Selective Na+/K+ separation from greenhouse wastewater concentrate streams using Donnan dialysis for sustainable water and nutrient recovery

“Greenhouse horticulture plays a crucial role in addressing global food demand; however, the discharge of nutrient-rich wastewater presents challenges for environmental protection and resource recovery. Effective sodium (Na⁺) removal is critical for reusing these streams in irrigation, as excess Na⁺ inhibits plant growth. While electrodialysis (ED) has shown potential, it currently lacks sufficient Na⁺/potassium (K⁺) selectivity, limiting K+ recovery, a valuable nutrient that accumulates in the concentrate stream. This study investigates Donnan dialysis (DD) as a selective and scalable approach for Na⁺/K⁺ separation from greenhouse wastewater concentrate. Lab-scale experiments using standard cation exchange membranes (CEM) achieved 77 % Na⁺ separation, reducing concentrations to 2–5 mmol/L, levels suitable for partial irrigation reuse. Higher draw concentrations improved Na⁺ removal but with diminishing returns beyond 200 mmol/L. A predictive model validated the experimental results within 10 % and identified key parameters influencing performance and scalability. Competing divalent ions (e.g., calcium (Ca²⁺)) reduced Na⁺ separation efficiency by up to 9 %, indicating the need for their exclusion or selective rejection. The use of monovalent-selective CEMs (MS CEMs) improved Na⁺/K⁺ selectivity threefold in the presence of divalent ions, though Na⁺ flux halved. Up-scaled DD stack runs demonstrated process scalability, achieving effective separation with residence times below 7 s. Real greenhouse wastewater trials confirmed Na⁺ removal to 4 mmol/L (CEM) and 2.95 mmol/L (MS CEM), although both remain above the stringent threshold of <0.5 mmol/L for high-quality irrigation. Modeling simulations further demonstrated that a 2-stage DD process could achieve compliance. This study demonstrates a novel, selective, and scalable DD approach for Na⁺/K⁺ separation, outperforming current methods and supporting minimal liquid discharge (MLD) systems for sustainable greenhouse horticulture." (Citation: Guleria, T., Rijnaarts, T., van den Broeke, J., et. El. - Selective Na+/K+ separation from greenhouse wastewater concentrate streams using Donnan dialysis for sustainable water and nutrient recovery – Water Research 285(2025)art. No. 124029 – DOI: 10.1016/j.watres.2025.124029) © 2025 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.