Correlation between the feed composition and membrane wetting in a direct contact membrane distillation process

Membrane distillation is a promising option for desalination owing to its advantages, but additional studies are still required before drawbacks such as membrane wetting can be resolved. To investigate the effects of organic and inorganic contents in feed solutions on membrane wetting, a lab-scale direct contact membrane distillation system was operated with different compositions of synthetic feed solutions. CaCl2 and MgSO4 were added to the feed solutions at different concentrations. Humic acids were injected into the feed solutions as organic additives. The hydrophilicity of humic acids was changed by ozonation with different ozone contact times. Membrane wetting was monitored by measuring the distillate flux and conductivity during operation. As a result, when the concentrations of CaCl2 and MgSO4 increased, membrane wetting was increasingly promoted. Once the membrane pores were wetted, salts could pass through from the feed side to the distillate side through the wetted pores, resulting in scale formation even on the distillate side of the membrane after being dried, which was confirmed by scanning electron microscopy. Without ozonation, humic acids were hydrophobic and induced severe membrane fouling, but this retarded the membrane wetting. With ozonation, humic acids were hydrophilic and accelerated the membrane wetting, which was more significant with longer ozone contact times. Hydrophilic ozonated humic acids reduced the liquid entry pressure of the membranes, enhancing the potential for membrane wetting.