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

Abstract

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.

Advanced Antiscaling Interfacial Materials toward Highly Efficient Heat Energy Transfer

“…They speculatively correlate the sliding angle with antiscaling properties of the CF 4 ‐MP‐PVDF surface. Interestingly, the further combination of physical purging will enable superhydrophobic membrane to treat hypersaline brine with sustainable performance . Thus, this work may provide a promising platform for understanding scaling on membrane distillation.…”

Understanding the fouling/scaling resistance of superhydrophobic/omniphobic membranes in membrane distillation

“…(2) Non-alkaline, as represented by CaSO4, which is pHindependent [26]; CaSO4 mainly forms rod-like structures, but needle-like and rosette-like ones have also been observed (Fig. 2c) [31,[44][45][46][47][48][49][50][51]. CaSO4 scaling has three forms as a function of temperature [44,52]: dihydrate (gypsum, CaSO4•2H2O), hemihydrate (bassanite, CaSO4•0.5H2O) and anhydrite (CaSO4) [34].…”

Unprecedented scaling/fouling resistance of omniphobic polyvinylidene fluoride membrane with silica nanoparticle coated micropillars in direct contact membrane distillation

“…Nevertheless, the hydrophobicity alone does not appear to be the only deterministic factor for fouling resistance as controversies over the relationship between superhydrophobicity and anti-fouling effects has been reported [16,29,30,40,56,57].…”

“…deposited fouling also induces nucleation or growth) [25], leading to progressive wetting and performance reduction [26]. Established fouling mechanisms unraveled a thermodynamic understanding of fouling phenomena, and provided a guidance on alleviating or eliminating fouling/scaling by process parameter [27,28] and membrane fabrication [17,19,[29][30][31][32][33].…”

“…Due to the negative surface charges in membrane surface, electrostatic repulsion to the matters in water which are also negatively charged contributes to fouling resistance. Studies in thermodynamic analysis indicate that increasing membrane hydrophobicity can deter the scaling formation as well as membrane wetting [21,43,[53][54][55], however, the results sometimes are controversial as there are still scenarios in which superhydrophobicity has a negligible or even detrimental effect on fouling mitigation [16,29,30,40,56,57]. Thereby, there still lacks definitive clarification on the anti-fouling mechanism for superhydrophobic or omniphobic membranes.…”