Effects of different oxygen conditions on pollutants removal and the abundances of tetracycline resistance genes in activated sludge systems.

The individual and combined effects of tetracycline (TC) and divalent copper (Cu2+) on the performance of activated sludge systems and the abundances of tetracycline resistance genes (TRGs) in activated sludge, under both aerobic and anaerobic conditions, were studied. Activated sludge systems received TC (0.2 mg L-1) and Cu2+ (5 mg L-1) separately or jointly under either aerobic or anaerobic conditions. The addition of TC did not affect the performance of activated sludge systems and the addition of Cu2+ and mixed TC/Cu2+ inhibited biological phosphorus removal. The TC removal efficiencies in systems under aerobic and anaerobic conditions were 98.4%-99.7% and 96.8%-99.9%, respectively, and Cu2+ promoted TC removal in activated sludge systems. The TC degradation product was 4-epitetracycline (ETC) in activated sludge systems under both aerobic and anaerobic conditions. The total relative abundances of TRGs (tetA, tetC, tetE, tetM, tetO, tetW, tetX and tetB(P)) in activated sludge showed opposite development trends under the two oxygen conditions and aerobic condition was beneficial to the attenuation of high-risk TRGs. The results of this study might improve evaluation of the combined effects of antibiotics and heavy metals on wastewater biological treatment systems.