Single and combined impacts of nickel and cadmium on the performance, microbial community and enzymatic activity of sequencing batch reactors

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 NH₄⁺-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.