Negative Pressure Membrane Distillation: A Novel Strategy for Wetting Mitigation

Abstract

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.

Hypercrosslinked Hydrogel Composite Membranes Targeted for Removal of Volatile Organic Compounds via Selective Solution-Diffusion in Membrane Distillation

“…Currently, extensive efforts have been devoted to developing advanced membranes such as superhydrophobic, omniphobic and Janus membranes or intensifying MD process via pressure or electric field assistance to deal with membrane wetting, fouling and scaling issues. ,− However, there is hardly MD membrane that can efficiently intercept volatile substances, restricted by the high volatility, hydrophobicity and small molecular size of VOCs. ,, Recently, nanocomposite membranes displayed good decomposition or interception of low concentration VOCs in the MD process with the assistance of additional oxidants, light and electric field. − Dual-layer thin film composite (TFC) membrane with VOCs rejection of ∼90% was developed for freshwater recovery from VOCs containing saline feed via the MD process . In addition, solar-driven evaporators were also designed to achieve VOCs–water separation using nanomaterial composite membranes (e.g., MOF and TiO 2– x ) and polymer membranes based on different interception mechanisms. − The above successful cases unlocked possibilities for the interception of VOCs in the evaporation process.…”