Removal Of Typical Trace Organic Pollutants In Wastewater By Constructed Wetlands:Performance And Mechanisms

In recent years,constructed wetlands(CWs),with unique advantages such as low construction cost,simple operation and maintenance,environmental friendliness,and sustainability,have been widely used for advanced treatment unit and secondary effluent purification.However,the occurrence of low concentration and high-risk trace organic pollutants(TOPs)including pharmaceutical,personal care products(PCPs),endocrine disruptors(EDCs),and pesticides,have attracted widespread attention.These TOPs have carcinogenicity,reproductive toxicity,neurotoxicity,endocrine disruption and other hazards,which will pose a huge threat to human health and aquatic ecological security.Therefore,it is essential to deeply understand the distribution characteristics,migration,and purification mechanisms of TOPs in CWs systems.The findings will be helpful for us to take scientific,reasonable,and effective treatment measures for TOPs removal,thereby effectively reducing the equivalent of TOPs in water cycle and reducing the potential ecological and health risks caused by TOPs to ecology and humans.Therefore,current thesis aims at the scientific problems “The removal characteristics and purification mechanisms of TOPs in CWs.Firstly,based on CW systems treating effluents from wastewater treatment plant(WWTPs),it clarified the migration and transformation characteristics of various types of TOPs in complete CWs systems,and then revealed the removal characteristics and degradation pathways of TOPs;Then,in order to identify the transformation and degradation characteristics of TOPs under a single removal process in CWs "black box" system,the adsorption characteristics of wetland substrates to typical TOPs,and physiological response and purification mechanisms of wetland plants to typical TOPs were studied respectively,based on laboratory batch experiments,in combination with on-site pilot-scale CWs systems;Finally,a concise summary of purification principles of TOPs in CWs systems was presented,and a decision-making framework for design of CWs guided by the removal of TOPs was proposed.Meanwhile,technical strategies to enhance the removal performance of CWs for TOPs were proposed,thereby providing theoretical,decisionmaking,and technical support for ensuring water quality safety and water ecological health.The main research conclusions of this thesis were provided in the following:(1)The reduction characteristics and removal pathways of TOPs in CWs,indicates that during cold seasons,CWs provide higher removal efficiency for pharmaceutical and endocrine disruptors,while the removal efficiency for TOPs such as flame retardants,bactericides,and pesticides is relatively lower.During warm seasons,the removal performance of six types of TOPs ranked in the following sequence: EDCs >pharmaceutical > PCPs > bactericides > pesticides > flame retardants.The CWs types has a significant effect on TOPs removal.The bidirectional cross flow unit(BCF)with special functional structure and operating mode presents the most efficient performance in removing pharmaceutical,PCPs,and EDCs with removal rates of 97.876 ± 4.894%,14.203 ± 1.136%,and 79.829 ± 3.991%,respectively.The removal efficiency of VSSF CWs for pharmaceutical,PCPs,and EDCs is significantly higher than that of HSSF CWs units,while FWS CWs achieve the highest removal efficiency of flame retardants(9.474 ± 0.758%).The mass balance and relative contribution studies imply that substrate adsorption and plant uptake contribute to 72.7% and 8.9% for pharmaceutical removal,50.8% and 41.0% for PCPs removal,14.6% and 1.3% to EDCs removal,and7.5% and 7.3% to pesticides removal,respectively.(2)The adsorption characteristics of wetland substrates for typical TOPs,shows that adsorption mechanism of volcanic rocks,zeolites,sands,and biochar on endocrine disruptors BPA is mainly mononuclear adsorption,while the adsorption of pharmaceutical SMX on rocks are mainly the physical and chemical adsorption.Wetland hydro-chemical conditions significantly changed the adsorption behavior of the four substrates for TOPs.The combination of humic acid and biochar provides new adsorption point for BPA,but inhibits the adsorption of BPA and SMX by volcanic rocks.The strong adsorption capacity of volcanic rock on pharmaceutical and PCPs in pilot-scale wetland substrate may be attributed to the strong hydrophobicity of TOPs and high organic carbon content of CWs substrate,while the adsorption capacity of volcanics,zeolite,sand,and other wetland substrates to bactericides and pesticides is small.Based on microbial diversity analysis and functional prediction,it was further found that substrate types would affect the structure,biological phenotype,and metabolic pathway of microbial communities.The order of abundance and diversity of substrate microbial communities follow this ranking: volcanics > zeolite > sand,indicating that volcanic provided the highest abundance and diversity of microbial communities w.In comparison with sand and zeolite,volcanics substrates are more conducive to biofilms formation in CWs systems,and volcanics substrate microorganisms have the strongest environmental stress tolerance.(3)The response characteristics and purification mechanisms of wetland plants to TOPs,suggests that the phytodegradation process of wetland plants Phragmites communis and Nymphaea tetragona to endocrine disruptors BPA and drug SMX conforms to a pseudo first order kinetics.In terms of the removal efficiency of BPA and SMX,the overall trend is that the wetland system of Phragmites communis,Thalia dealbata,Canna indica,Arundo donax emergent plants > the wetland system of Nymphaea tetragona floating plants.Endocrine disruptors BPA and drug SMX stress have caused varying degrees of growth inhibition on wetland plants.The Yield and q P index of plant photosynthesis have a significant positive correlation with the removal rate of TOPs(p < 0.05),indicating that the light energy conversion efficiency of photosynthesis can regulate the phytodegradation of TOPs.The pilot study found that,due to the developed root porosity and strong oxygen secretion of Canna indica,Arundo donax,the removal efficiency of CWs planted with Canna indica,Arundo donax for pharmaceutical was 60.8% and 51.6%,respectively,and the removal efficiency of EDCs was 51.6% and 38.9%,respectively,significantly higher than that of CWs planted with Thalia dealbata(57.5% and 20.2%).Among the three groups of plant rhizosphere samples,the aerobic phenotypes of rhizosphere microorganisms in Canna indica and Arundo donax were significantly enriched,followed by the aerobic phenotypes of rhizosphere microorganisms in Thalia dealbata.The tolerance of rhizosphere microorganisms in Arundo donax was the strongest,with the highest relative abundance of metabolic pathways,significantly higher than those in Canna indica and Thalia dealbata.(4)The purification mechanism and intensification strategy of TOPs in CWs,demonstrated that substrate adsorption,phytoremediation,microbial degradation and solar irradiation will interact with each other in complex CWs systems,and the physical and chemical properties of TOPs will also affect entire removal process,leading to changes in the relative contribution of different removal processes to the removal of specific TOPs.Pearson correlation statistical analysis shows that wetland types,hydraulic conditions,and environmental factors can affect the removal efficiency of TOPs in CWs to varying degrees.Based on the purification principle of TOPs in CWs,a design framework and technical strategies for enhancing the removal efficiency of CWs for TOPs removal are proposed.The intensification of CWs of for TOPs treatment include three aspects: chemical modification,redox environmental regulation,and biological manipulation.