Treatment of high salinity brines by direct contact membrane distillation: Effect of membrane characteristics and salinity

•Four PTFE membranes were examined in treating highly concentrated salt solutions.•Active layer of membrane is crucial to mass transfer coefficient and system thermal efficiency.•Flux decreases significantly as salt concentration increases to more than 2.0M.•High salinity KCl, NaCl and MgCl2 solutions (⩾2.0M) shows distinct trends over flux decline. Direct contact membrane distillation (DCMD) is one of the attractive technologies for high salinity brine treatment. In this study, four polytetrafluoroethylene (PTFE) membranes were examined in treating highly concentrated salt solutions. Results showed that non-supported membranes generally have a higher overall mass transfer coefficient but porosity seems to be the most important parameter controlling membrane flux and thermal efficiency. Supported membranes with large thickness had relatively higher thermal efficiency than small thickness. This can be attributed to their reduced heat loss through heat condition. In addition, KCl, NaCl and MgCl2 solutions showed distinct trends over flux decline at high salt concentrations (⩾2.0M). The difference in flux was largely due to the discrepancy in water activities of these solutions (KCl>NaCl>MgCl2). However, the effect of viscosity on permeate flux could not be neglected for MgCl2 at high salt concentrations as the suddenly increased viscosity could lead to serious temperature polarization. This study indicates that membrane distillation is a promising technology for high salinity brine treatment.