| In recent years,the phenomenon of black-odorous occurring in urban rivers has became a serious environmental and ecological problem which needs to be solved urgently.After the effective control of exogenous sources,the secondary pollution caused by the release of endogenous pollutants from the sediments of maladorous rivers has attracted increasing attention.Nitrogen nutrients are the primary endogenous pollutants in the remediation of maladorous rivers.As one of common used in situ measures for the remediation of malodorous river,aeration turbulence has an important effect on the sediment habitat and the migration and transformation of nutrients in sediments.Major Science and Technology Project of China(No.2009ZX07317-006)and the National Natural Science Foundation of China(No.41101471)supported the study.Gongye River in Shanghai and Shanxia River in Wenzhou were selected as the research objects.Based on field investigation,in-situ small-scale and pilot-scale experiments were conducted.The behavior of endogenous nitrogen in sediments under different artificial aeration turbulence conditions were studied systematically.Surfer and CFD(Computational Fluid Dynamics)methods were used to visualize and simulate the effect of aeration on the flow field and flow pattern characteristics,respectively.The relationships between the behaviors of endogenous nitrogen nutrients and the flow field were analyzed.In addition,DGGE(Denatured Gradient Gel Electrophoresis)was employed to analyze the response of major microbial communities in the fluid mud and the black mud under different aeration turbulence conditions.Based on the above studies,the treatment effects of the demonstration project of in-situ remediation of Gongye River and Shanxia River by using artificial aeration were evaluated.(1)The type of flow in simulating device under different aeration conditions were turbulent flow.In the small scale device,the average flow velocity of each run was 245mm/s,291mm/s,314mm/s and 358 mm/s,respectively,and the corresponding Reynolds number was 1101,1528,1761 and 1525,respectively.In the pilot scale device,the rotational speed of aerator impeller were 350rpm,230rpm,160rpm and 290 rpm,respectively,the corresponding average flow velocity of each run was 277 mm/s,181mm/s,133mm/s and 225mm/s,respectively and the corresponding Reynolds number was 2272,1625,1315 and 1481,respectively.(2)By combining the measured and simulated results,X-axial flow showed turbulence-tempestuous at two ends near the aerator in the small scale device and the turbulence has a gradually smooth along the flow direction in the pilot scale device.The flow pattern at the sediment-water interface of Z-axial in small scale device and pilot scale device were exhibited the most drastic turbulence.It was indicated that aeration turbulence promoted the endogenous nitrogen release from sediments,the flow velocity and Reynolds number could characterize the turbulence intensity and had a good consistency with the flow field characteristics of each run.In sum,flow velocity and Reynolds number can be used as a quantitative parameter to characterize the aeration process(3)Under aeration turbulence,the oxygen-transfer efficiency was mainly related to the flow velocity and there was a linear relationship between stable stage DO(Dissolved Oxygen)concentration and flow velocity.The oxygen-transfer efficiency of the pilot scale device was higher than that of the small scale device and the DO level of each run was close to the saturated level under the corresponding water temperature.Under aeration turbulence,the SOD(Sediment Oxygen Demand)had a linear relationship with the flow velocity when the flow velocity was less than 350 mm/s.The higher total endogenous pollutants load of the pilot scale device which led to the SOD value was smaller than that the small scale device.(4)Aeration turbulence promoted the nitrification in overlying water,ammonium was transformed to the nitrate and nitrate and diffuse into the fluid mud.In overlying water,the removal efficiencies of ammonium was related to the stable stage DO concentration and the flow velocity at 1528-1761 Reynolds number condition,while it had a positive correlation with SOD and Reynolds number at 2232-2479 Reynolds number condition.The ammonium and nitrate of the fluid mud and the nitrate of overlying water were both positively correlated to the Reynolds number and SOD,and turbulence intensity affected the mass transfer efficiency of DO and ammonium release of fluid mud;In black mud,nitrogen nutrient behavior was mainly associated with diffusion formed by concentration difference.In general,the Reynolds number was positive correlated to the nitrification rate in a scope of flow velocity of 303 mm/s-320 mm/s.(5)The intensity of denitrification was related to Reynolds number and SOD within a certain flow velocity range.In small scale device,it presented better denitrification when the flow velocity was in the range of 291 mm/s to 314 mm/s and the corresponding Reynolds number was in the range of 1528 to 1761.In pilot scale device,the corresponding flow velocity was in the range of 303 mm/s to 320 mm/s and the Reynolds number was in the range of 2232 to 2479.By fitting the nitrification-denitrification under different run in each device,the maximum denitrification rate was 42.29%when the flow velocity and Reynolds number was 285 mm/s and 2099,respectively.(6)In small scale device,the dominant species of bacteria and archaea in sediment had a positive correlation between Reynolds number、nitrification rate and denitrification rate.The diversity of bacteria was higher than archaea.In the pilot-scale device,nitrifying microorganism had a higher diversity and abundance in fluid mud within the flow velocity of 303 mm/s to 320 mm/s,which indicated that nitrification was mainly happened in fluid mud.The Nitrosomonas was the dominant species of AOB(Ammonia Oxidizing Bacteria)and had a positive correlation with DO and nitrification rate.In addition,Nitrosospira had a positive correlation with SOD、nitrification rate、flow velocity and Reynolds number.The dominant species of AOA(Ammonia Oxidizing Archaea)were Thaumarchaeota and Crenarchaeota,Thaumarchaeota and unculutred archaea had a positive correlation with flow velocity and Reynolds number,while Crenarchaeota had a positive correlation with nitrification rate.The diversity and abundance of AOA were higher than AOB under aeration turbulence condition.The dominant species of NOB(Nitrite Oxidizing Bacteria)was Nitrobacter,its diversity and abundance decreased at stable stage,which indicate that nitrite transformed to nitrate.The Nitrobacter had a positive correlation with Reynolds number and flow velocity.(7)Aeration turbulence facilitated the diffusion of nitrate to the fluid mud and black mud,which could also improve the intensity of denitrification in the sediment.DB(Denitrifying Bacteria)had a better diversity and abundance in the black mud of sampling point 2,and it became higher when the flow velocity was in the range of 303 mm/s to 320 mm/s.The dominant species of DB was Dechloromonas sp.,which was positively correlated with SOD,denitrification rate and nitrification rate.In summary,when the flow velocity was in the range of 303 mm/s to 320 mm/s,the succession of dominant microorganisms changed significantly,which was agreed to the optimal performance of nitrification-denitrification.Meanwhile,the dominant microorganisms were positive correlated with Reynolds number and flow velocity.(8)Based on the above simulation device research,The effect studies were conducted in order to evaluate the demonstration project of in-situ remediation of Gongye River and Shanxia River by using aeration turbulence.The result showed that the intermittent aeration could improve the denitrification rate,when the average flow velocity of Gongye River was 133.3 mm/s,Reynolds number was 1840,the denitrification rate was 46.3%±16.7%,while it was raised to 52.76%±5.9%in Shanxia River under the flow velocity was 169.4mm/s and Reynolds number was 2308.According to the CFD calculation result,the installation depth and its arrangement made the nitrification and denitrification coupled well,when the average flow velocity became closer to optimal values,the denitrification rate was better.Two demonstration projects achieved the anticipated goal when they ran stable. |
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