Removal Of Antibiotics And ARGs From Domestic Sewage By Constructed Wetlands: Removal Mechanism And System Optimization

A significant fraction of administrated antibiotics could reach the environment directly or indirectly as the parent form or its metabolite forms,which would pose a potential threat to aquatic and terrestrial organisms as well as human health.Antibiotic resistance is the most concerned and has become a global environmental issue.The potential threat of antibiobtic resistant bacteria(ARB)and antibiobtic resistance genes(ARGs)could be increased because of antibiotic residue.Wide detection of various antibiotics and ARGs in effluent and sludge from wastewater treatment plants(WWTPs)is a major cause for the ubiquitous occurrence of antibiotics and ARGs in various environmental compartments.Thus,a better wastewater treatment technology is needed for removing antibiotics and ARGs,and constructed wetlands(CWs)could be a good one.However,most of the previous studies focus on the removal of COD,BOD and nitrogen,the removal of antibiotics and ARGs is not satisfied,and its removal mechanism is still not known.Therefore,it is necessary to conduct optimization and upgrading of existing CWs,focusing on the removal of antibiotics and ARGs and its removal mechanism.This thesis investigated removal of antibiotics and ARGs by CWs and elucidated their removal mechanism through fieldwork and pilot system.This research includes:(1)constructing 12 mesocosm-scale CWs;(2)optimizing CWs including substrate types,different hydraulic loading rates(HLRs),CW types and plant species with the emphasis on antibiotics and ARGs removal;(3)investigating the influence of two stage CW systems and aeration on removal rates;(4)analyzing the removal mechanism based on the mass balance calculation and microbial diversity analysis.The main results obtained from this research are given as follows:(1)One stainless steel regulating pool(200 cm in height,145 cm in length and 150 cm in width)and twelve stainless steel mesocosm-scale CWs(80 cm in height,80 cm in length and 60 cm in width)were constructed outdoors inside the campus of Guangzhou Institute of Geochemistry(GIG),Chinese Academy of Sciences,in Guangzhou City,China.Every mesocosm-scale CW was built with an independent and adjustable water inlet,and influent flows were achieved by flow meters.Three water outlets and three sampling spots were designed within the CWs,and three sampling depths(0,0.25 and 0.50 m from bottom of the CWs)were designed for water and substrates sampling.The alternative substrates were oyster shell,zeolite,medical stone and ceramic,the designed HLR were 10,20 and 30 cm/d,the designed CW types were surface flow constructed wetland,vertical subsurface flow constructed wetland(up flow or down flow)and horizontal subsurface flow constructed wetland,and the alternative plant species were Thalia dealbata Fraser.,Iris tectorum Maxim and Cyperus alternifolius L..(2)The removal potential of antibiotics and ARGs in raw domestic wastewater by 12 mesocosm-scale horizontal subsurface-flow CWs planted Cyperus alternifolius L.with different design parameters(four substrate types: oyster shell,zeolite,medical stone and ceramic and three HLRs: 10,20 and 30 cm/d)were assessed.Comprehensively considering the removal rates and removal amounts,the CW with HLR of 20 cm/day and zeolite as substrate was the best choice for treatment of the target compounds in domestic wastewater.As demonstrated by laboratory sorption and biodegradation experiments,both sorption and biodegradation could contribute to the reduction of these wastewater contaminants in a horizontal subsurface flow constructed wetland,and biodegradation played a more important role.(3)The removal potential of antibiotics and ARGs in raw domestic wastewater by 6 mesocosm-scale CWs with different flow configurations(surface flow,vertical subsurface flow(up flow or down flow)and horizontal subsurface flow constructed wetland)or plant species(Thalia dealbata Fraser.and Iris tectorum Maxim).The HSSF-CWs and VSSF-CWs showed higher removal rates of pollutants than SF-CWs,and the presence of plants is beneficial to pollutant removal.In terms of removal mechanism,substrate adsorption(1.99 % ~ 4.29 %),plant uptake(1.86×10-5 % ~ 1.65×10-5 %)and biodegradation(73.7 % ~ 95.2 %)contributed to the reduction of various wastewater contaminants including the nutrients,antibiotics and ARGs,while the biodegradation played a very important role.(4)The removal potential of antibiotics and ARGs in antibiotics-spiked domestic wastewater by two-stage(vertical subsurface down-flow CW vs horizontal subsurface flow CW)constructed wetland systems with aeration was investigated.The results showed that two-stage CW systems had good performance of removing pollutants including nutrients,antibiotics and ARGs.Among them,the combination of aeration in the first stage and no aeration in the,second stage could be the perfect choice.In terms of removal mechanism,biodegradation(91.0 ~ 94.7 %)played the most important role in removing antibiotics based on mass balance calculation.Furthermore,Proteobacteria,Bacteroidetes,Actinobacteria and Firmicutes were the dominant microbial species,with their percentage more than 80 %.And concentrations of antibiotics,COD and nitrogen pollutants,as well as pH could significantly affect microbial diversity in constructed wetlands.(5)The removal of antibiotics and ARGs in mixed rural wastewaters by an integrated constructed wetland(ICW)consisting of a regulating pool,four surface and subsurface flow-constructed wetlands,and a stabilization unit was studied.The results showed that detected antibiotics(94.3 %)and ARGs(97.9 %)in rural wastewaters could be effectively reduced through the wetland systems.It is obvious that adsorption onto medium is an important aqueous-phase removal mechanism for the target pollutants as detected from the solid samples.The total pollution loading of antibiotics to the receiving environment via ICW effluent discharge was found relatively low(199 μg/day);therefore,ICW could be applied as an important treatment technology for the removal of antibiotics and ARGs.Our research focuses on the removal efficiency and removal mechanism of antibiotic and antibiotics resistance genes by CWs,through mesocosm-scale CWs optimization and strengthen,as well as an integrated constructed wetland investigation.The roles of biodegradation,substrate adsorption and plant uptake in the removal of pollutants by CWs were clarified.This study provides the scientific basis and data support for the application of wetland system in the treatment of antibiotics and resistance genes.