Research On Denitrification Effect,Mechanism And Demonstration Of Ecological Floating Bed In Deep Reservoir Of Southeast Hilly Mountains

As reservoirs are the important water supply sources for most cities,cyanobacterial blooms and odor problems caused by eutrophication seriously affect the security of drinking water reservoir water supply and become one of the main challenges for urban water security in China.Nitrogen is the key nutrient element that causes the proliferation of algae.In recent years,surface source pollution has gradually risen as the main type of pollution in reservoir water environment,becoming a key"bottleneck"that must be faced by reservoir safety operation and economic development.Deep-water reservoirs in the southeast hilly areas are of low purification capacity for nitrogen input from surface sources,thus it is urgent to purify the nitrogen pollution entering reservoirs from surface sources in the watershed by engineering means.However,land resources in mountain reservoirs are tight,the application of ecological floating bed technology can make up for the shortage of wetland purification space.This research takes Qiandao Lake,a typical deep-water nutrient-poor reservoir in southeast hills of China,as the research object.We investigated the effects of nutrient concentrations,cutting,light intensity and plant species on the denitrification efficiency and nitrogen removal mechanism of ecological floating bed through simulated experiments.Meanwhile,monitoring the in situ water purification effect of the constructed floating bed pilot demonstration,aiming to extract key technical indicators and optimize the management measures.This study is expected to break through the key technology of in situ nitrogen removal in nutrient-poor reservoirs,and provide scientific and technological support for the scale up of ecological floating beds in deep-water reservoirs..（1）A one-year water quality background survey of the whole reservoir surface water of Qiandao Lake clarified the spatial and temporal distribution of carbon and nitrogen concentrations in the reservoir,and provided a basis for setting the parameters of the ecological floating bed.The monitoring results showed that the annual average value of TN and TP concentration in the surface water was about 0.91 and 0.024 mg/L,and NO₃^--N was the main form of nitrogen,accounting for 65.47%.The annual average value of total organic carbon index,which is closely related to the denitrification rate,was only 1.68 mg/L,indicating that the overall denitrification capacity of the lake body is weak.The average value of nitrogen concentration had reached the standard of Class III water,especially the upstream was more serious,showing a trend of gradual decrease along the main channel of Xin’anjiang reservoir.Therefore,it is especially important to take engineering means to strengthen the denitrification capacity of Qiandao Lake.（2）The adaptability of floating bed plants to nitrogen and phosphorus concentrations was investigated through simulated culture experiments.Firstly,five seedling stage vegetables:Oenanthe javanica（O.javanica）,Ipomoea aquatica（I.aquatica）,Lactuca sativa（L.sativa）,Gynura cusimbua（G.cusimbua）and Petroselinum crispum（P.crispum）were selected through the screening pre-test in autumn and winter seasons,two plants of I.aquatica and L.sativa were selected that could maintain good growth in the low temperature water of Qiandao Lake.Subsequently,formal experiments were conducted to investigate the adaptation and the nitrogen removal effect of O.javanica and L.sativa by setting different gradients of nitrogen and phosphorus concentrations.The results showed that after 28 days of growth,the root system of O.javanica elongated rapidly with a net gain of about 22.7 cm.When O.javanica and L.sativa were grown at TN concentrations above 1.9 mg/L and 1.45 mg/L respectively,the biomass increased significantly.The TN removal capacity of both vegetables was positively correlated with N concentration,with O.javanica（y=18.20x-5.07,p<0.01）having a higher removal rate for N than L.sativa（y=7.82x-2.12,p<0.01）and being more influenced by the external water column N concentration.The experiment showed that O.javanica is preferred as the dominant pioneer vegetable species for floating bed planting in Qiandao Lake in autumn and winter,and combined planting with L.sativa is available in medium concentration waters..