# Membrane Engineering for Sustainable Development: A Perspective
## 2 Membrane Operations in Sustainable Development: Case Scenario  
### Desalination  
The concept of desalination (i.e., removal of salts and contaminants from water) is centuries old and has been practiced in various civilizations. Historically, the desalination has been achieved through distillation, also mentioned by [PERSON] in his Meteorologica. However, the major commercialscale developments in distillation took place during 1930s, in regions where freshwater sources were not enough to fulfil the local water demand. However, the developments in desalination sector have been fueled during the last few decades due to the increased population, growing urbanization, huge industrialization, increase in middle class population in the developing countries and depleted water sources due to increased abstraction and change in weather patterns. These developments have put pressure on conventional water sources in terms of its consumption and pollution. Consequently, much growth has been observed in the desalination sector. According to the International Desalination Association, in the first half of 2016, the amount of global contracted capacity in desalination plants was 95.59 million m\({}^{3}\)/day and the global online capacity was 88.56 million m\({}^{3}\)/day, a total increase of 2.1 million m\({}^{3}\)/day in new desalination capacity over 2015 [4]. The major distillation techniques adopted for desalination include multi-stage flash, multi-effect distillation, vacuum vapor compression etc. These techniques utilize mainly thermal input to produce the vapors from brackish or seawater which are then condensed to get pure water. The energy consumption of these techniques, however, is enormously high. Aside from that, the footprint and weight of the used equipment are very large and corrosion issues are very serious. To overcome the drawbacks of thermal desalination techniques, the membrane based desalination started emerging in 1950's [5]. The main breakthrough was the formation of asymmetric thin film membrane for reverse osmosis (RO) by [PERSON] and [PERSON] [6]. Over the period of time, major improvements in RO process have been taken place and the cost of RO process has reduced more than 50% only in the last two decades. Overall, the current RO process consumes ten folds less energy than its conventional thermal counterparts. The polymeric membranes, generally applied in RO, solved the corrosion issues associated with thermal desalination techniques. However, new environmental and economic goals are driving another revolution in desalination. Limited recovery factor of RO (50% typically), brine disposal issues, rigorous pretreatment requirements and the interest in reducing the energy consumption are further driving the interest in investigating better alternatives for desalination. New materials, technologies and rationales with the potential to go beyond the inherent barrier of RO are being investigated. The main new interesting membrane operations being considered to address the drawbacks of RO include membrane distillation (MD) and forwards osmosis (FO). The performance of the former is only marginally dependent upon the feed concentration and the operation can be carried out by using low-grade energy. Pretreatment requirements are supposed to be less stringent, thus reducing the use of chemicals. These operations can be integrated with the existing thermal and membrane based desalination techniques or as standalone process. MD, integrated at RO brine treatment, can achieve the objective of zero liquid discharge in desalination [7]. Despite its repeatedly stated advantages, the progress of MD at commercial scale has been slow. A timeline of MD developments is shown in Figure 1[8]. The figure indicates that the progress during first three decades had been very slow and most of the studies were of fundamental nature focusing at the basic understanding of the process and its various configurations.