CFD Modeling of an activated sludge bubble column

Abstract

The bubble columns are widely used in different industries such as wastewater treatment, chemical, petrochemical, and biochemical. The current design of the bubble columns including but not limited to the geometry and dimensions of the column, the location, size, and type of the gas spargers, gas superficial velocity, and fluid rheology relies on the assumptions about the gas and liquid behavior and the mixing performance. The main objective of this study was to evaluate the effect of superficial gas velocity and sparger type on the liquid and gas phase flow patterns, gas hold up, and mixing time through computational fluid dynamic (CFD) modeling and simulation. The CFD model was simulated the multiphase flow based on the Eulerian-Eulerian approach and validated by experimental measurements. The standard k-ε model was employed to predict the turbulent flow and circulations of the liquid phase. The activated sludge as a shear thinning non-Newtonian fluid has been modeled as a power-law rheological model. The trends of the mixing process were calculated by using the specious and transport model. The CFD model accurately predicted the flow pattern and mixing process. The results reveal that the increase of the superficial gas velocity increases the value of gas holdup. Moreover, the mixing time have been decreased by increasing the rate of superficial gas velocity. One of the most important results of this study highlighted the significant effect of the tracer injection location on the mixing time. The typical star shape sparger has also been modified in this study. The performance of the modified sparger was assessed and compared to the performance of the typical star shape in terms of liquid and gas flow pattern and mixing time. The modified sparger exhibited better performance in sparging the gas through non-Newtonian activated sludge. The validated CFD data was employed to investigate the effect of superficial gas velocity, type of gas sparger, and tracer injection location on the mixing time of the activated sludge bubble column. A statistical-based experimental design with full factorial design method was applied to evaluate the individual and interactive effects of the design parameters.