Abstract: Effluents from urban wastewater treatment plants (UWWTPs) discharged into water bodies such as the sea or ocean, offer a potential source of renewable energy through the salinity gradient (SGE) between seawater and treated water. The European project Life-3E: Environment-Energy-Economy aims to demonstrate an innovative process integrating renewable energy production with water reclamation. Using reverse electrodialysis (RED) technology, SGE can power tertiary wastewater treatment processes in coastal UWWTPs, offsetting energy costs associated with water regeneration and reuse. This study pioneers a pilot-scale RED system with a 20.125 m2 membrane area at a coastal UWWTP in Comillas, Spain. The initial tests with synthetic solutions in the up-scaled RED module have reached a peak power density of 1.39 W/m2. Under real environmental conditions, using natural seawater and treated water at ambient temperatures (289 ± 0.5 K), the system achieved a peak power density of 0.95 W/m2, outperforming previous setups in stability and efficiency. The results show competitive energy metrics, with an energy efficiency of 1.9 W/m2·m³LC and up to 38.2 Wh/m³LC generated. The treated water, with an inlet conductivity to the RED stack of <1 mS/cm, exits the pilot with a conductivity of around 4 mS/cm (measured under a load of 2A and a flow rate of 500 L/h), maintaining the quality standards for urban reuse. This study demonstrates the effective integration of RED technology into water reclamation stages, creating a self-sustained energy loop and enhancing the efficiency in water management. By harnessing blue energy and supporting sustainable water reuse, this research contributes to the global shift toward a circular water economy and critical sustainability goals.

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