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Synthetic zeolites with pretreated fly ash as a raw material were used to remove ammonium from wastewater using a hydrothermal method in this study. Two pretreatment methods of fly ash were used to compare the ammonium removal of zeolites: water-washing and pickling. In addition, the effects of several factors including the time, temperature, pH, adsorbent dosage, coexisting ions and initial concentration were investigated to gain insight into the adsorption rate, behavior and mechanism of synthetic zeolites for ammonium. N2 adsorption/desorption isotherms showed that the synthetic zeolite was a mesoporous material with a higher specific area (13.05 m2/g) than the values for raw fly ash (0.34 m2/g). The X-ray diffraction result suggested that the synthetic products mainly belonged to zeolite P and Y. The adsorption kinetic data fitted well with a pseudo-second-order model. The maximum ammonium adsorption capacity was 32.16 mg/g. The synthetic zeolites were also applied to adsorb the ammonium from real swine wastewater. The ammonium removal efficiencies in raw swine wastewater and effluent from the biochemical unit were 64.34% and 79.61%, respectively, which indicated that the synthetic zeolites have a good application for real ammonium wastewater.