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Membranes 2020, 10, 221 45 of 72 improve power density. Compared to conventional RED using similar NaCl streams, MEA-based RED at small scale attained higher power densities (up to 4.1 W/m2) [255]. Capacitive reverse electrodialysis (CRED) is another alternative of RED technology [273]. Similarly to MCDI, it stages capacitive electrodes of active carbon. Those generate electricity without redox reaction (about 1 W/m2 with small-scale 30-cell equipment), thus preventing the use of specific chemicals. However, electrodes would still need to be regenerated [274]. Similar average current densities were achieved by a capacitive concentration flow cell using CEMs and carbonized peat moss electrodes [275]. RED with flow electrodes (FE-RED) was developed to allow continuous operation by recirculating the electrodes. Its huge advantage is a constant electricity generation, however current productivity is limited due to maximum power densities not exceeding 0.26 W/m2 [276]. Other technological developments include reverse electrodeionization (REDI) showing a decreased system resistance compared to RED due to embedded IX resin in the dilute compartment, leading to 0.32–0.44 W/m2 power densities [277,278]. Reverse electro-electrodialysis (REED) was also developed to mitigate RED’s shortcomings and produce hydrogen for energy. Chen et al. [279] experimented on a REED stack using strong acid/base as electrolyte to boost electrochemical performances, in particular reduction of H+ in gaseous H2 at the cathode. They attained a 0.45-W/m2 power density and a H2 production rate of 0.55 mL·h−1·cm−2 electrode, but only 2% hydrogen energy recovery due to small scale limitations. RED with bipolar membranes are currently studied for potentially higher power densities [280]. Finally, a new nanoporous carbon membrane with mesoscopic thickness have been developed for reverse electrodialytic applications and reached a maximum power density of 67 W/m2 [281]. Despite showing increasing promises for SGP energy storage, these recent developments are still in need of further studies establishing larger-scale practicality. 5. Integration of ED Technologies in New Sustainable Strategies Current ED processes are making a growing contribution toward sustainable pure water production. Moreover, these are very promising for energy, wastewaters remediation, fine chemicals and food applications. In this part, we present recent intensification developments and strategies involving new and mature technologies, and discuss their eco-efficient potential. Eco-efficiency is a management tool that relates life cycle environmental indicators and product value to support decision making [282,283]. It aims at identifying scenarios that decrease environmental impact while increasing the product value. Indeed, to improve the eco-efficiency, one can choose a different supplier, change to a greener process or technology, or develop a product with a stronger valued market [284]. 5.1. Desalination and Salinity Gradient Power Recovery 5.1.1. Desalination Strategies for Ultrapure Water Desalination of sea water using conventional ED is a robust process with moderate energy requirement. ED represents 4% of the desalination capacity worldwide, while reverse osmosis (RO) stands for 64% and thermal distillation processes for about 30%. In addition to a more efficient energy use than for thermal processes, ED has several advantages compared to RO. Lighter pre- and post-treatments lead to longer membrane lifespan, while higher temperature tolerance and selectivity allow more flexibility. ED has also better water recovery [6,285]. In terms of cost effectiveness, ED is generally better for water with less than 5 g of total dissolved solids (TDS) per liter, while RO is more interesting for streams with concentrations higher than 5 g TDS/L [192]. Due to this complementarity and in order to reach deeper desalinations by maintaining the eco-efficiency of the process, researchers are studying the coupling of electrodialytic technologies with other operation units such as pressure-driven membrane filtration. Continuous electrodeionization leads to higher water purity than ED or RO due its enhanced conductivity (c.f. Section 4.8), however it requires less concentrated water as the feed stream (<30 ppm). Therefore, a viable strategy is to associate CEDI with RO, ED or both [205]. Ho and Wood [286] evaluatedPDF Image | Electrodialytic Processes
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