| Antibiotics play an important role in clinical treatment and in the livestock industry.Among them, tetracycline (TC) is the second most widely used antibiotics in the world. Unlikemany other pollutants present in the environment, many of the antibiotics cannot be completelymetabolized or eliminated and thus have a direct biological action on microbes. Abuse andoveruse of antibiotics result in the occurrence and spread of antibiotic resistant bacteria (ARB)andresistance genes (ARGs). Anaerobic-anoxic-oxic (AAO) process is a traditional method ofwastewater treatment and widely used in municipal sewage treatment plants. There are severalresearches focused on the migrating and transforming of TC in sewage treatment plant, however,the removal mechanism of TC and the impact of TC on the microbial community remains unclear.The removal performance of TC and its influence on the microbial community was investigated inthis study. Response surface methodology (RSM) was used to optimize the operating conditions ofAAO process by acquiring the maximum response of removal efficiencies of TC. Then theadsorption characteristics of TC on activated sludge were studied. In order to investigate thechanges of the microbial diversityand TC resistance under TC pressure, illumina high-throughputsequencing in combination with real-time quantitative polymerase chain reaction (PCR) was usedto evaluate the relative abundance and diversity of microbial community and tetracyclineresistance genes (TRGs) under different oxygen conditions. In addition, the evolution of nitrifyingbacteria was measured to explore the influence of TC on the nitrification in AAO system.The results showed that hydraulic retention time (HRT) and sludge retention time (SRT)of AAO process both had a significant effect on the removal of TC. According to the RSMexperimental results, SRT, the initial TC concentration and their interactions affected the removalof TC obviously. The removal efficiency of TC was about99%under the optimal conditionscomprising an SRT of15.5days, an HRT of9.9h and an initial TC concentration of283.3μg/L,which was close to the predicted value. The adsorption kinetics of different initial TCconcentration on activated sludge was in accordance with pseudo secondary model andchemisorption was the most important factor to control the whole adsorption rate. Under differenttemperature, TC adsorption on activated sludge presented different features: at10℃and25℃,it was more close to the Langmuir adsorption model while at40℃, it fitted well with Henrymodel. It showed that TC adsorption on activated sludge was endothermic reaction.According to the illumina high-throughput sequencing analysis, TC could change thestructures of the microbial community and tended to increase its diversity regardless of differentoxygen conditions in AAO process. The copy numbers of tet(A), tet(B), tet(C), tet(E), tet(M), tet(S) and tet(O) in the AAO with TC addition reached a higher level than those without TC. Significantcorrelations were observed for tet(A), tet(B), tet(C), tet(E), tet(M), tet(S) and tet(O)(P <0.05)while the relationship between tet(X) and the other detected TRGs were weakest (p>0.05). Theconcentration of efflux pump genes tet(A, B, C, E) was much higher than that of ribosomalprotection protein genes and enzymatic modification genes in sludge. TC would promote thenitrification in AAO system instead of inhibiting it and under trace TC pressure, part of nitrifyingbacteria tended to become tetracycline resistant nitrifying bacteria which carried TRGs. |
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