Quantitative Study on Crystallization-Induced Scaling in High-Concentration Direct-Contact Membrane Distillation

Unique Behaviors and Mechanism of Highly Soluble Salt-Induced Wetting in Membrane Distillation

Scaling-induced wettinggreatly limits the application of membrane distillation (MD) for the desalination of high-salinity feed. Although highly soluble salts (e.g., NaCl) have high concentrations in this water, their scaling-induced wetting remains overlooked. To unravel the elusive wetting behaviors of highly soluble salts, in this study, we systematically investigated the scaling formation and wetting progress by in situ observation with optical coherence tomography (OCT). Through examining the influence of salt type and vapor flux on the wetting behavior, we revealed that highly soluble salt-induced wetting, especially under high vapor flux, shared several unique features: (1) occurring before the bulk feed reached saturation, (2) no scale layer formation observed, and (3) synchronized wetting progress on the millimeter scale. We demonstrated that a moving scale layer caused these interesting phenomena. The initial high vapor flux induced high concentration and temperature polarizations, which led to crystallization at the gas-liquid interface and the formation of an initial scale layer. On the one hand, this scale layer bridged the water into the hydrophobic pores; on the other hand, it blocked the membrane pores and reduced the vapor flux. In this way, the decreased vapor flux mitigated the concentration/temperature polarizations, and consequently led to the dissolution of the feed-facing side of the scale layer. This dissolution prevented the membrane pores from being completely blocked, facilitating the transportation and crystallization of salts at the distillate-facing side of the scale layer (i.e., the gas-liquid interface), thus the proceeding of the wetting layer.

Membrane distillation crystallization technology for zero liquid discharge and resource recovery: Opportunities, challenges and futuristic perspectives

Water resources are getting limited, which emphasises the need for the reuse of wastewater. The conventional waste(water) treatment methods such as reverse osmosis (RO) and multi-effect distillation (MED) are rendered limited due to certain limitations. Moreover, the imposition of stringent environmental regulations in terms of zero liquid discharge (ZLD) of wastewater containing very high dissolved solids has assisted in developing technologies for the recovery of water and useful solids. Membrane distillation crystallization (MDCr) is an emerging hybrid technology synergising membrane distillation (MD) and crystallization, thus achieving ZLD. MDCr technology can be applied to desalinate seawater, treat nano-filtration, and RO reject brine and industrial wastewater to increase water recovery and yield useful solids. This manuscript focuses on recent advances in MDCr, emphasizing models that account for application in (waste)water treatment. MDCr has dual benefits, first the environmental conservation due to non-disposal of wastewater and second, resources recovery proving the proverb that waste is a misplaced resource. Limitations of standalone MD and crystallization are discussed to underline the evolution of MDCr. In this review, MDCr's ability and feasibility in the treatment of industrial wastewater are highlighted. This manuscript also examines the operational issues, including crystal deposition (scaling) on the membrane surface, pore wetting phenomenon and economic consequences (energy use and operating costs). Finally, opportunities and future prospects of the MDCr technology are discussed. MDCr technology can amplify natural resources availability by recovering freshwater and useful minerals from the waste stream, thus compensating for the relatively high cost of the technology.

Slippery for scaling resistance in membrane distillation: A novel porous micropillared superhydrophobic surface

Scaling in membrane distillation (MD) is a key issue in desalination of concentrated saline water, where the interface property between the membrane and the feed become critical. In this paper, a slippery mechanism was explored as an innovative concept to understand the scaling behavior in membrane distillation for a soluble salt, NaCl. The investigation was based on a novel design of a superhydrophobic polyvinylidene fluoride (PVDF) membrane with micro-pillar arrays (MP-PVDF) using a micromolding phase separation (μPS) method. The membrane showed a contact angle of 166.0 ± 2.3° and the sliding angle of 15.8 ± 3.3°. After CF₄ plasma treatment, the resultant membrane (CF₄-MP-PVDF) showed a reduced sliding angle of 3.0°. In direct contact membrane distillation (DCMD), the CF₄-MP-PVDF membrane illustrated excellent anti-scaling in concentrating saturated NaCl feed. Characterization of the used membranes showed that aggregation of NaCl crystals occurred on the control PVDF and MP-PVDF membranes, but not on the CF₄-MP-PVDF membrane. To understand this phenomenon, a "slippery" theory was introduced and correlated the sliding angle to the slippery surface of CF₄-MP-PVDF and its anti-scaling property. This work proposed a well-defined physical and theoretical platform for investigating scaling problems in membrane distillation and beyond.