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This study investigated the acute nickel toxicity on nitrification of low ammonia synthetic wastewater at 10,23,and 35℃.The nickel inhibition half-velocity constants (K1,Ni) for ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) based on Ni/MLSS ratio at 10,23,and 35℃ were 5.4 and 5.6 mg Ni/g MLSS,4.6 and 3.5 mg Ni/g MLSS,and 9.1 and 2.7 mg Ni/g MLSS,respectively.In addition,chronic toxicity of nickel to nitrification of low ammonia synthetic wastewater was investigated at 10℃ in two sequencing batch reactors (SBRs).Long-term SBRs operation and short-term batch tests were comparable with respect to the extent of inhibition and corresponding Ni/MLSS ratio.The μmax,b,and Ko of AOB were 0.16 day-1,0.098 day-1 and 2.08 mg O2/L after long-term acclimatization to nickel of 1 mg/L at 10℃,high dissolved oxygen (DO) (7 mg/L) and long solids retention time (SRT) of 63-70 days.Acute nickel toxicity of nitrifying bacteria was completely reversible.