# Reverse Electrodialysis: Potential Reduction in Energy and Emissions of Desalination
## 1 Introduction
The electric current (electron flux) and the electric potential yielded by the RED pile power the external load connected to the end electrodes [12]. RED enables the harnessing of energy from abundant yet largely untapped sources, as industrial effluents, thus providing energy and emission savings from an otherwise waste stream, conforming with the waste-to-wealth concept. Several authors have explored the energy retrieval from desalination concentrate effluents [9; 13; 14; 15; 16; 17; 18], and secondary treated wastewater effluents [19; 20; 21; 22], which bring higher power densities than the seawater/river water pair widely tested in previous works [19; 23; 24; 25]. While latest research advances have boosted the maturity level of RED, moving from lab-scale units to up-scaled prototypes and pilot plants [11; 26], full-scale RED progress will require further techno-economic and environmental assessments and field demonstrations in industrially relevant environments that prove its practical viability. Demonstration projects are limited to the RED pilot plant commissioned next to the Ettore-Infersa saltworks in Trapani (Italy) using saturated brine from the adjacent saltworks and brackish water from a shoreline well, realizing 330 W, and the RED pilot plant at Afsluitdijk (The Netherlands) using seawater from the Wadden Sea and freshwater from Lake Issel with a nominal capacity of 50 kW. To date, fundamental-based studies and experimental investigations have examined the implementation of RED to recover energy from desalinationconcentrate effluents assessing separately endogenous--e.g., RO configuration, water recovery rate, specific energy consumption and capacity--and exogenous--e.g., seawater conditions and electricity mix--factors relevant to the real RO desalination process, overlooking global variations. While these studies have provided insights on how (i) working conditions--concentration [13; 14; 15; 16; 18; 27; 28; 29], flow rate [15; 16; 28] and temperature [13; 16; 29] of feed solutions--(ii) configuration--i.e., RED acting as pre- and/or post-treatment to RO desalination [13; 15; 27]--and (iii) even the combination with other technologies to increase concentrate's salinity--e.g., membrane distillation [29], membrane capacitive deionization [18] and solar evaporation [14]--affect RED performance and, therefore, desalination SEC decrease and concentrate effluent dilution, consideration of all these factors at once has not yet been fully investigated and could pinpoint the appropriateness for full-scale RED implementation from the technical and environmental side. This work has been one of the first attempts to thoroughly examine the potential energy savings and environmental benefits for desalination stemmed from RED's energy recovered from brine effluents using a comprehensive modelling tool of the RED system from our research group. Our study delves into the relative influence of site-specific conditions and features relevant to real SWRO desalination plants to size the potential energy and emissions savings full-scale RED implementation could offer to the energy- and emission-intensive desalination industry. This work aims to ascertain decisive factors on the RED system's performance which, alongside particular features of desalination plants, bounds the ability of RED integration to cut down energy and emissions to desalination. The insights gained from our assessment may be useful in the prospective decisions to fully exploit the synergies of RED deployment in energy-intensive processes. Figure 1: Working principle of reverse electrodialysis (RED).