Influence Of Cattail Litter Addition On Nitrogen Removal Of Surface Flow Constructed Wetland

When applied in treating low-polluted water,constructed wetland?CW?is often difficult to obtain ideal nitrogen removal effect due to lack of carbon sources.In order to improve the denitrification effect,the common practice is adding chemical carbon source.However,addition of these carbon sources will result in the increase of operating cost and may even cause secondary pollution of water bodies.Therefore,seeking more ecologically natural and alternative carbon sources is of great practical significance for denitrification effect of CW system.In recent years,plant carbon source has drawn people's attention gradually.If aquatic plant litter in wetland can be effectively used as the internal carbon source during the wilting period,it can not only improve the nitrogen removal efficiency of CW system,but also consume the aquatic plant litter within the system,thus eliminating the need for outward handling and saving disposal cost.At present,studies on aquatic plant litter decomposition mainly focused on the impact on water quality of water bodies,systematic and specialized researches have not been conducted on utilization of the carbon source in wetlands.In view of the current situation,this thesis intends to conduct targeted research in the following aspects:?1?Comparative study on the decomposing process of three different types of aquatic plant litters commonly used in constructed wetland system was conducted.Water hyacinth,hydrilla and cattail were selected as representatives of floating,submerged and emergent plants to study the decomposition laws of their litters in water bodies and the influence on water quality.Meanwhile,by analyzing the variation range of water quality index in receiving water body,the type of aquatic plant litter which had less influence on water quality and the highest releasing C/N ratio was selected during the decomposing process.The results showed that the decomposition process of water hyacinth had the greatest influence on water quality of the receiving water body,followed by hydrilla and cattail.The increase of litter dosage enhanced the impact of the decomposing process on water quality,and the introduction of sediment reduced the impact of litter decomposing on water body.The releasing C/N ratio from water hyacinth,hydrilla and cattail litter decomposition to receiving water body were 2.6,5.3 and 20.3,respectively.Therefore,cattail litter was more suitable as additional carbon source for denitrification in CW system.In addition,the results of high-throughput sequencing showed that the microbial communities responsible for the decomposition of water hyacinth and hydrilla litter were relatively similar,which were different with cattail litter.After the sediment addition,the microbial communities responsible for the decomposition of water hyacinth and hydrilla litter were significantly changed,while the changes of cattail were not obvious.?2?The nitrogen removal performance of cattail-type surface flow constructed wetland?SFCW?system for low-polluted water was evaluated.The effect of cattail planting density,water depth and hydraulic retention time?HRT?on nitrogen and phosphorus removal of the system was studied by single factor experiment.Response surface methodology was used to analyze the interaction between factors and their effect on nitrogen removal efficiency of the system to obtain the optimal conditions.The results showed that the increase of cattail planting density and HRT could enhance nitrogen and phosphorus removal rate of the system,which was contrary to water depth.Among them,only increase of cattail planting density enhanced nitrogen and phosphorus removal loading of the system.The impact level on nitrogen removal efficiency of the system was water depth>HRT>planting density.The optimum conditions with higher nitrogen removal loading and water treatment quantity were cattail planting density of 20 plants/m²,water depth of 42 cm and HRT of 1d,the removal efficiencies of total nitrogen?TN?and nitrate nitrogen?NO₃^--N?were 20.49%and 20.15%,respectively,and their removal loadings were 1.12 and 0.85 g m^-2 d^-1,the treating capacity of the water was 0.126 m³ d^-1.?3?The effect of cattail litter addition on nitrogen removal performance of SFCW system was studied.By comparing the concentration of nutrients released from the cattail litter decomposition at different dosages and temperatures,the influence of litter dosage and water temperature on the litter decomposition was determined.By comparing the effect of different dosage methods on the pH,dissolved oxygen?DO?and denitrification efficiency of the water body,the optimum dosing method was determined to obtain higher TOC releasing with unit litter and nitrogen removal efficiency.In addition,the influence of cattail litter addition on the nitrogen removal performance of the system in different seasons was determined by examining the yearly operating performance of the system.Combined with the annual average yield of cattail litter,the potential applicability of cattail litter was analyzed.The results showed that the litter should be added in batches with the dosage of 0.1 g L^-1 d^-1.After the litter was added into the system,no obvious increase of TOC was observed in the effluent,the removal rate and removal loading of TN and NO₃^--N were significantly increased.In different seasons,increasing level of nitrogen removal by cattail litter addition was different with 21.57%in summer and 14.97%in winter.Aquatic plants planted within the wetland system can be fully harvested in the wilting period and placed directly in the system without the need for external transport treatment,which can achieve the recycling of aquatic plants in SFCW system.?4?The balance of carbon and nitrogen in the SFCW system was established.By comparing the releasing fluxes of greenhouse gases in the system before and after the cattail litter addition,the releasing rules of greenhouse gases under different seasons and the impact of litter addition on the greenhouse gas emissions were clarified.Meanwhile,by monitoring the form and content of carbon and nitrogen in each phase of the wetland system?mainly including plant phase,mud phase,water phase and gas phase?,the migration and conversion pathway of carbon and nitrogen in the wetland system was analyzed to establish carbon and nitrogen balance of SFCW system.Gas chromatography monitoring of greenhouse gases found that the release of N₂O,CH₄and CO₂ showed clearly seasonal change,except for the performance of CO₂ sinks in summer and autumn,the other are releasing sources,the releasing flux was summer>autumn>spring>winter.After the litter addition,the releasing flux of the three kinds of greenhouse gases were increased significantly.The main pathway for removing carbon from the system was the catabolism transformation of microorganisms,nitrogen removal pathway was the denitrification by microorganisms,and the litter addition increased the contribution ratio of carbon and nitrogen removal pathways to the total carbon and nitrogen removal by the system.