Wetting prevention in membrane distillation through superhydrophobicity and recharging an air layer on the membrane surface

- For the first time, air recharging for wetting prevention in a full MD setup was examined. - Air recharging combined with superhydrophobic membranes prevented wetting for low surface tension saline solutions with ~100% salt rejection. - Our results suggest that air recharging is critical for MD applications with feed waters containing organic compounds or surfactants.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 macro porous 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.