Field and CFD Analysis of Jet Aeration and Mixing

Field tests and three-dimensional computational fluid dynamics (CFD) modeling were conducted to evaluate effectiveness of mixing in a jet aeration system for a sequencing batch reactor (SBR). The SBR is part of the Blacks Ford Regional Water Reclamation Facility (BFRWRF) of the JEA utility in Jacksonville, Florida. Solids measurements were taken with the SBR tanks operating in both aerated and un-aerated pumped mixing modes. A density-coupled CFD model incorporating solids settling and transport as user-defined functions (UDF) was calibrated to the field solids profile data and used to identify the capacity of the system to maintain solids suspension with different power levels in both mixing modes. The modeling indicated that while the existing system was adequate to ensure complete mixing during aerated periods, the existing pumped mixing power level of approximately 7.7 W/m3 (39 horsepower per million gallons – hp/MG) would need to be increased to at least 30.8 W/m3 (156 hp/MG) to maintain a conventional mixing criterion during un-aerated mixing. As a follow up to the mixing evaluation, simulations were prepared in which density coupling of the solids transport model to the fluid momentum was turned off in the UDF. Results from these experiments indicated that the neutral density simulation (no density coupling) seriously over-predicted the degree of mixing seen in the calibrated density-coupled model. Since neutral density CFD evaluations for mixing are a common practice in the industry, this finding is significant.