Water reclamation from unconventional resources has attracted heightened interest worldwide. Membrane distillation (MD) is a promising technology for hypersaline wastewaters/brines reclamation. Since the key component in MD is the membrane, a myriad of research effort on MD has been devoted to developing MD membranes with ultrahigh vapor permeabilities. In this study, we perform a comprehensive analysis of the performance of practical MD operations to evaluate the benefits of developing MD membranes with ultrahigh vapor permeabilities. For coupon-scale MD operations (i.e., bench-scale systems), regardless of the membrane vapor permeability, the membrane vapor flux is limited by temperature polarization, and with the feed/distillate temperature of 60/20 °C under practical operating conditions, the membrane vapor flux can barely exceed 200 kg m–2 hr–1. For module-scale MD operations (i.e., large-scale systems), increasing the membrane vapor permeability by 15 folds only reduces the energy consumption by ∼26%, and despite the substantial saving of membrane area, the membrane cost needs to be considered. The results from our analysis demonstrate questionable benefits of membranes with ultrahigh vapor permeabilities in practical MD operations, casting doubt on the necessity of developing such membranes for MD applications. Last, we highlight the critical needs for robust MD membranes that can overcome challenges of membrane fouling, wetting, and scaling, shedding light on future MD membrane development.

As an emerging desalination technology for hypersaline wastewater treatment, membrane distillation (MD) faces a critical challenge of membrane wetting. The state-of-the-art wetting mitigation strategy in MD is to use novel membranes that are commercially unavailable and difficult to fabricate. This study proposes an operational mode, negative pressure MD (NPMD), as a novel wetting mitigation strategy in MD operations. Compared with conventional MD, NPMD can substantially enhance the wetting resistance of commercially available hydrophobic MD membranes. Specifically, in a conventional direct contact MD (DCMD) operation, a polyvinylidene fluoride (PVDF) membrane is easily wetted by a 0.1 mM sodium dodecyl sulfate (SDS) feed solution, while in NPMD, the PVDF membrane can remain unwetted with a 0.2 mM feed solution. By determining the liquid entry pressure (LEP) of the PVDF membrane using an impedance-based technique, the working mechanism of NPMD for wetting mitigation is illustrated, and such a mechanism is further confirmed by DCMD experiments using feed solutions containing ethanol. With a negative gauge pressure on the feed stream, the transmembrane hydraulic pressure becomes lower than the LEP of the PVDF membrane, thereby mitigating membrane wetting. As a simple yet effective wetting mitigation strategy, NPMD can be readily implemented in practice with commercially available hydrophobic membranes, showing vast potential to advance MD applications.

Figure 1. Schematic illustration of the fabrication of the TFC membrane with a hydrophobic membrane substrate.