Shear profiles inside gas sparged submerged hollow fiber membrane modules

Abstract

Gas sparging is a common strategy used to minimize and prevent fouling on membrane surfaces. Although there have been numerous studies on the hydrodynamic conditions and the mechanisms of fouling control in confined membrane systems such as tubular membranes, there is very limited literature available that describes the hydrodynamic conditions and mechanisms of fouling control in unconfined systems such as submerged hollow fiber membranes. The present study investigated bubble-induced shear profiles in submerged hollow fiber membrane modules using an electrochemical method for measuring shear forces. The effects of different operating conditions such as gas sparging rates, fiber packing densities, diffuser nozzle sizes, and module configurations (i.e. loosely versus tightly held) on shear profiles were studied. It was found that the hydrodynamic conditions in confined tubular membrane systems and unconfined submerged hollow fiber membranes are different, which implies that the mechanisms of fouling control in submerged hollow fiber membrane units may be different than those in confined systems such as the tubular membranes. Surface shear profiles during sparging were found to be substantially affected by the operating conditions (i.e. gas sparging rate, fiber packing density, diffuser nozzle size, and module configuration). The different shear profiles were due to the different bubble geometries and flow paths which resulted from the different operating conditions considered. For tightly held low and medium fiber packing density bundles the high number of shear events generated by small bubbles (i.e. small diffuser nozzle) resulted in more favorable hydrodynamic condition for fouling reduction and flux enhancement. The higher permeate fluxes that have been reported in loosely held submerged hollow fiber membrane systems likely result from the larger number of fibers (i.e. regions) that can be impacted by sparged bubbles in systems with loosely held fibers. The results from the present study provide a better understanding of the hydrodynamic conditions inside submerged hollow fiber membrane modules, and therefore a better understanding of possible mechanisms of fouling control via gas sparging.