Dynamic Mixing Analysis in Aeration Tanks: Modeling, Validation, and Complete Mixing

Abstract The biochemical degradation reaction of organic pollutant must be conducted in a complete mixing state, while an aeration tank operates in Continuous Stirred Tank Reactor (CSTR) mode. Due to the long reaction time, the mixing process could to be analyzed on large temporal scale. Here, a two-dimensional mixing model of an aeration tank was developed to analyze the mixing state. The feasibility was verified with the reported literature and the entrainment coefficient µ of bubble plume was calibrated, µ = 0.47. The mixing processes of instantaneous point source tracer in three square aeration tanks ( 270.0 m³, 1230.0 m³, and 2200.0 m³ ) with depth H = 5.5 m and diagonally arranged inlet and outlet, were calculated using the mixing model. The concentration distribution, output concentration profiles, concentration difference rate curves β(t), and complete mixing time tP were obtained. The model calculation showed that the spatial mixing scale of the aeration tanks dominated the mixing processes, and the calculated tP were 7.3, 24.4, and 39.1 min, respectively. The square aeration tanks operated in the complete mixing reaction state or CSTR mode, tP < < T, T – hydraulic retention time. The range tP for the rectangular aeration tanks with the same volume were obtained according to that of the corresponding square aeration tanks, and the rectangular aeration tanks with larger length L could not operate in the complete mixing reaction state or CSTR mode. The spatial mixing scale of the aeration tanks dominated the mixing processes and tP, and minimal spatial and temporal mixing scales tP in the square aeration tank. The mixing model revealed the large scale characteristic of the mixing in an aeration tank for improving the control of the operation, without the calculation of gas-water two- phase hydrodynamic on small spatial scale.