**Preface to "Thermal Performance of Membrane Distillation"**

Membrane distillation (MD) is a thermally driven membrane operation able to theoretically reject 100% of all non-volatiles contained in aqueous streams. It is based on the evaporation of the feed to be treated at the feed–membrane interface, the migration of the vapor/volatiles through the micropores, and the condensation and recovery of the permeated species at the distillate side. Membranes used are hydrophobic and microporous. The driving force of the process is the difference of partial pressure created across the membrane, and the temperature at the feed–membrane interface has been shown to have the greatest impact on the transmembrane flux. However, the temperature at the feed–membrane interface is usually lower than the feed bulk temperature because of temperature polarization phenomena, with a consequent decrease in the process efficiency. In addition, during MD, the feed is cooled inside the module, not only due to the evaporation but also due to the heat lost by conduction through the membrane matrix and the heat lost towards the environment. Therefore, the effective temperature for the evaporation is further reduced. This Special Issue focuses on the research efforts made to improve the thermal performance of MD, including the development of new module designs and heat recovery systems, the preparation of new types of membranes, the use of renewable energies and the integration with other membrane units. For instance, the analysis of heat and mass transport correlations, the development of membranes for localized heating, the design of new modules and condensation devices, the use of solar energy, the integration of different MD configurations and the thermal efficiency of MD in specific applications, are presented and discussed.

My most sincere thanks to all the Authors who contributed to the success of this Special Issue.

**Alessandra Criscuoli** *Editor*
