Research On The Mechanism Of Nitrogen Removal In Constructed Wetland Systems Under The Influence Of Benthic Animals

At present,the concentration of nitrogen and phosphorus in the tail water of wastewater treatment plants in China is high and the discharge is large.It intensifies the eutrophication of the receiving water body with limited self-purification capacity.In order to optimize the water quality of the receiving water,the tail water of the wastewater treatment plant still needs further in-depth treatment.However,due to the large amount of tail water in wastewater treatment plant,it is unrealistic to treat it by conventional treatment.Constructed wetland wastewater treatment system,as a new wastewater treatment technology,has been widely used in large,medium and small cities at home and abroad due to its advantages of low operation and maintenance cost,good economy and environmental benefits.The benthic fauna in wetlands are an important part of the natural wetland system.,to maintain the food chain,water quality indicators,wetland ecological system of material circulation have important influence,benthic fauna can promote nitrification and denitrification,the microbial activity and types,eventually reach the effect of nitrogen and phosphorus removal in constructed wetland.In this paper,based on the important position and feeding characteristics of benthic animals in constructed wetlands,a typical benthic animal,mussels,was introduced into surface flow constructed wetlands,through the establishment of a complete system of constructed wetlands,the removal effect of elements in the matrix of clam and wetland on TN,TP and COD of the effluent was analyzed.Using mussel to explore the effluent removal capacity of constructed wetland system for different nitrogen forms and the calculation of nitrogen exchange flux,and discuss the nitrogen removal mechanism of constructed wetland under the influence of mussel.Finally,taking the constructed wetland system with mussel added in winter as an example,the influence of mussel on the element composition in plant body,microbial abundance and community composition were discussed,and the mechanism of benthos on the nitrogen removal process in the constructed wetland was deeply discussed.The main research conclusions are as follows:(1)The increase of the number of mussels improved the adsorption effect of matrix on P,but had little effect on N.There was no significant difference in the change of P in mussels,and animal assimilation did not contribute to P removal.Adding mussels also improved the removal rate of TN,TP and COD pollutants in wetland system,and the removal rate of TN increased by 16%.The effluent concentrations of TP in CWs-2 and CWs-3 systems are 0.31±0.10 mg/l and 0.20±0.03 mg/l,and the removal rate of TP in CWs-3 system is 83.47±3.89%.COD concentration of effluent can be reduced to below20 mg/L.The constructed wetland system with mussels showed good ability of nitrogen and phosphorus removal.(2)There was no accumulation of NH4~+-N in the effluent of the three systems,and the NO3~--N removal effect was improved.The removal rates of NO3~--N in the system added with mussels are 30.18%?48.65%and 56.3%?68.98%,respectively.In the study of vertical distribution of water between typical periods,mussel can shorten the vertical distribution distance in the constructed wetland system and increase the nitrogen exchange flux.Then it can accelerate the conversion rate of NH4~+-N and NO3~--N,promote the vertical distribution of nitrification and denitrification,has a good water purification effect.The diffusion of pollutants between the overlying water and the interstitial water of the substrate can be promoted to improve the water quality of the constructed wetland.(3)In CWs-1,CWs-2 and CWs-3 systems,the C content of plant stems and leaves was 26.538 mg/g,27.241 mg/g and 33.833 mg/g,and the P content was 0.526 mg/g,0.582mg/g and 0.856 mg/g,respectively.The addition of mussel increased the content of C and P in plants.(4)Combined with high-throughput sequencing results,it was found that the mussel did not change the number of species in the wetland system,but increased the proportion of nitrogen-transforming microorganisms.The addition of mussel affected the abundance of microorganisms related to denitrification,promoted the denitrification and inhibited the nitrification in the underlying substrate.The presence of nitrifying microorganism and denitrifying microorganism in mussel indicates that the mussel body is also one of the places where nitrification and denitrification take place.