<p>Ion exchange resins (IERs) are widely used in water treatment to remove hardness ions such as calcium (Ca²⁺) and magnesium (Mg²⁺), but their long-term performance is constrained by fouling, inefficient regeneration, and high chemical consumption. This study systematically optimised brine-based regeneration by evaluating four key operational parameters: pre-treatment mode (swollen vs dried), resin-to-brine ratio (1:1–1:4), brining temperature (25–60&#xa0;°C), and NaCl concentration (1%, 5%, 10%). Dried resin exhibited approximately 14% shrinkage compared to the swollen state and achieved superior hardness removal due to reduced pore size and shortened diffusion pathways. SEM analysis confirmed structural damage and microbial fouling, underscoring the importance of pre-treatment. The optimal condition, dried resin regenerated with 5% NaCl at a 1:1 resin:brine ratio and 55&#xa0;°C, yielded the highest Ca²⁺ and Mg²⁺ removal, as verified by atomic absorption spectrometry and oxidation–reduction potential analysis. This optimised strategy enhances regeneration efficiency, lowers chemical demand, minimises brine waste, and extends resin service life. The findings provide a cost-effective and sustainable approach for high-purity water production, with strong relevance to industries such as pharmaceuticals, food and beverage, and power generation.</p> Graphical Abstract <p></p>