Unsteady-state shear strategies to enhance mass-transfer for the implementation of ultrapermeable membranes in reverse osmosis: A review

• Need to curb CP and fouling effects to exploit high fluxes of new RO membranes
• Review unsteady-state shear stress means for cost-effective, enhanced CP control
• Induce boundary layer disturbance and renewal without excessive pressure losses
• Assess techniques in terms of potential enhancement and power required
• Identify practical constraints to implementation and promising options

Advances in material science promise the development of a new generation of ultrapermeable membranes (UPMs) for reverse osmosis (RO) desalination and water reclamation, which will lead to reduced footprint and lower capital costs. However, due to the attendant increased concentration-polarization (CP) and membrane fouling effects, the higher fluxes are not possible unless the boundary-layer mass-transfer is enhanced to match the flux increase. In a conventional module, a two-fold increase in flux via UPM would require a four-fold increase in crossflow, generating a 12-fold increase in channel pressure drop. To overcome this, the application of unsteady-state shear to the membrane surface has the potential to be more energy-efficient than a steady-state high shear approach. Hence, this paper reviews a range of unsteady-state shear strategies, including gas sparging, vibrations, particle fluidization, and flow pulsations. Analysis shows that unsteady-state shear could allow for an enhancement of two- to five-fold at an incremental power cost of about 10% compared to the conventional RO desalination process. Some of the practical constraints to implementation are discussed and the promising options identified for further development. Novel modules and modes of operation could provide a challenge for material science and membrane preparation.

Plot of enhancement factor E, which is the ratio of permeability in the presence of unsteady-state shear to that in the absence, as a function of the average shear rate γave for four unsteady-state shear methods. Pulsatile flow is omitted because γave values are unavailable. Embedded in the top right hand corner is the specific power requirement for each method.Figure optionsDownload as PowerPoint slide