Removal Efficiency And Mechanisms Of Antibiotic Resistance Genes In Secondary Effluent By Different Oxidants Combined With Powdered Activated Carbon And Ultrafiltration

Antibiotic resistance genes?ARGs?are emerging environmental pollutants,which have been detected in large quantities in the secondary effluent of Wastewater treatment plants,posing a serious threat to the public health and ecological safety.In this study,sodium hypochlorite?NaClO?,ozone?O₃?and ultraviolet?Ultraviolet,UV?were used for pre-oxidation and combined with powdered activated carbon?PAC?and ultrafiltration?UF?to treat the secondary effluent.The removal efficiency of ARGs,class I integron?intI1?,16S rRNA and organic matters in the secondary effluent by the combined process such as NaClO-PAC-UF,O₃-PAC-UF,and UV-PAC-UF were studied.Additionally,the influence factors and the removal mechanisms of the three combined processes were investigated.The results showed that the optimal dosage of NaClO?based on available chlorine concentration?,O₃ and UV was 8.0mg/L,2.0mg/L and 40mJ/cm² respectively.The three combined processes could effectively reduce the content of ARGs,intI1 and 16S rRNA in the secondary effluent,and the removal amounts were 3.02⁴.07-log,2.67³.92-log,2.28³.68-log,respectively.Among them,the combined process of NaClO-PAC-UF had the best removal effeciency.The pre-oxidation of the three oxidants could enhance the removal efficiency of organic matters(DOC and UV₂₅₄)in the secondary effluent by the PAC-UF process.Among them,the process of NaClO-PAC-UF had the best removal effeciency for DOC and the removal rate was 57.5%.The process of O₃-PAC-UF had the best effeciency for UV₂₅₄ and the removal rate was 45.2%.The pre-oxidation of the three oxidants could make macromolecular organics in water into small molecular organics,which were easier to be adsorbed by PAC,so that the removal effeciency of organics in different molecular weight intervals in the membrane effluent of the combined processes were improved.Among them,NaClO-PAC-UF had a better removal effeciency of organic matters in each molecular weight interval.For the organic substances with fluorescent properties,the process of NaClO-PAC-UF has the best removal effeciency on tryptophan protein organics,fulvic acid organics,and soluble microorganism by-products.And the process of O₃-PAC-UF has the best removal effeciency on humic acid organics.The concentration of ARGs in the water was significantly positively correlated with the concentration of DOC,intI1,16S rRNA?p<0.05?.The removal of organic matters,intI1 and microorganisms in water all contributed to the reduction of ARGs.The decrease of pH and the increase of water temperature were beneficial to the reduction of ARGs in water by the combined process of NaClO-PAC-UF.The increase of pH and the decrease of water temperature were beneficial to the reduction of ARGs in water by the combined process of O₃-PAC-UF.And the decrease in pH and turbidity were beneficial to the reduction of ARGs by the the combined process of UV-PAC-UF.The pre-oxidation process of the three oxidants enhanced the removal of cellular and free ARGs in water.The cellular ARGs were not detected in the membrane effluent of the three combined processes,and the free ARGs removal amounts were 3.02³.28-log,2.87³.05-log,2.52².87-log,respectively.Among them,the process of NaClO-PAC-UF had the best removal effect.The oxidant of NaClO mainly produces HClO through hydrolysis to complete the oxidative decomposition of ARGs.The oxidation mechanism of O₃ to ARGs is mainly the direct oxidation of O₃ molecules,and the oxidation of OH·is auxiliary.And the oxidation mechanisms of UV to ARGs is mainly through the photochemical oxidation of UV,OH·plays a minor role.The three combined processes completed the removal of cellular and free ARGs in water through oxidative decomposition of oxidants,PAC adsorption,and UF retention.The three oxidants such as NaClO,O₃ and UV could help reduce the ARGs in the secondary effluent by the process of PAC-UF.The combined process of different oxidant and PAC-UF could be used as an effective method for the advanced treatment of secondary effluent and reduce the safety risks by the reuse of recycled water on the water environment and human health.