# Membrane Engineering for Sustainable Development: A Perspective
## 2 Membrane Operations in Sustainable Development: Case Scenario
### Energy Consumption and Production  
Separation, concentration and purification of molecular mixtures are the key requirements in process industry, underlying the importance of efficient separation in this sector. The state-of-the-art norms to achieve these objectives include distillation, extraction, crystallization, adsorption and ion exchange technology. These technologies are energy intensive in nature and generally mark a large footprint. To alleviate these drawbacks of the conventional processes, membrane operations have demonstrated their potential to perform the same operation with far less energy consumption [11]. Relatively less explored membrane operations, such as membrane distillation (MD) and forward osmosis (FO), are able to exploit the sources of low-grade energy to fulfil their operational energy requirements and thus, almost eliminating the motivation to consume high-grade energy [10; 12]. Additionally, the footprint, safety level, automation and monitoring, ease to scale-up and product quality of membrane processes is higher than the conventional counterparts. Aside reducing the energy consumption, new membrane operations are also capable to produce green energy by using sustainable resources. Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are being investigated at various scales to achieve this objective in techno-economic viable way [13]. The operation principle of PRO is the extraction of water from low to high salinity solution separated through a semipermeable membrane. The pressure generated at the high-salinity-solution compartment can be converted into electricity by using a hydro-turbine as shown in Figure 2. By exploiting the extractable global potential of 980 GW, the salinity-gradient-energy can fulfil 20% potential of current global energy demand [3]. PRO and RED can be integrated with the existing desalination capacities to use their brine as the high concentration solution. This improves the performance of PRO and RED and assists the desalination plants to handle the disposal issue of the brine [14]. The technical and economic feasibility of PRO for commercial scale applications is currently being investigated under the projects such as Mega-tone  
Figure 1: Timeline for membrane distillation (MD) development (diamond symbol indicates the milestone events; star symbol indicates the MD projects/pilot plant; gray circular symbol indicates the launch of technology developer companies with the location of their headquarters). Reproduced with permission from [8]. Copyright Elsevier, 2017.  
and Global MVP [3]. The first RED unit working with real streams in natural environment has been installed under REAPower project [15]. The unit had a membrane area of 50 m\({}^{2}\) and could give a power output of approximately 40 W (1.6 W/m\({}^{2}\) of cell pair) by using real solutions (brackish water and saturated brine). The initial results were promising and no significant technical issues were observed. Inspired from these results, the plant was upscaled to a capacity of 1 kW. To achieve this objective, the membrane area was increased to 400 m\({}^{2}\)[16]. With the nominal capacity of 1 kW, the plant could achieve power capacity of 700 W using artificial brine and brackish water. The power capacity reduced to 50% when real solutions were used. Commercial scale implementation of PRO and RED is hampered by several factors. The fundamental issue for both technologies is the unavailability of suitable membranes. For PRO, the membranes must exhibit high permeability, low reverse salt diffusion, minimum fouling and very high mechanical strength to withstand against the applied pressure. PRO process is severely limited by internal concentration polarization (ICP) in the porous membrane support, i.e. : (i) the accumulation of rejected solute from low concentration feed when the dense active layer is oriented towards the draw solution (i.e., support facing the feed water) or (ii) the dilution of draw solution concentration when the active layer is facing the feed water.