Membrane distillation (MD) is a rapidly emerging water treatment technology; however, membrane pore wetting is a primary barrier to widespread industrial use of MD. The primary causes of membrane wetting are exceedance of liquid entry pressure and membrane fouling. Developments in membrane design and the use of pretreatment have provided significant advancement toward wetting prevention in membrane distillation, but further progress is needed. In this study, a broad review is carried out on wetting incidence in membrane distillation processes. Based on this perspective, the study describes the wetting mechanisms, wetting causes, and wetting detection methods, as well as hydrophobicity measurements of MD membranes. This review discusses current understanding and areas for future investigation on the influence of operating conditions, MD configuration, and membrane non-wettability characteristics on wetting phenomena. Additionally, the review highlights mathematical wetting models and several approaches to wetting control, such as membrane fabrication and modification, as well as techniques for membrane restoration in MD. The literature shows that inorganic scaling and organic fouling are the main causes of membrane wetting. The regeneration of wetting MD membranes is found to be challenging and the obtained results are usually not favorable. Several pretreatment processes are found to inhibit membrane wetting by removing the wetting agents from the feed solution. Various advanced membrane designs are considered to bring membrane surface non-wettability to the states of superhydrophobicity and superomniphobicity; however, these methods commonly demand complex fabrication processes or high-specialized equipment. Recharging air in the feed to maintain protective air layers on the membrane surface has proven to be very effective to prevent wetting, but such techniques are immature and in need of significant research on design, optimization, and pilot-scale studies.

Although membrane distillation offers distinctive benefits in some certain areas, i.e., RO concentrate treatment, concentrating solutions in the food industry and solar heat utilization, the occurrence of wetting of the hydrophobic membrane hinders its potential industrial applications.Therefore, wetting prevention is a vital criterion for the treatment of solutions with lower surface tension than water. The present work examines the effect of recharging air bubbles on the membrane surface for the wetting incidence when a surfactant (sodium dodecyl sulfate, SDS) exists in a highly concentrated NaCl aqueous solution. This study shows that the presence of the air bubbles on the surface of the superhydrophobic membrane in a direct contact membrane distillation setup inhibited the occurrence of wetting (~100% salt rejection) even for high concentrations of the surface-active species (up to 0.8 mM SDS) in the feed solution while no undesirable influence on the permeate flux was observed. Introducing air into the feed side of the membrane displaces the liquid which partly tends to penetrate the macroporous structure with air bubbles and therefore increases the liquid entry pressure, and in addition, the simultaneous use of a superhydrophobic membrane enhances the solution contact angle.

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