The impairment of joint tetracycline and copper oxide nanoparticle exposure on activated sludge

• Both individual and combined exposures of TET and CuO-NPs deteriorated the characterizations of the sludge. • CuO-NPs would decrease the activity of denitrification-associated enzymes. • Although TET individually did not decrease the nitrogen removal, TET could exacerbate CuO-NPs’ effects on nitrogen removal. • Both TET and CuO-NPs decreased the abundance of denitrification genera (e.g., Dechloromonas, Thiothrix , and Hyphomicrobium ). The increasing usage of two emerging contaminants (i.e., tetracycline (TET) and copper oxide nanoparticles (CuO-NPs)) has raised wide concerns about their potential impacts on the efficiency of wastewater treatment. However, the joint effects of TET and CuO-NPs on activated sludge processes have rarely been documented. This study investigated the sludge characteristics and bioactivities under both individual and joint exposure to TET (1 mg/L) and CuO-NPs (2 mg/L) for 60 days. The results demonstrated that both individual and joint exposures of TET and CuO-NPs deteriorated the flocculation ability and dewatering of sludge by inducing non-filamentous bacteria. The joint exposure showed higher effects on sludge when compared with individual exposure. Sludge characterization analysis attributed this deterioration partly to the increased extracellular polymeric substance (EPS) secretion (especially loose-bound EPS). In addition, exposure to CuO-NPs would result in an inhibition of total nitrogen removal due to impairment of the denitrification process (i.e., nitrate reductase, nitrous reductase, and electron-transfer associated enzyme), but not for TET. The inhibition of nitrogen removal could be exacerbated when simultaneously exposed to CuO-NPs and TET. 16S rRNA analysis further revealed that the long-term joint exposure to TET and CuO-NPs resulted in a significant decrease in microbial community richness, diversity, and associated functional bacteria abundance. The results of this study suggest that joint exposure to TET and CuO-NPs results in the deterioration of sludge function efficiency.