Localized heating to improve the thermal efficiency of membrane distillation systems

Membrane distillation (MD) is a thermal-based membrane operation with high potentialfor the treatment of aqueous streams. However, its implementation is limited and only few examplesof MD pilots can be found in desalination. One of the reasons behind this is that MD requires thermalenergy for promoting the evaporation of water, which implies higher energy consumption withrespect to pressure-driven membrane operations, like reverse osmosis (RO). Recently, among thedifferent methods investigated to improve the thermal efficiency of MD, attempts for obtaining alocalized heating of the feed, close to the membrane surface, were carried out. This work reviewsexperimental activities on the topic, dealing with both modified membranes, used under solar irradiationor coupled to an electric source, and specifically designed heated modules. The main resultsare reported and points of action for further optimization are identified. In particular, although atan early stage, this type of approach led to improvements in membrane flux and to a reduction ofenergy consumption with respect to conventional MD. Nevertheless, long tests to ensure a stableperformance time, the optimization of operating conditions, the development of methods to controlfouling issues, and the identification of the best module design, together with the scale-up of membranes/modules developed, represent the main research efforts needed for future implementation oflocalized heating strategy.