In this study the influence of highly concentrated reagents (NaCl, 22.36%; HCl, 20%; NaOH, 32%) on the mineralogy (e.g. structure and mineral chemistry) and NH ₄ ⁺-exchange capacity of a natural zeolite (clinoptilolite) is investigated. Treatment was executed in packed bed columns and accompanied by liquid and solid analytics (ICP-MS, XRD, EPMA, BET, ion exchange). Ammonium uptake was tested in batch experiments with model solutions using different concentrations of NH ₄Cl and real effluents from mechanical sludge dewatering (sludge liquor). It turned out that the Si/Al-ratio of the zeolite minerals measured by EPMA changes to some extent because of dissolution of Si and Al during treatment. Acid treatment leads to a remarkable increase in the specific surface (from 16 to 77 m ² g ^-1 after treatment). NH ₄ ⁺-exchange isotherms revealed a significant decrease in equilibrium loadings with HCl-treatment and an increase with NaOH-treatment (+25%) at concentrations > 1300 mg NH ₄ ⁺ L ⁻¹ compared to untreated zeolite. At lower concentrations (<1300 mg NH ₄ ⁺ L ⁻¹) treatment with NaCl is best. Basic treatment leads to the formation of an altered outer rim on zeolite particles enriched in Na and depleted in K. Compared to model solutions, ammonium uptake from sludge liquor (712 mg NH ₄ ⁺ L ⁻¹) was significantly lower for NaCl treated zeolite but higher for HCl and NaOH treated samples, probably as a result of the complex composition of real effluents leading to potential cross-interactions that influence NH ₄ ⁺ ion exchange on treated zeolites.