The partial nitrification (PN) performance under high ammonium concentrations was evaluated in an airlift reactor (ALR). The ALR was operated for 253 days with stepwise elevation of ammonium concentration to 1400 mg/L corresponding nitrogen loading rate of 2.1 kg/m(3)/d. The ammonium removal rate was finally developed to 2.0 kg/m(3)/d with average removal efficiency above 91% and nitrite accumulation percentage of 80%. Results showed that the combined effect of limited DO, high bicarbonate, pH and free ammonia (FA) contributed to the stable nitrite accumulation substantially. The biomass in the ALR was improved with the inception of granulation. Precipitates on biomass surface was unexpectedly experienced which might improve the settleability of PN biomass. Organic functional groups attached to the PN biomass suggested the possible absorbability to different types of pollutant. The results provided important evidence for the possibility of applying an ALR to treat high strength ammonium wastewater. (C) 2015 Elsevier Ltd. All rights reserved.