Comparative Study of Ammonium Nitrogen Removal Characteristics of Different Types of Resins in Combined Sewer Overflow Containing Hardness Ions

This study systematically evaluated the performance of different cation exchange resins for ammonium nitrogen (NH4+–N) removal from combined sewer overflow (CSO) in the presence of competing hardness ions. Strong acid resins with sulfonic acid functional groups exhibited markedly higher NH4+–N adsorption capacities and affinity compared to weak acid resins and chelating resins. Adsorption kinetics followed a pseudo-first-order model (R2 > 0.99), reflecting rapid NH4+–N uptake at accessible sites. After ten adsorption-regeneration cycles, NH4+–N adsorption capacities of 001 × 7, D061, and D072 resins decreased by approximately 0.8 mg/g, demonstrating good reusability. The brittleness caused by the high cross-linking degree of macroporous resins led to cracking after frequent ion exchange, whereas the 001 × 7 resin maintained a stable morphology and moderate swelling rate, enhancing its reusability. Breakthrough curve analysis confirmed that competing hardness ions accelerated the NH4+–N breakthrough in fixed-bed columns. Nevertheless, the strong acid gel-type resin 001 × 7 maintained a higher NH4+–N adsorption capacity and longer breakthrough time. In dynamic adsorption-regeneration experiments, up-concentration factors (UCF) of resin 001 × 7, D061, and D072 were 17.1 ± 0.4, 13.8 ± 0.3, and 14.3 ± 0.3, respectively. Overall, resin 001 × 7 demonstrated superior comprehensive performance for NH4+–N removal under hardness ions’ interference, providing a robust scientific basis for the design of efficient and sustainable NH4+–N removal systems for CSO treatment.