Study On Mechanisms Of Enhanced Pollutants Removal In Constructed Wetland Added With Typical Benthic Fauna

With the rapid development of the economy and society in China,the problems of environmental pollution and ecological imbalance in aquatic environment are still serious.Thus develop the efficient and ecological wastewater treatment technology is urgent.Constructed wetland(CW)is a kind of ecological treatment technology that uses natural process to achieve wastewater treatment.It has the advantages of low cost of construction and operation,convenient management and function of environmental beautification.The CW has prominent advantages in wastewater treatment in developing regions.However,the low efficiency of pollutant removal in winter limits the continuous and stable operation of CWs.The removal pathway of nitrogen and phosphorus in CWs is limited,and as a deep water purification process,the low ratio of carbon to nitrogen in the influent restricts its efficient operation.The substance exchange between water and substrate is limited,which causing the hypoxia in substrate.Benthic faunas are widely distributed and have strong stain resistance.They have direct effect on the removal of pollutants through their predation and gathering.Also,they can indirectly promote the pollutants removal and optimize the environment by their effects on plants and microorganisms.Therefore,the introduction of benthic faunas into CWs is of great significance for the stable operation and the enhancement of pollutants removal in CWs.To solve the problems of CWs,most of the researches optimized the microbes,substrate and plants,but ignored the synergy between benthic faunas,plants and microorganisms.This paper aimed to increase the biodiversity of CWs,based on the bioturbation activity of benthic faunas,and built a "plant-animal-microbe" system to strengthen the operation of CWs.Firstly,the typical benthic fauna Tubifex tubifex(T.tubifex)is introduced into the surface-flow CW(SFCW)system,and the feasibility of T.tubifex to enhance the removal of pollutants from CWs was explored.Its effect on the migration and transformation of carbon,nitrogen and phosphorus,and microbial community structure were studied.The impact and mechanism of T.tubifex on carbon conversion was well studied.Based on the feeding characteristics of benthic fauna,the mussels were further introduced to construct a combined CW system to explore the interaction between benthic fauna and plants,substrate and microorganisms.Finally,based on the pollutants removal mechanisms of T.tubifex and mussels,the promotion and mechanisms of the typical benthic faunas on the removal of typical organic pollutant polycyclic aromatic hydrocarbons(PAHs)in CWs were investigated.The main conclusions obtained are as follows:(1)T.tubifex enhanced the removal efficiencies of nitrogen and phosphorus in CWs,improved the plants' absorption of nitrogen and phosphorus,promoted the vertical transfer of nutrients and improved the abundance of functional microbes.The removal efficiencies of TN and TP were 22.3%and 27.4%higher in T.tubifex added CW in summer,respectively.The T.tubifex improved the NO3--N removal efficiency from 54.2%to 82.6%,and the NH4+-N removal efficiency increased from 84.6%to 90.9%than the control group.The nitrogen and phosphorus concentrations in substrate increased by 0.53×<10-4 and 0.23x10-6 g/g DW with the effect of T.tubifex,respectively.T.tubifex improved the plants' absorption of nitrogen and phosphorus by 11.4%and 30.0%,respectively.In winter,the T.tubifex can still increase the NO3--N removal efficiency of CWs by 40.9%,but limit the removal efficiency of NH4+-N,so the TN removal efficiency only increased by 22.9%,the TP removal efficiency improved by 7.40%with T.tubifex.The T.tubifex changed the abundance of functional microbes in the CWs,increased the number of 16S rRNA,denitrifying bacteria(nirS and nirK)and DNRA functional bacteria(nrfA),but the number of nitrifying bacteria(amoA and NSR)decreased slightly.