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.
Sun et al. (Tue,) studied this question.