The inhibitory effects of zinc oxide nanoparticles (ZnO NPs) and impacts of N-acyl-homoserine lactone (AHL)-based quorum sensing (QS) on biological nitrogen removal (BNR) performance have been well-investigated. However, the effects of ammonia nitrogen (NH 4 + -N) concentrations on NP toxicity and AHL regulation have seldom been addressed yet. This study consulted on the impacts of ZnO NPs on BNR systems when high NH 4 + -N concentration was available. The synergistic toxic effects of high-strength NH 4 + -N (200 mg/L) and ZnO NPs resulted in decreased ammonia oxidation rates and dropped the nitrogen removal efficiencies by 17.5% ± 0.2%. The increased extracellular polymeric substances (EPS) production was observed in response to the high NH 4 + -N and ZnO NP stress, which indicated the defense mechanism against the toxic effects in the BNR systems was stimulated. Furthermore, the regulatory effects of exogenous N-decanoyl-homoserine lactone (C 10 -HSL)-mediated QS system on NP-stressed BNR systems were revealed to improve the BNR performance under different NH 4 + -N concentrations. The C 10 -HSL regulated the intracellular reactive oxygen species levels, denitrification functional enzyme activities, and antioxidant enzyme activities, respectively. This probably synergistically enhanced the defense mechanism against NP toxicity. However, compared to the low NH 4 + -N concentration of 60 mg/L, the efficacy of C 10 -HSL was inhibited at high NH 4 + -N levels of 200 mg/L. The findings provided the significant application potential of QS system for BNR when facing toxic compound shock threats.