Microporous polyvinylidene fluoride (PVDF) membranes were prepared by immersion precipitation of dimethylformamide/PVDF solutions, dissolved at 50°C, 60°C, 70°C, and 80°C in a nonsolvent bath. The formed membranes (M50, M60, M70, and M80) exhibited symmetric structures consisting of spherical particles whose size increased from ~1 μm for M50 to ~3 μm for M80. The desalination capabilities of the membranes were assessed via the direct contact membrane distillation process. For the case of desalting 3.5% NaCl(aq), very high rejection ratios (>99.7%) were achieved for all tested membranes under the operation conditions: Thot stream=52°C, Tcold stream=18°C, circulation rate=0.7 l/min, while the highest permeation flux obtainable was 16.3 l/m2h (LMH). When the feed temperature or circulation rate was raised, the permeation flux increased correspondingly. On the other hand, raising the salt concentration resulted in a strong decrease of the permeation flux. For the membrane M60 operated over the period of 48 h, the flux decreased slightly from 11.6 to 10.5 LMH, while the conductivity increased from 1.5 to 27 μs/cm indicating the occurrence of slight surface wetting during this extended operation period.
