FSMBR is a promising wastewater treatment and reclamation technology that is gaining increasing attention in recent years. In its operation, air bubbles are injected to rise in the narrow gaps formed by the flat sheets, typically at a spacing between 3-12 mm. The bubble rising behaviour within the narrow gap, yet has not been fully understood, plays a vital role in FSMBR performance and durability by affecting the oxygen dissolution, enhancing mass transfer, suppressing the membrane fouling and contributing to the energy cost. In this work, we used the high-speed camera to systematically investigate the bubble rising feature in a narrow channel with the spacing of 5 mm and compared to that in unbounded conditions. By varying the bubble size from 0.2 mm to 10 mm, we were able to extract the bubble terminal velocity, bubble shape and rising trajectory from the recorded videos. As expected, the terminal velocity of bubbles larger than 1 mm is generally 10-20% lower in narrow channels than in the unbound liquid. Interestingly, the terminal velocity of bubbles around 4 mm is roughly equivalent to that in unbounded liquid, which can be attributed to the wall effect on the bubble wake structure and the bubble-wall collision effect on the zig-zag rising trajectory. By analysing the rich phenomena (e.g., bubble deformation, trajectory) observed for bubbles larger than the gap spacing, the shear stress of bubbles exerted on the wall is also discussed. The outcome obtained from this study is expected to provide good guidance for the optimization of bubbling technology in FSMBR design.

flat-sheet membrane bioreactor,bubble dynamics,bubble terminal velocity,shear stress