Zinc oxide (ZnO) nanoparticles (NPs) have been reported to induce adverse effects on organisms. The impacts of ZnO NPs on nitrification and the nitrobacteria community in activated sludge were investigated in a simulated SBR. It revealed that ZnO NPs at low concentrations (5 and 10 mg L À1 ) slightly inhibited nitrification. At these concentrations the activity of ammonia monooxygenase (AMO) and nitrite oxidoreductase (NOR) as well as the cell membrane integrity of Nitrosomonas europaea were almost unaffected. Concentrations of 20 mg L À1 and 50 mg L À1 ZnO NPs had significantly adverse effects on the activity of AMO and NOR and on the transformation of NH 4 + -N to NO 2 À -N and NO 2 À -N to NO 3 À -N. Analysis by denaturing gradient gel electrophoresis (DGGE) revealed that higher concentrations of ZnONPs significantly inhibited the growth of the typical ammonia-oxidizing bacteria (AOB) that were mainly responsible for oxidation of ammonia to nitrate. Moderate concentrations of ZnO NPs could accelerate the growth of some types of denitrifying bacteria and promote the growth of some pathogenic bacteria.Moderate and high concentrations of ZnO NPs could obviously destroy the integrity of the cell membrane of Nitrosomonas europaea. These findings meaningfully assessed the adverse effects of ZnO NPs on activated sludge in wastewater treatment.

10 and 20 mg L−1 ZnO NPs damaged the integrity of cell membranes and the microbial community and affected phosphorus removal.