Membrane distillation (MD) is a thermally driven membrane process capable of desalinating hypersaline brine. It has a great potential to become a competitive technology for treating a variety of hypersaline wastewaters that are too challenge for conventional desalination technologies (e.g. reverse osmosis) to treat. However, in many cases these hypersaline wastewaters are also of a complex composition and contain contaminants that can readily fail the MD process by fouling and/or wetting the MD membrane. In addition, mineral scaling is an unavoidable challenge to overcome if MD is to be employed for high recovery of water and mineral resources. Our research aims to overcome these challenges that MD faces when used for treating challenging waste brines, and thereby significantly expand the (feed water) compatibility of MD and enable MD in application areas where it did not but can potentially excel. We will achieve this goal by working on the following subtopic areas.
- Membrane with special wettability We have been developing and testing membranes with special wettability, including omniphobic membranes and Janus membranes with different combinations of wetting properties, to mitigate membrane wetting and fouling in MD. So far, we have acquired systematic understanding regarding the compatibility between membranes with different special wettability and feed solutions of different compositions, which is summarized in the figure below.
- Early detection of imminent MD membrane failure We have recently developed a novel method based on single frequency impedance (SFI) to detect pore wetting in MD. The principle of this approach is based on the assumption that air gap (or the gas-filled pores) within the MD membrane is equivalent to a capacitor in an alternating electric field. We have found that the SFI technique enables significantly earlier detection of imminent pore wetting before distillate conductivity starts to increase (i.e. the membrane has already been wetted). Not only this technique allows early detection of imminent pore wetting, it also facilitates the elucidation of mechanisms of pore wetting that cannot be unveiled by measuring the distillate conductivity. We are now actively exploring the potential of this SFI technique in early detection and mechanistic elucidation of other membrane failure including fouling and scaling.
- Mechanisms of membrane fouling, wetting, and scaling To better manage membrane failure in MD, it is of crucial importance to acquire in-depth and mechanistic understanding of different membrane failure phenomena. We use different techniques such as oil-probe force spectroscopy (based on tensiometer), colloidal probe force spectroscopy (based on Atomic Force Spectroscopy), and SFI. Some of the questions to be answered include, but are not limited to, (1) whether oil droplets spread on the membrane surface alone or they also wick into the membrane pores; (2) whether pore wetting occurs simply because of the reduced liquid surface tension or also due to the modified surface energy of the polymeric membrane; (3) whether scaling (or mineral precipitation) occurs mostly in the bulk or at the membrane surface.
System-scale Analysis of Desalination and Processes
One important overarching goal of most ongoing research on desalination is to make it more affordable. The major effort in the frontier of desalination research has focused on developing novel desalination processes, materials, and system configurations that may potentially lead to desalination with lower cost and higher performance. Relatively little has been done, surprisingly, to systematically evaluate how these innovations can enhance the kinetic and energetic efficiencies, and thereby reduce the overall cost of desalination. System-scale analysis is essential to understanding the thermodynamics, kinetics, and energy efficiency of desalination processes and to evaluating how bench or even microfluidic scale innovations can translate to energy and cost saving in large operating desalination systems. System-scale analysis is also of critical importance to optimizing system design and operation for various desalination processes as it quantifies the relationship between energetic and kinetic efficiencies. The desalination (and salinity-gradient energy generation) processes we have been analyzing include:
- Engineered osmosis (reverse osmosis, forward osmosis, and pressure retarded osmosis)
- Membrane distillation
- Capacitive Deionization
We are also actively exploring other research areas (not necessarily as systematically as those listed here) and will continue to update this page as progress is made.