforced by tracer diagnostics and field validation, provides a practical framework for optimizing hydraulic performance in full-scale aeration systems.