Numerical investigation on mixing behaviour in the contact zone of an industrial-scale dissolved air flotation tank

Abstract

The mixing behaviour of the wastewater and the recycle bubbly water in the contact zone could affect the separation performance of dissolved air flotation systems. In this study, the mixing characteristics in the contact zone of an industrial-scale dissolved air flotation tank were investigated using the computational fluid dynamics method. At first, the influence of the turbulence model on the simulation results was clarified. It is demonstrated that the realizable k-ε model could predict the gas holdup distribution with better accuracy, and the maximum relative deviation between the predicted gas holdup and the experimental results is only 10%. The influence of the geometry and operating parameters on the mixing process was then studied numerically. It is found that the mixing performance in the contact zone could be improved by decreasing the height position of the nozzle inlet in a certain range. The reason is that microbubbles disperse better in the bottom region and a double circulation flow state forms in the contact zone when the nozzle inlet is installed at a low height. Additionally, the mixing process could be further enhanced by decreasing the diameter of the nozzle inlet. As to the operating conditions, increasing the recycle ratio could not only increase the gas holdup but also improve the mixing performance in the contact zone. When the recycle flow rate remains constant, increasing the wastewater flow rate will lead to a decrease in gas holdup and deterioration of the mixing effect.