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The performance, microbial enzymatic activities and the microbial community of sequencing batch reactors (SBRs) were evaluated under the single and combined nickel (Ni2+) at 20 mg/L and cadmium (Cd2+) at 10 mg/L. The single and combined Ni2+ and Cd2+ had no adverse impacts on the COD removal, whereas the NH 4 +-N removal efficiency declined sharply from about 99% to 34.42% and 42.67% under the single Ni2+ and combined Ni2+ and Cd2+. Compared with the absence of Ni2+ or Cd2+, the specific oxygen uptake rate (SOUR), ammonia-oxidizing rate (SAOR), nitrite-oxidizing rate (SNOR), nitrite-reducing rate (SNIRR) and nitrate-reducing rate (SNRR) declined by 24.09%, 56.63%, 51.50%, 58.01% and 52.09% under the combined Ni2+ and Cd2+, which were slower than the sum of those under single Ni2+ and Cd2+. The dehydrogenase, ammonia monooxygenase, nitrite oxidoreductase, nitrate reductase and nitrite reductase activities showed the similar varying trends to the SOUR, SAOR, SNOR, SNIRR and SNRR, suggesting that the combined Ni2+ and Cd2+ displayed antagonistic inhibition on the nitrogen removal rates and microbial enzyme activities. The combined Ni2+ and Cd2+ declined the microbial diversity and richness less than the sum of those under single Ni2+ and Cd2+. The relative abundance of Nitrosomonas , Nitrospira and identified denitrifying bacteria displayed some changes under single and combined Ni2+ and Cd2+. These findings would contribute to better understand the combined impacts of multiple heavy metals on biological wastewater treatment systems. Unlabelled Image • Combined Ni2+ and Cd2+ showed less inhibition on nitrogen removal than Ni2+ and Cd2+. • Combined Ni2+ and Cd2+ inhibited the nitrogen removal rate less than Ni2+ and Cd2+. • Combined Ni2+ and Cd2+ displayed antagonistic inhibition on the enzymatic activity. • Combined Ni2+ and Cd2+ showed less inhibition on microbial diversity and richness. [ABSTRACT FROM AUTHOR]