Water-Energy Nexus: Membrane Engineering Towards a Sustainable Development.

Sustainable development is linked to the achievement of several different objectives, as outlined by the 17 Sustainable Development Goals (SDGs) defined by the United Nations. Among them are the production of clean water and the combat of climate change, which is strictly linked to the use of fossil fuels as a primary energy source and their related CO₂ emissions. Water and energy are strongly interconnected. For instance, when processing water, energy is needed to pump, treat, heat/cool, and deliver water. Membrane operations for water treatment/desalination contribute to the recovery of purified/fresh water and reducing the environmental impact of waste streams. However, to be sustainable, water recovery must not be energy intensive. In this respect, this contribution aims to illustrate the state of the art and perspectives in desalination by reverse osmosis (RO), discussing the various approaches looking to improve the energy efficiency of this process. In particular, the coupling of RO with other membrane operations, like pressure-retarded osmosis (PRO), reverse electrodialysis (RED), and forward osmosis (FO), as well as the osmotic-assisted reverse osmosis (OARO) system, are reported. Moreover, the possibility of coupling a membrane distillation (MD) unit to an RO one to increase the overall freshwater recovery factor and reduce the brine volumes that are disposed is also discussed. Specific emphasis is placed on the strategies being applied to reduce the MD thermal energy demand, so as to couple the production of the blue gold with the fight against climate change.