| Background and objectivesDue to its satisfactory therapeutic and growth promoting effects,antibiotics are used in in both clinical treatment and animal husbandry,with high production and usage in China.However,antibiotics cannot be metabolized completely in human and animal bodies;they are excreted as parent compounds,metabolites,or conjugates via urine and feces into sewer systems and are ultimately discharged into wastewater treatment plants.It has been reported that wastewater treatment plants cannot effectively remove antibiotics,resulting in antibiotics being released into the environment via effluent discharge and sludge disposal.As antibiotics are continually released,potentially resistant to degradation,highly soluble in water,and adsorbed onto soil and sediment,these"pseudo-persistent" compounds are present in the environment for extended periods.Antibiotics can migrate and transform in the environment and be taken up by plants and animals and then transferred into human beings through food chain,posing a threat to ecosystems and even public health.The type and dosage of antibiotics used dynamically change with the seasonal differences in disease prevalence.In addition,the environmental parameters and microbial communities in biological treatment processes are dynamic during different periods.Therefore,the dynamic distribution characteristics of antibiotics in different seasons deserve attention.At present,researches on antibiotics in wastewater treatment plants are mostly focuses on the residue and removal of antibiotics,lacking continuity studies across different seasons,and insufficient in revealing the spatiotemporal dynamic changes of antibiotics.In this context,the seasonal distribution,dynamic evolution,and removal pathway of antibiotics and their ecotoxicological risk,resistance selection risk,and human health risk were studied at a municipal wastewater treatment plant in Jinan.This study will help regulate and optimize the operation of wastewater treatment plants to achieve effective antibiotic removal during different seasons,and provide theoretical support for implementation of antibiotic pollution management and control.MethodsIn February,May,August,and November 2018,120 wastewater samples and 24 sludge samples were collected.The separation and detection of the target antibiotics(sulfadiazine,sulfamethoxazole,levofloxacin,ciprofloxacin,enrofloxacin,norfloxacin,doxycycline,roxithromycin,chloramphenicol,and metronidazole)were performed with a high-performance liquid chromatography-tandem mass spectrometer.The distribution characteristics in different seasons and treatment stages as well as removal characteristics of the target antibiotics were analyzed.Ecotoxicological risk,resistance selection risk,and human health risk of the target antibiotics were evaluated through risk quotient method.Result(1)Detection characteristics:all the target antibiotics were detected in wastewater and sludge samples,with concentrations ranging from ND to 1981.42 ng/L and ND to 5506.44 μg/kg dw,respectively.The detection rates of levofloxacin(73.8-91.7%)and roxithromycin(73.8-91.7%)were high,while that of sulfonamides(73.8-91.7%)was low.Levofloxacin(ND-1981.42 ng/L in wastewater and ND-5506.44 μg/kg dw in sludge)and ciprofloxacin(ND-587.50 ng/L in wastewater and ND-750.73 μg/kg dw in sludge)have the highest concentrations,and the concentrations of enrofloxacin(ND291.50 ng/L in wastewater and ND-34.74 μg/kg dw in sludge)and doxycycline(ND32.99 ng/L in wastewater and ND-41.58 μg/kg dw in sludge)were lowest.The daily per-capita mass loads of the target antibiotics in this study showed low to moderate levels in both domestic and international comparisons.(2)Spatiotemporal distribution characteristics:there were significant differences in the concentration of most antibiotics among different seasons.Generally,antibiotic concentrations in winter were statistically higher than that in other seasons.In spring and summer,the concentration of antibiotics showed a trend of first decreasing,then increasing,and then decreasing along the wastewater treatment processes.However,in autumn and winter,there was no significant trend of antibiotic concentration increasing first and then decreasing from anaerobic tank to secondary sedimentation tank.The aqueous phase removal of antibiotics varied from-40.47 to 100%.The removed mass proportions of sulfadiazine,sulfamethoxazole,roxithromycin,and metronidazole were higher(41.46-97.45%),while the mass proportions of norfloxacin,doxycycline,levofloxacin,chloramphenicol,ciprofloxacin,and enrofloxacin adsorbed on sludge were higher(22.51-219.36%).(3)Risk assessment:for ecotoxicological risk,the RQ value of levofloxacin from the effluent to Daphnia was 1.89×10-2,all others are less than 0.01.For resistance selection risk,levofloxacin(2.58 × 10-2)and ciprofloxacin(2.21 × 10-2)in the effluent and ciprofloxacin(1.35×10-2)and metronidazole(6.98×10-2)in the sludge had RQ values greater than 0.01.For human health risk,the RQH values of antibiotics in the effluent were all less than 0.01.Conclusion(1)Antibiotic contamination is common in wastewater and sludge from wastewater treatment plants.Among the target antibiotics,levofloxacin has high detection rates and concentrations,and exhibits low ecotoxicological and resistance selection risk.(2)Antibiotic concentrations in winter were mostly higher than that in other seasons.In addition,the concentration of antibiotics varied along the wastewater treatment processes,with different trends among seasons,mainly during the biological treatment stage.The antibiotic removal efficiencies in the wastewater treatment plant were overall at a moderate to effective removal level.Sulfonamides,roxithromycin,and metronidazole were mainly removed through biological processing,whereas fluoroquinolones,doxycycline,and chloramphenicol were removed through sludge adsorption.(3)In addition to levofloxacin,ciprofloxacin in the effluent and sludge and metronidazole in the sludge may facilitate the selection of antibiotic-resistant bacteria in the environment.The antibiotic residue discharged from the wastewater treatment plant posed negligible human health risk. |
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