Distribution of surface shear forces and bubble characteristics in full-scale gas sparged submerged hollow fiber membrane modules

Abstract

Gas sparging is commonly used in submerged membrane systems to induce shear forces at the membrane surface, which removes the accumulated foulants. However, limited knowledge exists regarding the magnitude and distribution of these forces, especially in submerged hollow fiber membrane systems. The present study focuses on addressing this current knowledge gap by mapping the shear forces induced by gas sparging in full-scale submerged hollow fiber membrane modules. The shear forces were observed to be highly variable over time and heterogeneously distributed within the system, ranging from 0.1 to over 10Pa. The shear force measurements and the bubble counts indicated that the narrow gap between the membrane modules likely provided sufficient resistance to direct a substantial amount of the sparged bubbles to the side of the modules rather than between the membrane modules, and that regardless of the sparging flow rate, the amount of sparged bubbles that rose between the modules was constant. Increasing the sparging flow rate simply directed more of the sparged bubbles to the side of the membrane modules. As a result, the shear forces were heterogeneously distributed within the system. No correlation was observed between the shear force and bubble count or rise velocity. The distribution of sparged bubbles, the bulk liquid flow and the surface shear forces in the system were highly affected by the system geometry (e.g. module spacing, tank configuration and diffuser nozzle size).