(2)The predation and excretion,and bioturbation activity of T.tubifex promoted the migration and transformation of carbon,nitrogen and phosphorus,and changed the microbial community structure.The T.tubifex transformed the formation of phosphorus,increased the proportions of particulate and colloidal phosphorus in CW by 15%and 11%,respectively,which promoted the sedimentation of phosphorus to substrate and the absorption of phosphorus by plants.The T.tubifex also changed the formation of organic carbon,and decompose the refractory organic carbon into methyl acetate and sulfur compounds,which can be directly used by denitrifying microorganisms as carbon source.T.tubifex increased the proportions of DOC by 31%,and also improved the contents of humic organic carbon,which further increased the bioavailability of organic carbon,and provided sufficient electron acceptor for microbes,which could solve the problem of low nitrogen removal efficiency caused by low ratio of carbon to nitrogen in CW.The T.tubifex optimized the microbial community structure in CW,affected the degradation of organic carbon by changing carbon degradation bacteria.The proportion of denitrifying bacteria increased by 21.0%,the proportions of ?-Proteobacteria and Bacteroides were improved,the NO3--N removal efficiency was triple than control group,which increased the N2O emission.The promotion of DNRA process with T.tubifex was one of the reasons for low removal efficiency of NH4+-N.(3)Due to the filter-feeding capacity,the mussels could also improve the treatment performance of pollutants in CWs.The combined CWs with mussels and T.tubifex increased the stability of the ecosystem and operation of the CWs through the interaction between benthic fauna,plants and microbes.The NH4+-N removal efficiency in CWs added with mussels was up to 89.1%,improved the little lower NH4+-N removal efficiency with T.tubifex in winter,and realized the simultaneous removal of NO3--N and NH4+-N in CWs.The CW with co-effect of mussels and T.tubifex can achieve the annual TN and TP removal efficiencies by 63.4%and 35.5%,stabilized the annual pollutant removal rate.With the co-effect of T.tubifex and mussels,the sedimentation of pollutants to the substrate was promoted.Compared to the control group,concentrations of nitrogen and phosphorus in substrate of CWs with T.tubifex and mussels increased by 28.0%and 54.7%,respectively.Meanwhile,the absorption of nitrogen and phosphorus by plants increased by 30.6%and 27.9%,respectively.Due to the enhancement of denitrification by T.tubifex and mussels,the amount of N2O released from the CWs increased slightly,but the total amount of N2O produced by two benthic faunas was less than 0.60%in the removal of TN.The bioturbation activity and microorganisms in two benthic faunas assimilated the microbial community in CWs.The denitrifiers in the intestinal tract of T.tubifex and mussels,and the high proportions of Firmicutes and Bacilli improved the wastewater purification.These microbes could survive under extreme environment with high tolerance,which ensure the operation of CW.(4)T.tubifex and mussels changed the distribution of PAHs in CWs.The absorption of five PAHs by the plants increased by 7.6%and 43.1%in mussels and T.tubifex added CWs than the control group,respectively,which increased the proportion of plant absorption in PAHs concentrations from 10.6%to 15.2%and 11.4%,respectively.The two benthic faunas improved the root microenvironment by promoting plant growth and substrate oxygen transport through bioturbation,which promoted the degradation of 3-rings PAHs in the substrate.By exploring the removal of five typical PAHs in the CWs with these two benthic faunas,it was found that T.tubifex and mussels increased NO3--N concentration in substrate,which provided electric acceptor for PAHs degradation.Due to the filter-feeding activity,the PAHs concentration accumulated in mussels achieved 16.1 ?g/g FW.Also,the contents of PAHs in the substrate at 0-10 cm depth under water-substrate interface decreased by 29.2%than the control group.The bioturbation activity of T.tubifex promoted the precipitation of PAHs to substrate,and thus the PAHs contents in the substrate at 0-5 cm depth under the water-substrate interface increased by 19.3%than control group.The PAHs pollutions limited the NH4+-N removal in CWs,decreased the activity and abundance of nitrifying bacteria.The mechanisms of migration and transformation of PAHs with T.tubifex and mussels provides a new idea for the enhancement of PAHs removal in CWs.