The effect of Ni(2+) concentrations on ammonium oxidation was studied in a batch and partial bionitrification reactor (PBNR). The nitrification rates up to the concentration of 0.1 mg Ni(2+)/l were close to those without Ni(2+). After testing the operational conditions in the PNBR, the highest NO(2)-N/NO( x )-N ratio was achieved at the DO concentrations of 2.0 mg/l and pH 9.00. The PNBR was operated at steady state (NH(4)-N loading rate and NO(2)-N/NO( x )-N ratio were 405 g m(-3) day(-1) and 0.74, respectively) before exposure to Ni(2+). The removal efficiency of NH(4)-N and NO(2)-N/NO( x )-N ratio in the effluent waters was increased by adding low concentrations of heavy metals to the PBNR. The average number of aerobic mesophilic bacteria at the biofilm surface and in the water in the void volume of PNBR were 1.0 × 10(4) CFU/g and 1.4 × 10(5) CFU/ml, respectively.