Research On Synergistic Effects And Mechanisms Of The Aeration Coupled With Constructed Wetland System For River Water Treatment

River water treatment is one of the hot issues of great concern in China.Surface flow constructed wetlands are usually applied for the water treatment of rivers due to their low cost,easy maintenance,ecological restoration,and landscape construction.However,there exist some problems such as large footprint,limited water purification capacity,and shortened lifespan by many factors.In addition,river water not only contains the conventional pollutants but also the pharmaceutical pollutants which are potentially harmful to aquatic ecosystems and human health.Therefore,it is urgent and crucial to improve the surface flow constructed wetland technology in order to enhance the removal efficiencies of conventional pollutants and also to realize the removal of pharmaceutical pollutants.In this study,the aeration technology was coupled to the surface flow constructed wetland for the treatmen of river water inferior to Class V.Single factor and response surface analysis experiments were conducted to determine the optimal aeration conditions of the coupled technology.Under the optimized conditions,the aeration coupled wetland technology was applied for the treatment of the river water contaminated by pharmaceuticals.The removal performance of pharmaceuticals was evaluated,and the responses of reed growing and physiological indicators,the root exudates,and the rhizosphere and bulk microorganisms were explored.Eventually,the microbial mechanisms associated with the coupled technology for river water treatment was revealed.The main research findings are as follows:（1）Four factors including aeration rate,hydraulic retention time,aeration intermission time,and aeration position were selected for the single factor experiments.The superior aeration conditions were obtained as:aeration rate 0.60 L·（min·L）^-1,hydraulic retention time 2d,aeration intermission time 2 h（per 1-hour aeration）,and aeration position 0.4（aeration depth/water depth）.The removal efficiencies of conventional pollutants were 32.0%±1.8%of TN,55.8%±3.3%of NH₄⁺-N,55.2%±2.2%of TP,and 47.0%±5.0%of COD.（2）The response surface analysis experiments with four factors at three levels were carried out on the basis of the single factor experiments.The optimized aeration conditions were determined as:aeration rate 0.45 L·（min·L）^-1,hydraulic retention time 1 d,aeration intermission time 2 h（per 1-hour aeration）,and aeration position 0.5（aeration depth/water depth）.Under the optimal conditions,the removal efficiencies of TN,NH₄⁺-N,TP and COD were 55.6%±7.2%、78.7%±4.0%、68.1%±5.9%、53.4%±4.2%,respectively.The relative errors between the experimental and predicted values were less than 5%.（3）The optimized technology was used to treat the river water polluted by pharmaceuticals at a level of 150 ng·L^-1and 1000 ng·L^-1,respectively.Among the seven target pharmaceuticals,ketoprofen（KTP）,propranolol（PRP）,trimethoprim（TMP）,roxithromycin（RTM）,ibuprofen（IBP）and valsartan（VST）（except for diclofenac DCF）were oberseved better removal performance compared to the control experiments without aeration.Synergistic effects were found for both the treatment of RTM and IBP.As for the conventional contaminants,synergistic effects were also found for the removals of TN,NH₄⁺-N and TP.Although the COD removal did not have a synergistic effect but its performance was still better than that of the control experiment.（4）Aeration promoted the growth and photosynthesis of reeds.Glucose,galactose,fructose,arabinose,oxalic acid,citric acid,acetic acid,glutamic acid,glutamine and asparagine in the root exudates of reeds actively respond to the conditions of aeration and oxygenation.From leaf stage to rapid growth stage,the release rate of these substances is significantly higher than that without aeration.Aeration reduces the abundance and diversity of rhizosphere microorganisms,but as plants grow,The advantages of some resistant bacteria such as Proteobacteria,Actinobacteria,Patescibacteria,Cyanobacteria,Verrucomicrobiota,especially Azohydromonas,Vogesella,Angustibacter,and Pseudomonas,are gradually becoming obvious;by analyzing the correlation between root exudates and rhizosphere microorganisms,it can be seen that Proteobacteria,Actinobacteria,Patescibacteria,Cyanobacteria,Verrucomicrobiota,and the Azohydromonas,Vogesella,Angustibacter and Pseudomonas and Glucose,Galactose,Arabian One or more of sugar,oxalic acid,citric acid,acetic acid,glutamic acid,glutamine,and asparagine are significantly positively correlated,indicating that in the aeration-coupled wetland system,aeration promotes plant secretion of more glucose,Galactose,arabinose,oxalic acid,citric acid,acetic acid,glutamic acid,glutamine and asparagine,thereby promoting the growth of Proteobacteria,Actinobacteria,Patescibacteria,Cyanobacteria,Verrucomicrobiota,especially Vogesella,Pseudomonas,Azohydromonas and Angustibacter,which increase the pollution（total nitrogen,ammonia nitrogen,total phosphorus,COD and pharmaceuticals）Degradation efficiency.