Toward a universal framework for evaluating transport resistances and driving forces in membrane-based desalination processes

Abstract

Desalination technologies using salt-rejecting membranes are a highly efficient tool to provide fresh water and augment existing water supplies. In recent years, numerous studies have worked to advance a variety of membrane processes with different membrane types and driving forces, but direct quantitative comparisons of these different technologies have led to confusing and contradictory conclusions in the literature. In this Review, we critically assess different membrane-based desalination technologies and provide a universal framework for comparing various driving forces and membrane types. To accomplish this, we first quantify the thermodynamic driving forces resulting from pressure, concentration, and temperature gradients. We then examine the resistances experienced by water molecules as they traverse liquid- and air-filled membranes. Last, we quantify water fluxes in each process for differing desalination scenarios. We conclude by synthesizing results from the literature and our quantitative analyses to compare desalination processes, identifying specific scenarios where each process has fundamental advantages.

Pressure-driven distillation using air-trapping membranes for fast and selective water purification

“…The lack of a measurable change in salt rejection as the thickness decreased was consistent with our understanding that selectivity in the air-trapping membranes is attributable to the gas-liquid phase change transport mechanism. Because of their low thicknesses, the water permeabilities of membranes in this work were notably higher than previous work in osmotic and membrane distillation systems, where membranes are tens of micrometers thick and generally have water permeabilities one to two orders of magnitude lower ( 21 ). While the aim of this study was not to produce membranes that outcompete commercial RO membranes in terms of water permeability, our measurements provide experimental evidence that it is possible for air-trapping membranes to reach the water permeabilities needed for efficient water desalination ( 27 ).…”

“…This pressure-driven distillation process can retain the high energy efficiency and small footprint of RO but also achieve complete removal of nonvolatile species and tolerate harsh feedwaters ( 20 ). Pressure-driven distillation distinguishes itself from existing thermal membrane distillation since it does not require heat energy and is distinct from osmotic distillation, which requires a secondary separation step ( 21 ). The feasibility of desalination via pressure-driven distillation is evident from theory, but demonstration of the system has not been possible due to a lack of appropriate membranes for the process ( 22 ).…”

Nanocomposite Hydrogel Engineered Janus Membrane for Membrane Distillation with Robust Fouling, Wetting, and Scaling Resistance

“…1 Desalination of brackish water and seawater provides a feasible pathway to alleviate the freshwater scarcity. 2 However, vast quantities of concentrated wastewater are produced as a byproduct. 3 Additionally, a large volume of freshwater has been consumed and converted to hypersaline wastewater associated with intensive industrial activities.…”

Water Desalination via Pressure-Driven Distillation with Chlorine-Resistant and Large-Area Polymeric Membranes

“…14 Modeling work has shown that the energy efficiency of PD is likely similar to that of RO since both are driven by hydraulic pressure, and detrimental heat transfer effects in PD are minimal for all realistic membrane properties. 15,16 Despite the potential benefits of PD in desalination and other separations, few studies have experimentally investigated the process, in part due to the challenge of operating airtrapping membranes at pressures above 5 bar without wetting. Those studies that have been conducted have gleaned important insights on the process, showing that PD is experimentally viable, that PD membranes can reach water permeabilities comparable to RO membranes, and that PD can highly reject boron, urea, and other contaminants.…”

“…PD is also similar to membrane distillation (MD) in that transport occurs in the vapor phase through porous hydrophobic membranes; the main difference between PD and MD is that pressure is the driving force in PD rather than temperature, making PD a more energy efficient method of separation . Modeling work has shown that the energy efficiency of PD is likely similar to that of RO since both are driven by hydraulic pressure, and detrimental heat transfer effects in PD are minimal for all realistic membrane properties. , …”