Study On Sediment Jet-flow Dredging With Low Disturbance And Residual Wastewater Treatment Via Constructed Wetland In Eutrophic Lake

Lakes are an important part of freshwater resources in China.As the the rapid development of national economy and the continuous promotion of urbanization,excessive pollutants are poured into natural water body,leading to ecological environment destructions in lakes.Hence,the control of lake eutrophication becomes one of the key tasks in Chinese environmental governance,mainly based on the technical route of“exogenous pollution control,endogenous pollution elimination,ecological water diversion and ecological remediation”.Ecological dredging is a common approach for endogenous pollution management at present in China.Nevertheless,in order to reduce pollutant diffusion and to alleviate the disturbance on benthic community,the low disturbance dredging technology should be developed.Besides,the advanced treatment of residual wastewater from sediment sediment also deserves further investigation.Given the consideration above,this study firstly developed a jet-flow dredging pump with low disturbance based on hydromechanics Coanda effect,where the working mechanism and parameter optimization were intensively studied.Secondly,in order to realize the efficient decontamination of residual wastewater,the pollutant removal performance duting the treatment via constructed wetlands and its bioaugmentation strategy were investigated,where the mechanisms were further elucidated.The major results were described as follows:?1?Based on hydromechanics Coanda effect,the jet-flow dredging pump with low disturbance was designed in this study.The performance of jet-flow dredging pump was evaluated by Computational Fluid Dynamics?CFD?technique.The flow fields of three dredging pump cases with different inlet pressures were simulated and analyzed,and the relationships between the different inlet pressures and the average outlet flow rates were obtained.The analysis results demonstrated that Q_hV_h?the product of bottom flow rate and average velocity?could be used to evaluate the dredging capacity of the jet-flow pump and the optimal operating distance.After the structure and size of dredging pump were determined,the flow characteristics had little relation with the inlet pressure.The jet flows from the nozzle under the dredging pump directly moved into the exit cavity along the arc edge of member 2and joined the jet flows in the upper nozzle,where there was no flow separation.The lower nozzle jet drived the fluid at the bottom of the pump to flow out of the mouth,and had little disturbance to the flow field outside the pump.The corner shape of member 2 made the fluid in the cavity flow to the lower nozzle and the upper nozzle after separation at the corner.The analysis results on internal flow field and external flow field indicated that the key parameters affecting the performance of dredging pump were nozzle clearance,outlet radius,bottom distance and component shape.?2?The sensitivity of design parameters to dredging performance and the interaction between the parameters were analyzed by using the experimental design method.The sensitive parameters that affected the bottom scour momentum and work efficiency were the bottom distance and the exit radius,and the coupling effects between the design parameters were also significant.The response surface model of design parameters and performance was further established.The optimal results were obtained by the optimization algorithm.The bottom scour momentum and operation efficiency of the optimal scheme were 8.05 m⁴/s² and32.1%,respectively,higher than other cases in the experimental design.Then,in order to verify the design effect and performance of the low-disturbance dredging pump,the prototype of the dredging pump was designed and processed,and the pool test was carried out.The test results showed that the jet-flow pump could achieve the ecological dredging with low disturbance.There was no obvious pollutant diffusion,and the maximal mud content was up to 70%.When the upper nozzle clearance was unchanged,the lower nozzle clearance increased,and the outlet flow did not change much,and the outlet/inlet flow ratio decreased.The increase of nozzle size could reduce the mud content.The ptterns obtained from the tank test was basically consistent with the numerical simulation analysis.?3?The pilot-scale vertical flow constructed wetland?VFCW?and horizontal flow constructed wetland?HFCW?were used to treat the residual wastewater from dredging process,where dissolved oxygen?DO?distributions,decontamination performances and key enzymes activities were intensively compared under different influent loads.The results