# Practical Methodology for a Three-Dimensional-Printed Hybrid Desalination System
## 1 Introduction  
Desalination is an ancient and extensively studied process that separates or extracts salt from water, reducing salt content or producing fresh water [1, 2]. Modern desalination technologies encompass both thermal and membrane processes, with electrodialysis (ED) being a notable example [3, 4, 5]. While essential for meeting the increasing demand for potable water, desalination processes have a significant environmental impact. Globally, approximately 142 million m\({}^{3}\) of brine is produced daily, posing a major environmental challenge [1]. Additionally, desalination is energy-intensive, with the global desalination capacity requiring over 850 million tons of oil annually, leading to the emission of 76 million tons of CO\({}_{2}\), contributing to climate change [6]. In recent years, there has been an increasing trend toward integrating renewable energy technologies into desalination processes [3, 7]. This integration is driven by the desire to reduce greenhouse gas emissions, minimizeresource use, and mitigate excessive energy consumption, representing more than 50% of desalination process prices [3]. As the costs of these technologies continue to decrease, hybrid desalination systems are expected to become more economically viable, offering more sustainable and cost-effective long-term solutions. One of the renewable energy sources that can synergize with desalination systems is osmotic energy, also known as salinity gradient energy (SGE) or blue energy. SGE is an innovative renewable energy technology that exploits the energy potential generated when two bodies of water with varying salt concentrations merge [8; 9]. The concept was first introduced by [PERSON] in 1954 [10], who showed that salinity differences can be converted into electrical energy using ion exchange membranes. Since then, SGE has garnered significant research interest, with key technologies such as pressure-retarded osmosis, reverse electrodialysis (RED), and capacitive mixing emerging as the most promising approaches for efficiently capturing this energy source [11; 12; 13; 14]. These technologies offer a sustainable and untapped avenue for renewable energy generation, particularly in coastal regions with abundant fresh and saltwater bodies. RED and ED reactors are electrochemical flow reactors consisting of several internal compartments separated by alternating anion exchange membranes (AEMs) and cation exchange membranes (CEMs). At the ends of the cell, they feature an electrode system that includes an electrode rinse solution (ERS). However, while the RED reactor is a galvanic system that generates energy by harnessing the spontaneous movement of ions down their concentration gradient, the ED system requires an external energy source to drive the desalination process by forcing ions to migrate against the chemical potential. Figure 1a illustrates a RED unit cell, where a high-concentration saline solution (H) and a low-concentration solution (L) are separated by IEMs, creating a chemical potential difference. This gradient drives the diffusion of anions (Cl\({}^{-}\)) through AEMs and cations (Na\({}^{+}\)) through CEMs, generating an ionic current. Electrodes at the ends of the cell convert this ionic current into electrical energy through redox reactions. Figure 1b shows an ED reactor. In this case, a saline feed solution (F) flows through the compartments, and an external power source polarizes the electrodes. The electric field causes anions and cations to migrate toward the anode and cathode, respectively. The selective membranes reduce the salt concentration in one compartment, producing desalinated water (D), while the adjacent compartment accumulates the removed salts, resulting in a brine solution (B). Several hybrid desalination systems combining RED and ED have been developed, with both external (RED-ED) and internal configurations (RED/ED). Externally integrated systems, like those proposed by [PERSON] et al. (2017) and [PERSON] et al.