Study On Hydraulic Condition Optimization And Nitrogen Migration And Transformation Mechanism Of Constructed Wetland For Purification Of Mariculture Tail Water

The mariculture industry plays a vital role in the development of fishery production in China.In recent years,the mariculture industry has developed rapidly,and the tail water of mariculture in some areas has been discharged improperly,resulting in eutrophication in some areas and affecting the sustainable and high-quality development of fisheries.Constructed wetland is an artificially controlled sewage treatment technology developed from natural wetland system,which has the advantages of low cost,stable treatment effect,high ecological benefit and so on.In this study,the compound vertical subsurface flow constructed wetland was selected as the research object to explore the effect of the optimization of hydraulic conditions of the marine constructed wetland system on the removal efficiency of COD,NH₄⁺-N,NO₂^--N,NO₃^--N,TN and TP in the tail water of Japanese flounder culture.The tail water of mariculture was marked with ¹⁵N stable isotope,its direction was traced,the transformation path of labeled nitrogen pollutants in the seawater constructed wetland system was studied,and the nitrogen mass balance equation was established to clarify the contribution of plants,substrates and microorganisms to the nitrogen removal in the constructed wetland.For insight into the mechanism of microorganism in the system using high-throughput sequencing technology to water layers in artificial wetland system matrix sequencing,and plant rhizosphere microbes in depth study of microbial population diversity,community structure diversity and function of denitrification bacteria screening analysis,combined with the purification efficiency of constructed wetland system,explore the seawater artificial wetland nitrogen removal mechanism.The research results will provide some theoretical and technical support for the application of constructed wetland in the actual seawater recirculating aquaculture.The conclusions of the study are as follows:（1）At present,the larger areas of culture in Qingdao are sea cucumber pond culture,shrimp and swimming crab pond co-culture and industrial fish culture.Because the sea cucumber pond culture does not need feeding,the water quality of the culture pond generally meets the first class of water discharge requirements of"Mariculture Water discharge requirements（SC/T9103-2007）".The mixed culture of shrimp and swimming crab in the pond is mainly fed to live blue clam,and the water is not changed or changed in a small amount during the culture period.on the whole,the water quality of the culture pond meets the water discharge requirements such as"Mariculture Water discharge requirements",but because the water exchange is less,the total nitrogen and total phosphorus in the pond water quality will sometimes approach or exceed the water discharge limits of the"Mariculture Water discharge requirements".A large amount of compound feed is fed in industrial fish culture,and the emissions of organic particles such as feces and residual bait are usually about 3%of the body weight of farmed fish,and the values of nitrogen and phosphorus nutrients are on the high side in varying degrees.（2）In this study,the concentration of COD,NH₄⁺-N,NO₂^--N,NO₃^--N and TN in the effluent of constructed wetland system ranged from 1.34 to 4.33 mg/L,0.10 to 0.24mg/L,0.02 to 0.12 mg/L and 0.01 to 0.42 mg/L,respectively TP concentration ranges from 0.71 to 1.98 mg/L,and the average removal rates of COD,NH₄⁺-N,NO₂^--N,NO₃^--N,TN and TP are 45.27%,67.71%,94.86%,89.38%,85.83%and 67.77%,respectively Constructed wetland has a good effect on tail water removal,and the concentrations of COD and inorganic nitrogen in tail water of Paralichthys olivaceus treated by seawater constructed wetland all meet the first-class discharge standard of tail water of marine aquaculture（SC/T9103-2007）.（3）After adding stable nitrogen isotopes,the seawater constructed wetland system had an ideal effect on the removal of NO₃^--N.After 21 days of continuous operation,the concentration of mariculture tail water decreased from 0.74mg/L to（0.050.03）mg/L,removal rate reached 92.81%.Before the operation of the seawater constructed wetland system,the total mass of ¹⁵N isotope input was 120mg.After 21 days of continuous circulation of the system,the total amount of ¹⁵N isotope absorbed by plants in the downward pond and upstream pond was 10.44mg,and the total adsorption amount of ¹⁵N isotope by substrate was 7.75mg.The total amount of ¹⁵N isotope removed by microbial reaction in mariculture tail water was 72.92mg,and the ¹⁵N isotope content in fish was 18.22mg.The content of ¹⁵N isotope in fish feces is 9.36mg,and the absolute mass of ¹⁵N isotope not removed in water is 1.31mg.The contribution rates of plant absorption and substrate adsorption to NO₃^--N removal in seawater constructed wetland system were 8.70%and 6.46%respectively,the contribution rates of microorganisms to NO₃^--N removal were 60.77%,the amount of nitrogen absorbed by fish was 15.18%,the contribution of residual bait feces was 7.80%,and the unremoved NO₃^--N in water was 1.09%.Microorganisms were the main contributors to NO₃^--N removal in constructed wetlands.The proportion of microbial removal is more than 50%of the total nitrogen removal in the system,the contribution of plants to nitrogen removal in constructed wetlands is less than 10%,and the contribution of substrate to nitrogen removal in constructed wetlands is greater than that of plants.（4）The richness and diversity index of microbial community in plant root system and substrate layer of the seawater constructed wetland in this study are very high,which indicates that the vertical subsurface flow constructed wetland system provides a suitable ecological environment for the growth and reproduction of microbial community.There are differences in microbial diversity and species richness between plant roots and substrate layer in the same seawater constructed wetland system.The ecological environment of substrate layer and plant root plays an important role in affecting the similarity of microbial community structure in constructed wetland system.The microbial population richness of plant roots in descending and upstream ponds is lower than that in substrate layers.The microbial diversity in the substrate layer of the downpool increased at first and then decreased from the bottom to the top,while the microbial diversity in the upstream pool decreased layer by layer from top to bottom.The similarity coefficients of plant roots and microbial samples in each basal layer are in the range of 0.65-0.9.At the gate level,the constructed wetlands mainly include Bacteroidetes（37.17%）,Proteobacteria（33.41%）,Desulfurobacteria（9.64%）,Firmicutes（5.42%）,Planctomycetes（3.01%）,and Actinobacteria（2.58%）,Chloroflexi（1.65%）,the dominant phyla were Bacteroidetes,Prote0bacteria and Desulfurobacteria.The system class level of small-scale constructed wetland mainly includes Bacteroidia（33.74%）,Alphaproteobacteria（19.61%）,Gammaproteobacteria（13.76%）,Desulfobulbia（5.68%）,bacilli（3.97%）.Through the identification and analysis of microbial gene function in plant root and basement layer samples,15 genera with a total abundance of 2.2290%were found,and denitrifying bacteria were absolutely dominant.The highest abundance was Bacillus,the abundance was 1.4703%,the abundance of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria was low,and the abundance of ammonia-oxidizing bacteria was 0.2999%.The abundance of nitrite oxidizing bacteria is 0.1580%.The results show that the constructed wetland has a good removal effect on the tail water of Paralichthys olivaceus,and the best hydraulic compound is 0.3 m/d and the best influent is 100L.Nitrogen removal in seawater constructed wetland system is mainly realized by the action of microorganisms.The contribution of microbial denitrification is as high as 60.77%,the abundance of denitrifying bacteria in the microbial community is high,and denitrifying bacteria are dominant.