Improving Nitrogen Removal Efficiency Of Constructed Wetland By The Configuration Of Warm And Cool Season Plants

At present,constructed wetlands are widely used in the treatment of various kinds of sewage and waste water.However,constructed wetland for deep purification of tail water has a serious C/N imbalance problem,the lack of carbon source limits the ability of nitrogen removal,which leads to the poor nitrogen removal efficiency of wetland.In addition,the growth of aquatic plants and the activity of microorganisms were significantly correlated with temperature,it is a challenge to remove excess nitrogen in the constructed wetland under the condition of low temperature in winter.A large number of studies have shown that the addition of plant carbon sources can effectively improve the nitrogen removal efficiency of constructed wetlands,but the effect of plant carbon sources is greatly affected by the adding methods,adding amount and adding time,and few studies have combined the two aspects of the improvement effect of plant carbon sources on the purification effect of constructed wetland and the decomposition process of plant material under natural conditions.Therefore,this paper designed warm-season species of Hydrilla verticillata and cold-season species of Oenanthe javanica combined floating constructed wetland system,monitored the dynamics of the tested plants,tested the changes of dissolved oxygen(DO)and chlorophyll content in water body of wetland system,quantified the effects of the decomposition of H.verticillataon the nitrogen removal efficiency under the condition of low temperature in autumn and winter,to verify the feasibility of enhancing the nitrogen removal rate of C/N unbalanced water body by the configuration of warm and cool season plants.And using stable isotope 15N labeling technique to study the effect of H.verticillata residue decomposition prophase on the migration and transformation of nitrogen in wetland system,to provide theoretical guidance for constructed wetland ecological purification project.The results showed that:(1)O.javanica showed good low temperature adaptability in the autumn and winter floating constructed wetland system.After the temperature dropped to 5 ?,the trunk and the underground part of O.javanica remained normal except the ground dry weight decreased in different degree in each treatment group due to some of the leaves were yellow and withered.During the experiment,the root activity of O.javanica in each treatment group tended to decrease.LN(oligotrophic water + O.javanica treatment group)?LA(oligotrophic water + O.javanica + H.verticillata treatment group)?HN(eutrophic water + O.javanica treatment group)?HA(eutrophic water+O.javanica + H.verticillata treatment group)groups were 31.73 ?g.(1g·2h)-1?26.67 ?g·(1g·2h)-1?63.73 ?g·(1g·2h)-1?35.2 ?g·(1g·2h)-1 respectively at the end of the experiment.(2)H.verticillata decomposition and carbon release process can be divided into two stages.At the beginning of the experiment,H.verticillata just entered the decay period and decomposed very quickly.At the end of the experiment,due to the decrease of temperature,the decomposition slowed down,eventually H.verticillata dry weight decreased by 57%.In the four study batches,the COD value of LA and HA group was significantly higher than LN and HN group respectively,this shows that the decomposition of H.verticillata in autumn and winter period can release large amounts of organic matter and can be used as carbon source to improve the content of organic carbon of the wetland system.(3)H.verticillata excessive decomposition can increase the chlorophyll content and decrease the oxygen content of the water in the wetland system.During the whole experimental period,the chlorophyll content of LA and HA group was significantly higher than that of LN and HN group respectively;in the first and second batches,the dissolved oxygen concentration of LA and HA group was significantly lower than that of LN and HN group respectively;and during the experiment,the underground dry weight and root activity of O.javanicawere generally lower in LA and HA group,which is related to H.verticillata decomposition and consumption of dissolved oxygen.So root respiration of O.javanica was affected.(4)It is feasible to improve the nitrogen removal efficiency of low C/N wetlands by the configuration of warm and cool season plants.In the four study batches,the average C/N ratio of HA group was significantly higher than that of HN group,the maximum value of HA group was 2.36 times more than that of HN group.Compared with HN group,the removal efficiency of TN was increased by 23.03%,10.9%,18.55%and 22.93%respectively,and the removal efficiency of NO3--N was increased by 38.28%,20.74%,17.87%and 17.06%respectively in HA group.The results showed that the decomposition of warm-season plants in autumn and winter can improve the C/N ratio of the floating constructed wetland system,promote denitrification,and improve the TN and NO3-N removal efficiency of the wetland system.(5)H.verticillata residue decomposition prophase had a certain impact on the migration and transformation of nitrogen in wetland system.On the one hand,H.verticillata residue decomposition prophase can weaken plant absorption and removal of ammonia nitrogen by affecting plant growth.On the other hand,it can promote the denitrification and is beneficial to remove t Nof the system.At the same time H.verticillata residue can reduce the concentration of NH4+-N in water by adsorption,will not cause nitrogen pollution of water body.