Water scarcity is a grand challenge that has always stimulated research interests in finding effective means for pure water production. In this context, reverse osmosis (RO) is considered the leading and the most optimized membrane-based desalination process that is currently dominating the desalination market. In this review, various aspects of RO desalination are reviewed. Theories and models related to concentration polarization and membrane transport, as well as merits and drawbacks of these models in predicting polarization effects, are discussed. An updated review of studies related to membrane modules (plate and frame, tubular, spiral wound, and hollow fiber) and membrane characterization are provided. The review also discusses membrane cleaning and different pre-treatment technologies in place for RO desalination, such as feed-water pre-treatment and biocides. RO pre-treatment technologies, which include conventional (e.g., coagulationflocculation, media filtration, disinfection, scale inhibition) and non-conventional (e.g., MF, UF, and NF) are reviewed and their relative attributes are compared. As per the available literature, UF, MF and coagulation-flocculation are considered the most widely used pre-treatment technologies. In addition, this review discusses membrane fouling, which represents a serious challenge in RO processes due to its significant contribution to energy requirements and process economy (e.g., flux decline, permeate quality, membrane lifespan, increased feed pressure, increased pre-treatment and membrane maintenance cost). Different membrane fouling types, such as colloidal, organic, inorganic, and biological fouling, are addressed in this review. Principles of RO process design and the embedded economic and energy considerations are discussed. In general, cost of water desalination has dropped to values that made it a viable option, comparable even to conventional water treatment methods. Finally, an overview of hybrid RO desalination processes and the current challenges faced by RO desalination processes are presented and discussed.