（3）To investigate the effects of cutting management on plant growth and nitrogen removal efficiency,we selected the mature submerged plant Myriophyllum aquaticum（M.aquaticum）and emergent plant O.javanica as the subjects.The results showed that the relative growth rate of O.javanica accelerated after cutting,and the dry weight doubled around 25 days growth,while some leaves withered and plant height decreased were observed in the late stage of uncut O.javanica;The submerged part of M.aquaticum shrunk after cutting,and it was difficult to recover in this state and water quality background.Cutting treatment significantly promoted nitrogen uptake by the above-water portion of O.javanica,with the percentage increasing from13.85%to 25.37%;Cutting of the above-water portion of stems and leaves in M.aquaticum affected the normal growth of the submerged root system and reduced the overall nitrogen removal efficiency.However,cutting had a negative impact on the short-term plant root denitrification rate,reducing the water column△[N₂]of O.javanica and M.aquaticum by about 13.74%and 21.77%,after cutting.The experiment showed that floating-bed O.javanica should be cut in time to promote nitrogen uptake by plant growth;While cutting may be an irreversible damage to M.aquaticum,it is recommended to harvest the whole plant at the end of the growth period and not in the middle.（4）In situ simulations and comparative experiments were conducted to investigate the growth conditions and nitrogen removal efficiency of I.aquatica and M.aquaticum under different light intensities and nutrient concentrations in summer.The results showed that the growth of I.aquatica was significantly affected by nutrient salts and light conditions,and the biomass of I.aquatica under nutrients enrichment（TN=2.37 mg/L,TP=0.046 mg/L）was 1.6times higher than that of the in-situ water body（TN=0.66 mg/L,TP=0.028 mg/L）,while the growth under 40%shading was 1.5 times more than that without shading;The growth of M.aquaticum was less affected by either light or nutrient conditions.The nitrogen removal capacity of I.aquatica was significantly higher than that of M.aquaticum,with nitrogen removal efficiencies of 213.30 mg/（m²·d）and 44.23 mg/（m²·d）for I.aquatica（40%shading）and M.aquaticum（75%shading）under optimal conditions,respectively.I.aquatica was mainly based on plant assimilation to reduce nitrogen,accounting for more than 70%of the total nitrogen removal.The shading of 40%enhanced nitrogen removal through significantly increased assimilative nitrogen uptake and root denitrification rate of I.aquatica.To M.aquaticum,plant assimilation and denitrification each accounted for 50%.Both plants increased the denitrification potential of the water column,and the root denitrification rate of I.aquatica[5.79μmol N₂/（L·h）]was higher overall than that of M.aquaticum[4.12μmol N₂/（L·h）].In general,I.aquatica was more suitable for water bodies with higher nitrogen concentrations,and that 40%and 75%shading for I.aquatica and M.aquaticum floating beds respectively in summer would have better performance.（5）A 3000 m² ecological floating bed demonstration base was constructed in situ in Pingshan Cove of Qiandao Lake,and conducted in situ demonstration research on ecological floating beds.We developed a new ecological floating bed structure,which has good support for plant growth and will not be impacted by waves,ensuring stable in-situ operation of the ecological floating bed.The ecological floating bed in the demonstration base is planted with O.javanica and M.aquaticum in the cold season,I.aquatica and M.aquaticum in the warm season,which has been operating stably for 18 months,able to play a better economic,environmental and ecological benefits.The demonstration results showed that all three plant rotations showed good growth throughout the year.I.aquatica grew the fastest with a growth rate of 89.81 g/（m²·d）and the best uptake efficiency of nitrogen and phosphorus,201.52mg/（m²·d）and 12.31 mg/（m²·d）,respectively.Compared with the control area,TN and TP concentrations were reduced by 8.86%and 10.85%on average.Ecological floating beds could significantly inhibit algal proliferation,with total algal biomass reduced by about 34%～44%.The cultivation of I.aquatica and M.aquaticum had a significant enhancing effect on the zooplankton present in Qiandao Lake,with biomass being 9.22 and 6.47 times higher than that in the control area,respectively.More importantly,the application of ecological floating beds provided a more harmonious and stable habitat for all levels of organisms.Dense root systems of floating bed plants provided substrate for fish spawning and attachment sites for small individual snails.Meanwhile,ecological floating beds also attract a large number of birds to feed and rest.Thus,in accordance with the idea of source reduction,process interception and reservoir purification,seven ecological floating beds are planned in Qiandao Lake to strengthen the inlet rivers and reservoir in situ de-nitrogenization and phosphorus removal capacity,so as to deeply improve the water quality of Qiandao Lake.