The Pollution Of Antibiotic Resistance Genes In Wastewater Treatment Systems

Antibiotics have been widely used in human and veterinary medicines to treat and control many bacterial infections, but misuse or overuse of antibiotics contributes to the emergence and spread of antibiotic resistance genes (ARGs) in the environment. ARGs are emerging environmental contaminants and pose a threat to public health. In urban regions, large amounts of antibiotics, as well as ARGs, are excreted and reach wastewater treatment systems. In addition, other factors in wastewater treatment systems, such as gene cassettes, integrons, and heavy metals, which play important roles in the exchange of resistance as well as contribute to resistance retention and dissemination, are also detected in wastewaters. So wastewater treatment systems are regarded as important reservoirs for various ARGs encoding resistanceIn this study, four tetracycline resistance genes (tetM, tetO, tetQ and tetW) and two sulfonamide resistance genes (sulⅠ and sulⅡ) were evaluated in4municipal wastewater and8rural domestic sewage treatment systems with different wastewater-receiving capacity and treatment processes in Zhejiang areas using polymerase chain reaction (PCR) and real-time quantitative polymerase chain reaction (qPCR).Overall, high concentrations of ARGs and intll gene were found in the influent samples from different wastewater treatment systems in Zhejiang areas. In the influents, high concentrations of ARGs and intll gene were found, and the total tet concentrations (copies per milliliter) were higher (～1order of magnitude) in the influents of municipal wastewater treatment systems than in those of rural domestic sewage treatment systems. As for the sul genes and intll gene, significant differences were not found between rural domestic sewage and municipal wastewater treatment systems. In addition, significant correlations (R2=0.712, P<0.05for tetQ; R2=0.394, P<0.05for tetO) between the absolute concentrations and wastewater-receiving capacity were observed. Statistical analysis revealed a positive correlation (R2=0.756, P<0.05) between the concentrations of sull and intll, whereas the gene numbers of tetM and sull were strongly correlated with16S rDNA (R2=0.552, P<0.05for tetM;R2=0.448, P<0.05for sulⅠ).Significant reductions in ARGs were observed in municipal wastewater treatment systems, and the removal of tet genes, sul genes and intll gene reached1.52～2.54, 0.93～1.95,1.34-1.86orders of magnitude, respectively. Compared with the ARGs removal in municipal wastewater systems, the removal efficiency of ARGs in rural domestic sewage treatment systems was lower and varied with the treatment process. In site B and E, a multi-soil-layering system was used and the removal of ARGs was the least. A multi-stage anaerobic biological filter process was used in sites D, H, and G, and the decrease in ARGs was almost the same among systems D, H, and G (P>0.05). In site A, C, and F, with different wastewater-receiving capacities and treatment processes, similar and better removal efficiencies (P>0.05) were observed. It is interesting to note that the presence of a constructed wetland with plants is a common feature of system A, C, and F.The study of the performance of ARGs in different treatment stage of WWTPs showed that the primary treatment processes and anaerobic/oxic processes in the second treatment stage have little effect on the removal of ARGs, and the reduction of ARGs is likely connected to sludge sedimentation, implying that the removal of ARGs in WWTPs was partly rely on removal of bacteria and biomass. And high concentrations of ARGs and intll gene were found in the biosolids samples from municipal wastewater treatment systems.In advanced treatment process, constructed wetland proved to have significant contribution to the removals of ARGs (1.3～2.1orders of magnitude for tet genes,1.5orders of magnitude for sull gene), and intI1gene (about1.7orders of magnitude); biological aerated filter followed (1.0～1.21orders of magnitude for tet genes, about0.65orders of magnitude for sul and intll genes). Only very little change in concentrations of ARGs was observed between pre-and post-UV disinfected effluent samples (0.5-0.7orders of magnitude for tet genes, about0.3orders of magnitude for sul and intll genes). Statistical t-test between log values of ARGs removed in constructed wetland and UV disinfection effluent process show a significant difference (p<0.05). Further researches in the constructed wetland found the subsystem III also played an effective and better role in the removal of ARGs. The relationship between concentrations of ARGs in the wastewater samples and corresponding sediment samples were examined, and statistically significant correlations were observed (tetW:R2=0.881, P<0.05; tetM:R2=0.791, P<0.05; sull:R2=0.879, P<0.05).Compare the removal values of ARGs and intll in WWTPs and constructed wetland, the constructed wetland has a good capacity for removing ARGs close to the ones obtained in biological treatment processes, and a better reduction of relative gene abundance was found constructed wetland. These results provide insights into the occurrence and removal of ARGs in wastewater treatment systems in both rural and urban areas in China.