# Reverse Electrodialysis: Potential Reduction in Energy and Emissions of Desalination
## 1 Introduction  
The rising water demand and the steady decline in conventional water resources are urging the use of non-conventional ones, as desalinated, re-used or reclaimed water [1; 2]. Over the period of 2014 to 2040, the energy use in the water sector is expected to more than double, primarily driven by desalination [3]. Even though membrane technology's advances, installation of energy recovery devices and use of more efficient pumps have well declined the energy to drive desalination over the last four decades [4; 5; 6], seawater reverse osmosis (SWRO) desalination remains an energy-intensive source ofwater [4; 5]. Indeed, the specific energy consumption (SEC)--the energy consumed per cubic meter of water desalted--of current state-of-the-art SWRO plants ranges from 2.5-6.0 kWh m\({}^{-3}\) depending on several site-specific factors such as seawater composition and temperature, permeate quality standards, brine management, production capacity and reverse osmosis (RO) configuration [6; 7]. The high specific energy use, thereby, thwarts widespread use and environmental sustainability of desalination, as this process relies heavily on conventional power sources. Given that global desalination capacity is projected to increase at a steep pace in the coming years [1; 3], and fossil fuels dwarf the current electricity portfolio [8], the shift to low-emissions decentralized renewable power sources with little water needs is decisive in moving forward the gradual decarbonization of the desalination industry. Unlike solar and wind, innately intermittent and stochastic sources, challenging a stable and safe energy supply, salinity gradient is a clean and steady renewable energy source that may offer an integrated solution to the desalination sector as it reinforces the phaseout of conventional water-intensive energy sources. Salinity gradient energy (SGE), released when two solutions at different concentration spontaneously mix, is converted into useful work through reversible mixing of these two streams in engineered processes; pressure-retarded osmosis (PRO) and reverse electrodialysis (RED) are the most promising technologies among those developed to date [9; 10; 11]. Both are somewhat close to commercialization, but the high cost and current performance of membranes along with fouling issues prevent their widespread deployment. PRO uses semi-permeable membranes that allow transportation of water, but not ions, from a low-concentration solution (e.g., seawater, river water, brackish or wastewater) to a high-concentration solution (e.g., seawater, desalination brines) to pressurize water that generates electricity driving mechanical turbines, while RED is an emerging electrochemical process that uses ion-exchange membranes (IEMs) to directly draw electric power from the flow of ions and IEM electric potential difference. The advantage of RED over PRO is that it directly transforms SGE into electricity with no intermediate energy conversion stages (i.e., mechanical into electric). RED is also less prone to fouling issues, and thus can reasonably sustain performance over longer periods. In RED (Figure 1), the high- and low-salinity solutions (HC and LC, respectively) flow through a series of alternate cation-exchange membranes--CEMs, permeable to cations--and anion-exchange membranes--AEMs, permeable to anions--kept apart by spacers. Each alternate layer of HC and LC compartments adjacent to CEMs and AEMs makes up the cell pair. Several basic repeating units (or cell pairs) stacked in series between two endplates housing the electrodes form the RED pile. The driving force--i.e., the salinity gradient--along with IEM selectivity cause cations and anions to move across the ion-selective membranes from the high salinity side to the low salinity one towards the cathode and the anode, respectively, where redox reactions transform the ionic flux into an electron flux. The chemical potential difference between the concentrate and diluate solutions also gives rise to an electric potential difference over each membrane.