Understanding the effect of transverse vibration on hollow fiber membranes for submerged forward osmosis processes

External concentration polarization (ECP) remains a pervasive challenge for submerged forward osmosis (FO) systems, as it critically reduces the water flux of membranes. In this study, we propose to use transverse vibration on hollow fiber membranes to effectively reduce the ECP and increase the FO water flux performance. The effects of different vibrating amplitude and frequency were investigated. Also, the vibration effect was evaluated under different draw solution concentrations, membrane orientations and draw solute types. Our results show increased water flux with increasing vibrating intensity (i.e., amplitude and frequency) while maintaining a constant specific reverse solute flux (Js/Jw). At the highest amplitude (1.2 cm) and frequency (3 Hz) used, the mass transfer coefficient is increased from 0.7 × 10−5 m s−1 (at no vibration) to 1.8 × 10−5 m s−1, which falls within the optimal flux range as determined from our FO modelling results. The increment in mass transfer coefficient is found to be closely related to the improved hydrodynamic conditions due to vibration, and less dependent on the membrane orientation and draw solution concentration. In addition, both modelling and experimental results show that maintaining a reasonably high mass transfer coefficient is especially important for FO process with severe ECP conditions, such as when dilutive ECP is present (i.e., active-layer-facing-feed-solution orientation) or when a high water flux is applied (usually in the active-layer-facing-draw-solution orientation). However, the severe ECP arising from large draw solutes (e.g., larger sized polymer with low diffusivity and high viscosity) might not gain greater benefits from the vibration or improved hydrodynamic conditions.