Vacuum membrane distillation for desalination: Scaling phenomena of brackish water at elevated temperature

Membrane fouling or scaling is one of the main operational problems plaguing membrane distillation (MD) process during desalination. Specifically, scaling by the inorganic compounds is a serious and also common problem in MD, particularly at high temperature. In lieu of this, the effects of temperature, inorganic compounds and the membrane matrix on the membrane fouling phenomena require further investigation. In this work, a commercial hydrophobic polypropylene (PP) hollow fibre membrane with a smaller pore size (0.1 µm) was performed for its scaling and wettability propensity under the elevated feed temperatures condition. Brackish water that contains inorganic compounds was used as feed. Based on the membrane fouling in the brackish water jar test study, it was found that the deposits on the membrane surface were predominantly magnesium hydroxide [Mg(OH)₂], magnesium silicates (MgSiO₃), magnesium sulphate (MgSO₄), silica (SiO₂), and organic matter traces. The critical temperature for the significant deposition of Mg-based foulants on the membrane surface is 333 K. In the VMD experiments with feed temperature of 333 K, the commercial PP hollow fibre with smaller pore size (0.1 µm) and lower contact angle (102°) suffered more severe magnesium-based surface scaling compared to the membrane with larger pore size (0.2 µm) and higher contact angle (128°). These findings are important for the subsequent operational design of the membrane in vacuum membrane distillation (VMD) for the desalination process.