High Na⁺ levels are detrimental for most crops. Selective membranes provide the possibility for the selective removal of Na⁺ while preserving beneficial ion species. The challenge is to separate two ion species of the same charge. This study evaluates the implementation of an electrodialysis (ED) system equipped with a supported liquid membrane (SLM) and a commercially available monovalent cation-selective membrane (CIMS) in the treatment of greenhouse drainage water. The SLM shows a (minimum) K⁺ over Na⁺, Ca²⁺ and Mg²⁺ permselectivity of 9, 15 and 30, respectively. Whereas the CIMS holds a high K⁺ over Ca²⁺ and Mg²⁺ permselectivity of 10 and 16, respectively, the K⁺ over Na⁺ permselectivity is just 1.3. With the experimentally obtained membrane characteristics at hand, the treatment of drainage water was simulated by a two-steps process with the two membrane types operating in series. Using real-life operational parameters, analysis revealed the optimal configuration and the ability to recover 96% of the K⁺ and approximately 80% of the water, Ca²⁺ and Mg²⁺. Summarized, this study not only shows the efficient separation of two ion species of the same valance but also the implementation of this technology in a real-life application.