The Behavior Of Antibiotic Resistance Genes In The Process Of Anaerobic Digestion Of Sludge

In recent years, the study of antibiotic resistance genes in water environment is relatively more systematic and comprehensive, while the study in sewage treatment plant is underway. The residual sludge contains high concentrations of antibiotic resistance genes, but few researches are focus on effective treatment and disposal of sludge which in order to reduce its environmental risk. There are studies which have shown that anaerobic digestion can effectively reduce the resistance genes in sludge. Therefore, the residual sludge of municipal wastewater treatment plant is taken as the main object of the study, and sequencing batch anaerobic digestion is the main experimental method. We tend to observe the behaviors of antibiotic resistance genes under different conditions(Temperature, initial pH) and the correlations between them.In the process, liquid chromatography-electrospray tandem mass spectrometry (LC-MS/MS) was used to detect and analyze the concentrations of 8 kinds of antibiotics in sludge samples. The abundances of tetracycline resistance genes, sulfonamide resistance genes and class 1 integron (intIl) were quantified by real-time polymerase chain reaction.Operating temperatures of anaerobic digestion were 15℃,30℃,36℃,50℃ and 60℃. The results showed that the degradation rate of antibiotics increased as the temperature increasing. The average removal rates of total antibiotics under 15℃, mesophilic digestion(30℃,36℃) and thermophilic digestion(50℃,60℃) were 45%, 59% and 78%, respectively. Besides, the removal efficiency of antibiotic resistance genes (ARGs) was significantly affected by temperature. The reductions of total tet genes under 15℃, mesophilic digestion and thermophilic digestion were 0.52 log, 0.90 log and 1.50 log, while for sul genes the reductions were 0.56 log,0.78 log and 1.31 log, respectively. However, a recovery in the quantities of six of these genes (tetM, tetO, tetQ, tetX, sulⅠ, sulⅡ) were observed on the 50th day of the digestion which were 0.49 log,0.84 log,0.54 log,0.33 log,0.18 log and 0.07 log, respectively.Anaerobic digesters were operated at different initial pH(3,5,7,9,11). The results showed that initial pH had little influence on the removal of antibiotics and ARGs. Some antibiotics have been degradated, and the average removal rate of total antibiotics in all digesters was 42%. The reductions of tet genes in the control group and the digesters operating at different initial pH(3,5,7,9,11) were 0.65 log,0.96 log,0.75 log,0.62 log,0.86 log and 0.98 log, respectively, among which more genes reduced in digesters operating at initial pH=3 and initial pH=11. While for sul genes, the quantities were not efficiently reduced but increased about 0.18 log.Moreover, tetQ was the only gene detected which has negative correlation with nutrient elements, while some genes were found to have positive correlations with them. SCOD, NH3-N, TN and TP were found to have no correlations with tetM, tetX and sul Ⅱ, while all significantly correlated with tetA, tetG, tetL and tetW. intI1 was found to have significant correlations with NH3-N, TN and TP (R2=0.495,0.459, 0.587, P<0.05).