Fate Of Antibiotics And Antibiotic Resistant Genes During Low-temperature Thermal Hydrolysis-anaerobic Sludge Digestion Process

In recent years,the pollution of antibiotics and antibiotic resistance genes（ARGs）has attracted more and more attention.The sludge of wastewater treatment plant contains a large number of antibiotics and ARGs.The most commonly used method of sludge treatment is anaerobic digestion,in which sludge pretreatments can improve the digestion performance.However,there are still very limited studies on the fate of antibiotics and ARGs in the process of pretreatment-anaerobic digestion.In this study,variation of antibiotics and ARGs during sludge pretreatment and the low-temperature thermal hydrolysis-anaerobic digestion processe were investigated.Furthermore,the distribution of antibiotics inside and outside the cell was also investigated in raw sludge,pretreated sludge and digested sludge.The antibiotics in the sludge mixed liquor were mainly adsorbed on the sludge cells.The migration of antibiotics in the sludge was promoted from the adsorption state to the dissolved state under 70℃-120℃of pretreatments.The solubilization multiples of norfloxacin,ciprofloxacin,ofloxacin and azithromycin could reach 7.4,27.2 4.5 and 54.4 times at 120℃-30min pretreatment condition,respectively.Antibiotics were found in the sludge cells,with 0.46-1.99 C_IPS/C_EPSPS ratios for quinolones and 8.91-15.97 C_IPS/C_EPSPS ratios for macrolides.The total amount of quinolones and azithromycin was increased at 120℃pretreatment conditions,due to the release of antibiotics inside the sludge cell.DNA structures can be damaged under thermal hydrolysis leading to the reduction of ARGs.Under the 120℃pretreatment conditions,the concentration of total ARGs and class one integrons could be reduced by 2-3 orders of magnitude,while tetB,tetC,tetQ and tetS genes could not be detected.Anaerobic digestion could promote the migration of antibiotics from adsorption state to the dissolved state while having high removal rates of norfloxacin,ciprofloxacin,chlortetracycline,azithromycin and roxithromycin,with a high removal efficiency of 49.51%-73.05%.The degradation of norfloxacin,ofloxacin and tetracycline could be promoted under 120℃pretreatment conditions.After pretreatments,the total absolute concentration of ARGs significantly reduced by99.30%-99.93%,but the concentrations of tetB,tetM and tetQ genes were rebounded during anaerobic digestion.Partial of target ARGs had significant correlation with intⅠ1,and 16S rRNA genes,respectively,while tetB,tetO and tetQ genes had significant correlation with chlortetracycline.The results showed that horizontal gene transfer,biomass and antibiotics were important factors on the spread of ARGs.