Effect of dry-out on the fouling of PVDF and PTFE membranes under conditions simulating intermittent seawater membrane distillation (SWMD)

Hydrophobic membranes are perceived to be chemically stable and resistant to dry-out during intermittent membrane distillation (MD) operations. However, distillate quality deterioration has been reported in field tests of solar-powered MD systems. This work investigates the effects of salt deposition, resulting from membrane dry-out, on the properties of two types of commercial hydrophobic membranes commonly used for MD purposes. The membranes are made of polytetrafluoroethylene (PTFE) and polyvinylidenefluoride (PVDF). The intermittent operation was simulated by a series of wet/dry cycles using seawater at MD operational temperatures. Membrane properties including surface contact angle, gas permeability, and mechanical strength were assessed, along with other structural characteristics like pore size distribution. The morphology of the salt-exposed membranes was also investigated using SEM and EDS. Atomic force microscopy (AFM) measurements were carried out to assess surface morphology and quantify surface roughness, in relation to the fouling process. AFM was combined with the use of a CaCO₃ colloid probe to assess adhesion forces between CaCO₃ and the membranes. DCMD experiments were conducted to study the post-fouling performance of the membranes in the MD process. Significant scaling was detected starting from the first week of seawater exposure, as evidenced by all the characterization techniques used. PVDF and PTFE membranes were observed to behave somewhat differently under these fouling conditions. DCMD experiments proved that membrane wetting was exacerbated by intermittent operation.