Membrane distillation hybrids for water production and energy efficiency enhancement: A critical review

With an ever-increasing demand in energy, constrained by strict environmental regulations, process development faces stringent design requirements further limited by intrinsic properties of inherent materials. Process hybridization is now considered as an improvement path to several limitations. Complementarity between processes is the essence of the hybridization concept, with the ultimate goal to design more eco-friendly, energy efficient process combinations delivering higher throughputs and boosting the thermodynamic limits of the existing mature technologies. Market size of membrane-based separation processes, widely used in desalination, water treatment and purification, is forecasted to grow significantly in the next decades. While desalination market is mainly shared between thermal processes and reverse osmosis (RO), advanced water treatment and purification rely mostly on membrane technology. Among the large span of available techniques stands membrane distillation (MD), to which a tremendous research effort has been dedicated during the last two decades. Although praised for its numerous advantages, this thermally-driven separation process still cannot withstand large production rates while maintaining energy efficiency. Hybridization of MD with existing technologies and other emerging processes is therefore at the leading edge. This literature review presents the state-of-the-art MD hybrids with different separation processes including RO, pressure retarded osmosis, forward osmosis, mechanical vapor compression, electrocoagulation, electrodialysis, multi-stage flash, multi-effect distillation, crystallization and adsorption with a focus on water production and energy efficiency enhancement. Each of these processes has advantages at the cost of more or less severe drawbacks and its association to MD offers improvement opportunities. Each variant is thoroughly reviewed with major contributions and knowledge gaps highlighted. Perspectives and recommendations are emphasized in each case. Latest developments in MD and its energy consumption and optimization are also reported.