showed that the influent load increase caused the reductions of DO levels and contaminant removal efficiencies of chemical oxygen demand?COD?,total nitrogen?TN?,NH₄⁺-N and organic nitrogen,but it had no remarkable effect on the removal of NO₃^--N and total phosphorus?TP?.The interior DO concentrations of VFCW were higher than that of HFCW,indicating that a vertical hydraulic flow pattern was more conducive to atmospheric reoxygenation.Although VFCW and HFCW systems exhibited comparable removal capacities for TN,NO₃^--N and TP,VFCW had a remarkable superiority for chemical oxygen demand?COD?and organic nitrogen degradation.Nevertheless,because ammonia oxidation was the rate-limiting step,NH₄⁺-N produced from organic nitrogen degradation was not effectively removed,thus causing a significantly higher NH₄⁺-N concentration in VFCW effluent.During the entire operation period,the average removal efficiencies of COD,organic nitrogen,NH₄⁺-N,TN,NO₃^--N and TP by VFCW and HFCW were 52.4%vs.43.9%,77.3%vs.47.5%,41.1%vs.56.6%,54.7%vs.56.7%,89.3%vs.90.8%and 40.6%vs.41.2%,respectively.The activities of protease,urease and phosphatase declined with the increasing depth of substrate layers,and the attenuations were particularly pronounced between the upper and middle layers.In the upper layers,the enzymatic activities of VFCW were significantly higher than that of HFCW,respectively with 37.61 vs.29.82?protease?,6.97 vs.5.18?urease?and 9.88 vs.6.37 U/g-substrate?phosphatase?at the influent load of 200 L/d.Moreover,the activities of the three enzymes were positively correlated with DO concentrations.?4?The combined constructed wetland systems were adopted to improve the treatment efficieny of dredging residual wastewater.The COD removal contribution in the first stage of VFCW-HFCW combined process was significantly higher than that in the second stage,while the COD removal contributions between the first and the second stage in the HFCW-VFCW process were comparable.Although the NH₄⁺-N removal in the first stage of VFCW-HFCW was lower than that of HFCW-VFCW,the removal contribution in the second stage of VFCW-HFCW was nearly as twice as that of HFCW-VFCW.The organic nitrogen conversion in VFCW-HFCW was dominated by the first stage,while that in the second stage of HFCW-VFCW was slightly higher than its first stage.During the operation of VFCW-HFCW,organic nitrogen was more effectively converted to NH₄⁺-N.VFCW-HFCW was superior to HFCW-VFCW in total nitrogen?TN?removal,which was attributed to the difference of TN removal contribution in the second stage.Taken together,VFCW-HFCW was superior to HFCW-VFCW in the decontamination of COD,NH₄⁺-N,TN and organic nitrogen,and the effluent COD and TP concentration could reach the Surface Water Standards II and III,respectively.However,the effluent NH₄⁺-N and TN concentration only reached the Standard V,and NH₄⁺-N accounted for 75%of TN.Besides,the second stage of VFCW-HFCW suffered from bioavailable carbon source shortage,and its nitrogen removal based on nitrification and denitrification was limited.Thus,the introduction of autotrophic nitrogen removal approach was proposed in order to enhance nitrogen removal.?5?The sufficient nitrogen removal was achieved via bioaugmentation strategy,by introducing exogenous anaerobic ammonium oxidation?Anammox?mixed culture into VFCW-HFCW system.The effluent water quality was improved from Surface Water Standard V to III.There were fifteen Anammox species identified in the Anammox enriched culture,while only four species could be steadily planted in the constructed wetland system,and they were mainly affiliated to the Ca.Brocadia and Ca.Kunenia genus.The ¹⁵N Isotopic labeling analysis indicated that the Anammox activity in the system was significantly enhanced by the bioaugmentation.The Anammox contribution in the first stage?VFCW?and the second stage?HFCW?were improved from 5.84%and 15.23%to 27.87%and 46.82%,respectively.After introducing the Anammox enriched culture,the richness of dominant species in the system was improved,where the relative abundance of Planctomycetes in VFCW and HFCW was respectively improved from 1.58%and 2.75%to 6.45%and 11.69%.The functional gene quantities of ammonia oxidizing bacteria?AOB?were significantly higher than that of nitrite oxidizing bacteria?NOB?,indicating that partial nitrification should the primary reaction in the VFCW-HFCW system.Besides,most NOBs were affiliated to Nitrospirae rather than Proteobacteria,and the denitrificans were dominated by nirS type.Taken together,by the bioaugmentation strategy,the collaboration of short-cut nitrification,denitrification and Anammox was the major mechanism to achieve the sufficient nitrogen removal in VFCW-HFCW system.