EAGER: Water Continuity on the Performance of Osmotically Driven Membrane Processes

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Osmotically-driven membrane processes (ODMPs) leverage the difference in chemical potentials between two solutions that are separated by a semipermeable membrane (i.e., osmotic pressure) to move water from the low solute concentration side (feed water) to the high solute concentration side (draw solution). This ability to transport water through a membrane via osmotic pressure, without applying an external hydraulic pressure driving force, can be used to supply clean water and produce renewable energy directly from saline waters. However, low water flux across the membrane limits the performance of ODMPs and their potential to meet global water and energy needs. A phenomenon called internal concentration polarization (ICP) has received much of the blame for the low water flux under an osmotic pressure-driving force alone. The prevailing theory is that a strong ICP develops in the membrane’s porous support layer and diminishes the effective osmotic pressure driving water across the active (skin) layer relative to the total osmotic pressure of the membrane draw solution. Yet, membranes designed to minimize or eliminate ICP and, thus, improve water flux have yielded few technological breakthroughs. This project will consider the controversial idea that the collapse of water continuity across the membrane, instead of ICP, is mainly responsible for the low water flux under osmotic pressure. Successfully demonstrating that water continuity within the membrane is a key factor in ODMP performance has the potential to transform the field of membrane-based water treatment and purification. The project will also provide opportunities for graduate student training and STEM outreach through the Texas Tech University Chapter of the Society for the Advancement of Chicanos and Native Americans in Science. The goal of this project is to demonstrate that the loss of water continuity inside the membrane of an ODMP is a principal factor contributing to low water flux. A custom membrane system will be constructed such that hydraulic pressure can be independently manipulated on both the feed water and draw solution sides and that the pressure in the feed chamber can be measured when it is sealed and detached from the feed water tank. The membrane system will be used to observe the buildup of negative pressure in the sealed feed chamber under osmotic pressure. Under typical ODMP operating conditions, the system is expected to exhibit a process of negative pressure buildup and abrupt dissipation, wherein the negative pressure is relieved by the collapse of water continuity (i.e., cavitation). In addition to demonstrating the breakup of water continuity, the study will systematically examine the relationship between water flux in ODMPs and the status of water continuity as a function of operating conditions and membrane material. The independence of the hydraulic pressures applied to the feed water and draw solution sides will also be evaluated. The research is expected to generate new insights into osmotic pressure-driven water transport mechanisms across membranes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.