Highly Effective-Scaling Mitigation in Membrane Distillation Using a Superhydrophobic Membrane with Gas Purging

Membrane distillation (MD) is a thermal desalination process with the capability of harnessing low-grade waste heat to treat hypersaline brine. For this reason, MD has been actively explored as a promising technology for brine management and zero-liquid discharge (ZLD). The major and inevitable challenge with conventional hydrophobic MD membranes, however, is membrane scaling, i.e., the formation and deposition of mineral crystals on the membrane surface that eventually leads to process failure. By performing comparative MD experiments in this study, we show that a superhydrophobic membrane or gas purging can slightly alleviate gypsum scaling, but neither of them is an effective strategy for achieving sustained MD performance against gypsum scaling. However, the synergistic combination of both superhydrophobic membrane and periodic gas purging is extraordinarily effective in mitigating gypsum scaling in MD, enabling MD to concentrate a highly saline feed stream by 5-fold without suffering flux decline due to scaling that is always observed with a commercial hydrophobic membrane. Energy dispersive X-ray spectroscopy reveals the formation of crystal "anchors" inside the pores of the commercial hydrophobic membranes but not those of the superhydrophobic membrane, which explains the different effectivenesses of purging in mitigating scaling for the two membranes. The long-term flux stability offered with this scaling mitigation scheme is important for MD to be applied for brine management and ZLD.