Study On Purification Of Eutrophic Scenic Water And The Mechanisms With Constructed Wetlands

Different kinds of constructed wetlands(CWs)were constructed,to investigate treatment performance of CWs treating erutrophic scenic water,select suitable plants, substrates and CWs,and improve the performance and stability of CWs in winter. Purification efficiency,impact factors and pollutants concentrations of different types of CWs were studied.Then plant growth characteristic,nitrogen and phosphorus uptake capacity and its contribution to the removal of nutrients were investigated. Nitrogen and phosphorus transportation,and variation of microbial diversity,activity and community structure in integrated vertical-flow constructed wetlands(IVFCW) were discussed.The experimental results showed that 2d are the most optimal hydraulic retention time(HRT)for CWs treating eutrophic scenic water.Four types of CWs(Cyperus alternifolius/gravel bed horizontal subsurface flow constructed wetland(HFCW), Canna edulis/gravel bed HFCW,gravel bed IVFCW,and zeolite-shale bed IVFCW) all had better treatment performance under the optimal HRT.The removal efficiency is 80.2-92.3%for turbidity and 92.2-98.3%for CHLa,and 56.6-77.4%,73.0-95.0%, 66.7-94.9%for COD_Cr,TP,PO₄³⁺-P.58.1-85.3%of TN and 31.4-79.5%of NH₄⁺-N were removed.Pollutants removal rate of CWs reached to the highest in summer,followed by that in autumn,spring and winter.Seasonal variation and the type of CWs had significant effect on the removal of nitrogen,and type of CWs had more significant effect than plants.The nitrogen removal rate was improved in winter when constructed wetland was filled with zeolite.The nitrogen removal rate was higher and more stable in IVFCW than that in HFCW.Compared with HFCW,IVFCW was suitable for the purification of eutrophic scenic water.Nitrification and denitrification intensity in different layers of CWs were studied. The result showed that nitrification intensity decreased gradually from upper layer to lower layer in HFCW and from down-flow system to up-flow system in IVFCW. Denitrification intensity had no significant difference in four types of CWs. Nitrification intensity in IVFCW was better than that in HFCW,which was in accordance with the nitrogen removal efficiency.Pollutants concentration,pH and DO decreased along the direction of flow in all the four types of CWs and pollutants removal mainly happened in the front of CWs. Pollutants were first removed by infiltration and interception of SS in the front (down-flow system)of CWs,and then removed by plant uptake and microbial decomposition.The pollutants removal rate was low in the back(up flow system)of CWs since low dissolved oxygen and lack of organic carbon and pollutants release.Plants growth period can be divided into adaptation phase,increase phase,stable phase and senescent phase.Compared with Canna edulis,Cyperus alternifolius had longer growth period,higher aboveground biomass(2.438kg DW/m²)and uptake amount of nitrogen and phosphorus(36.49g/m² and 5.73g/m² respectively),suitable to be wetland plants for the purification of eutrophic scenic water.Content and uptake amount of nitrogen and phosphorus were different in different kinds of plants and the same kind of plants in different types of CWs.The contents of nitrogen were higher in plants growing in HFCW than that in IVFCW, and the contents of phosphorus had little variation in plants.The contents and amounts of nitrogen and phosphorus in Cyperus alternifolius were higher than that in Canna edulis.Total nitrogen and total phosphorus distribution in Cyperus alternifolius and Canna edulis organs can be summarized as follows:leaves>stems, and the contents were significant different in different plant organs.Plant height,aboveground plant biomass,and uptake amount of nitrogen and phosphorus decreased along the direction flow in CWs.The plant aboveground biomass,content and uptake amounts of nitrogen and phosphorus in the front half of CWs were higher than that in the back,and that were higher in down-flow system in IVFCW than that in up-flow system.The amounts of nitrogen removed by plant harvesting were about 33.64%, 16.49%,16.40%and 16.37%of removed TN in Cyperus alternifolius/gravel bed horizontal subsurface flow constructed wetland(HFCW),Canna edulis/gravel bed HFCW,gravel bed IVCW,and zeolite-shale bed IVCW respectively,and the amounts and dehydrogenase activity decreased gradually and then slightly increased near the surface of up-flow system.The diversity and spatial distribution of microbial community structure in IVCW was investigated using PCR-DGGE.The results showed that there existed some common kinds of microorganisms and specific microorganisms in different layers in constructed wetland because of the changes of environmental factors and nutrient levels.The diversity of bacteria community was the highest in the upper layer of down-flow system,and the similarity in down-flow system was higher than that in up-flow system.From down-flow system to up-flow system,the diversity of bacteria community decreased and the structure similarity gradually increased.because of the integrated influence of competition of plant rhizodeposition,heterotrophic bacteria, DO and ammonia concentration,there were significant spatial differences in the activity and diversity of ammonia-oxidizing bacteria(AOB)along the flow direction of wetland.The activity of AOB were 0.79mgNO₃^--N·(kg substrate·h)^-1near the surface of down-flow system in IVCW.From the surface of down-flow system to up-flow system,the activity of AOB decreased gradually,and slightly increased near the surface of up-flow system.The spatial variation of diversity of AOB showed similar change trend with the activity in IVCW.Most of AOB were belong to oligotrophic bacterium in IVCW,and the population of ammonia-oxidizing bacteria showed a higher percentage of Nitrosomonas-like sequences from the wetland samples.Uncultured beta proteobacterium,Comamonas sp.,Nitrosomonas oligotropha were also detected.The variation of the AOB community demonstrated slightly spatial pattern in IVCW,which might be related to different wetland environment.