Studies On Water Purification Performance Of Deep Bed Constructed Wetland Based On Autotrophic Denitrification

Water resource is an important basic natural resource and strategic economic resource for social development.Since the reform and opening up,with the rapid growth of China’s economy,the contradiction between the process of urbanization and modernization and the shortage and pollution of water resources has become increasingly prominent.Urban sewage is the second source of water in water-scarce cities,and the recycling of sewage is an important way to alleviate the shortage of water resources,protect the ecological environment,and realize the recycling of sewage,which plays an important role in water resources conservation and water pollution control.As a kind of sewage ecological purification technology with low infrastructure investment,low operating cost and beautiful landscape,constructed wetland has been paid more and more attention to its interception and degradation of nitrogen,antibiotics and other pollutants,and improve the natural purification function of urban water.In recent years,the deep bed constructed wetlands that optimize the "space-performance" configuration of constructed wetland has attracted extensive attention.However,with the increase of the depth,the number of microorganisms at the bottom of the constructed wetland decreases sharply,and the shortage of carbon sources has intensified,resulting in the nitrogen removal effect of the system does not improve but shows a downward trend,which limits the further research of the deep bed constructed wetland.In this paper,different types of autotrophic denitrification enhanced deep bed constructed wetlands were constructed to explore the nitrogen transformation process in the vertical dimension of autotrophic denitrification enhanced deep bed constructed wetland,and the enhanced nitrogen removal mechanism of autotrophic denitrification deep bed constructed wetland was studied by using microbial molecular ecology and isotope tracing technology.The removal effect of typical antibiotics(SMX)in deep bed constructed wetland enhanced by autotrophic denitrification was explored,and the ecological effect of deep bed constructed wetland was evaluated by means of degradation product toxicity prediction and eco-toxicity experiment.The influence of environmental factors,nutrients and REDOX indexes on antibiotic removal in deep bed constructed wetland was quantified.The removal mechanism of SMX by autotrophic denitrification deep bed constructed wetland was revealed,which provides theoretical basis and technical support for the promotion and application of deep bed constructed wetland in urban sewage reuse.The main conclusions are as follows:(1)The introduction of autotrophic denitrification improved the nitrogen removal efficiency of deep bed constructed wetland by 16.57%to 40.95%.The removal effect of TN from high to low is Fe-CW>E-CW>P-CW>Mn-CW>C-CW;C minus CW.Through the analysis of nitrogen morphology along the deep bed constructed wetland,it was found that the heterotrophic denitrification process was dominant in the upper quartz sand layer,and the removal rates of NO3--N by E-CW,P-CW,Mn-CW and Fe-CW were 83.87%,76.20%,66.58%and 43.28%,respectively.In the middle layer of quartz sand,the accumulation of NO3--N appeared in all systems.Although the underlying functional substrate layer reduced microbial richness,functional bacteria were significantly enriched at both phyla and genus levels,proving that further nitrogen removal was achieved through the underlying functional substrate layer.(2)The introduction of autotrophic denitrification can significantly promote the reduction of N2O release flux.The results of 15N isotope tracing showed that the emission of N2O in the deep bed constructed wetland mainly occurred in the process of NO3--N reduction.Fe/Fe2+as electron donor in Fe-CW reduced N2O emission by 39.58%.The difference in the contribution rate of NO3--N to N2O production in Mn-CW,P-CW and E-CW is mainly due to the different ORP of the substrate microenvironment.(3)The autotrophic denitrification enhanced deep bed constructed wetland significantly improved the removal efficiency of SMX.Among them,the removal rate of SMX by the introduction of electrochemical methods(Fe-CW and E-CW)was more than 70%.Meanwhile,the relative abundance of all ARGs(sul1,sul2,intl1)in the effluent and substrate layer of Fe-CW was significantly lower than that of other systems,showing the selection pressure of least resistant bacteria and resistance genes compared with other systems.The correlation analysis of SMX removal in each system showed that the SMX removal mechanism was obviously different in different deep bed constructed wetlands.SMX removal in Fe-CW and P-CW showed a significant correlation with nutrient indices(TN,TP),while SMX removal in E-CW showed a significant correlation with environmental factors(ORP).(4)Microbial degradation was the main removal mechanism of SMX in all deep bed constructed wetlands(9.90-34.71%),followed by substrate adsorption(9.12-22.23%)and plant absorption(4.27-10.10%).Fe-CW and E-CW significantly improved the species richness of the deep bed constructed wetland system.At the phylum level,the relative abundance of Proteobacteria,Actinobacteria and Firmicutes increased,and at the genus level,Pseudomonas,which has a strong ability to degrade antibiotics,was enriched.In addition,the eco-toxicity and genotoxicity risks of Fe-CW and E-CW were also significantly lower than those of other experimental systems,which made Fe-CW and E-CW the optimal choices for the removal of typical antibiotics and ensuring their ecological effects in deep bed constructed wetlands.