Integration of a continuous electrodeionisation process with a desalination plant for UPW production

In the context of the energy transition, the growing interest in green hydrogen production raises new challenges, particularly the limited availability of freshwater, which is a key resource for electrolysis. Industrial-scale electrolysers require ultra-pure water with electrical resistivity between 0.5 and 18 MO·cm [1]. Given its abundance, seawater can be desalinated and post-treated to produce low-conductivity water without exacerbating existing water scarcity issues. In recent decades, continuous electrodeionisation (EDI) has emerged as a water-polishing technology for the production of ultrapure water suitable for electrolysers [2]. EDI combines the principles of electrodialysis and ion exchange, eliminating the need for chemical regeneration. In this work, the integration of an EDI process with a conventional desalination plant was studied, with particular focus on investigating the optimal operating conditions for minimising the energy consumption of the EDI process while maintaining high water recoveries. For the experimental campaign, a pilot-scale EDI stack (MK-3MiniHT) was employed. The stack comprises 10 repetitive units (cell pairs), each composed of cation and anion exchange membranes (CEMs and AEMs) separated by a mixed bed of strong Ion exchange resins (IERs). Electrical power is supplied to the stack via a DC power supply, enabling ion transport across the membranes toward the oppositely charged electrodes, thus generating a highly deionised (dilute) stream and a corresponding concentrate stream. The experimental campaign was designed to explore the role of current density in determining the main feature of the produced ultrapure water and energy consumption, while also analysing its influence on current efficiency. In particular, the feed conductivity was 12 µS cm–1 and the flowrates of the dilute, concentrate, and electrode rinse solution (ERS) were set at 600, 50 and 70 Lh–1, respectively. The applied current density ranged between 12 and 57 A m–2. Observing the resulting trends of outlet conductivities, two different operating regimes can be recognised. Working at low current densities, the results show an accumulation of ions in the system, suggesting that the stack is operating in an adsorption regime (namely, enhanced transfer regime). Conversely, increasing the current density, a critical current can be identified, above which the stack works in an electro-regeneration regime. This study identifies an optimal combination of flowrates and current density that minimises the economic impact of the EDI step when integrated with the upstream desalination plant, while ensuring the water quality required for electrolyser operation.