Optimizing slug bubble size for application of the ultra-thin flat sheet membranes in MBR: a comprehensive study combining CFD simulation and experiment

Optimizing the slug bubble size specifically for ultra-thin flat sheet membranes in MBR systems can effectively enhance the scouring force and improve fouling control efficiency, thereby further advancing their targeted and widespread application. In this study, a three-dimensional model was developed based on the practical application to investigate the impact of slug bubbles on scouring performance in ultra-thin flat sheet MBR systems, encompassing their evolution, disturbance level, and shear stress. A membrane fouling probability index for quantifying the distribution of membrane fouling, along with a turbulence intensity index have been proposed. The findings revealed that the 20-mL slug bubble induced the highest disturbance level in the surrounding fluid, characterized by an instantaneous peak velocity of 0.63 m/s at the local system level, conducive to bubble scouring. And exerted the greatest shear stress effect, achieving the most effective reduction in membrane contamination, with a maximum shear stress of 1.82 Pa. The experimental validation conducted during the operational cycles confirmed that the scouring effect of 20-mL slug flow yielded in a maximum proportion of 48.16% within the low fouling probability region. The results provided evidence supporting the assertion that specific aeration conditions producing 20 mL of bubbles resulted in minimal membrane fouling, ensuring a more pronounced scouring effect. The combination anythsis of slug bubble characteristics and behaviors, integrating theoretical and experimental approaches, implied that 20 mL was the optimal bubble size in ultra-thin flat sheet MBR, which fulfilled the optimal air scouring effect.