Influence of ZnO-NPs and TC accumulation on nitrogen removal in bench-scale sequencing batch reactors

Abstract

This study investigated the individual and synergy effects of tetracycline (TC) and zinc oxide nanoparticles (ZnO-NPs) accumulation on both short-term and long-term nitrogen removal processes in the bench-scale sequencing batch reactors (SBR). During short-term (10 hour) analysis, the removal rates of NH4+-N in 0.1 mg·L−1 ZnO-NPs (98.3%), 5 mg·L−1 TC (97.5%), and the co-addition reactor (96.9%) were significantly higher (P < 0.05) than the control reactor (94.4%) at minimum dosages. Ammonification processes were inhibited by the increasing dosage of ZnO-NPs (≥ 5 mg·L−1) and TC (≥ 15 mg·L−1) due to the cumulative toxic interference. Ultraviolet (UV) radiation with the co-addition of ZnO-NPs and TC promoted NH4+-N removal, and nitrification processes might be enhanced by UV exposure through the produced hydroxyl radicals (·OH) from ZnO-NPs. In the long-term operation, the effect of 0.1 mg·L−1 ZnO-NPs on nitrogen removal was negligible even over 15 days. Although NH4+-N removal rates were initially decreased, they recovered with the prolongation of dosages and operating time due to self-adjustment and the existence of extracellular polymeric substances (EPS) in activated sludge. Fourier transform infrared (FTIR) analysis revealed that the absence of —CH3 may explain the inhibition of NH4+-N removal during TC accumulation. Research results can be served as indications for the understanding of the synergistic effects of ZnO-NPs and TC on the performance of biological treatments used in wastewater systems.