Membrane Distillation Process

The water stress that we have been experiencing in the last few years is pushing towards the development of new technologies for the purification and recovery of water. Membrane Distillation (MD) is based on the use of hydrophobic microporous membranes that prevent the passage of aqueous feed as liquid through the micropores, allowing the transport of water vapor and volatiles only, thanks to a difference of partial pressures established across the membrane. In this way, high-purity distillates can be produced starting from a variety of aqueous streams, like effluents coming from textile/agrofood/pharmaceutical industry, olive mill wastewaters, waters contaminated by heavy metals, sea, and brackish waters. Some studies on the application of MD for the purification of radioactive wastewaters and of urine have also been carried out.With respect to Reverse Osmosis (RO), which is limited by the osmotic pressure and sometimes shows low rejection values for elements like As(III) and Boron, MD is able to produce fresh water from high-concentrated streams and provides 100% theoretical rejections for all non-volatiles present into the aqueous feeds.Despite these advantages, MD is far from a significant application at industrial scale, due to some still pending issues:need of developing membranes with high hydrophobicity and liquid entry pressure values, able to be stable when treating real streams in long-term runs;need of developing modules with reduced thermal and mass transfer resistances;need of reducing the specific thermal energy consumption.