Study On The Flow Pattern And Nitrogen And Phosphorus Removal Characteristics In Oxidation Ditch A~2/O Process

The A²/O process is currently the most widely applied process for denitrification and dephosphorization in urban sewage treatment,and the reactors of each unit that realize A²/O are mainly multi-pool series complete mixed reactors or traditional plug flow reactors.The oxidation ditch cleverly combines a plug flow reactor and a complete mixing reactor,and is widely used in various sewage treatment systems.The oxidation ditch type A²/O realizes the A²/O process through the oxidation ditch type reactor,which is a new attempt for the biological nitrogen and phosphorus removal treatment of urban sewage.This subject takes the oxidation ditch A²/O process of Xi’an No.10 Wastewater Treatment Plant as the research object.Through the investigation and analysis of the water quality of the inlet and outlet water,the operation effect is evaluated;the flow rate of the mixed liquid in the ditch is measured to analyze its hydraulic power.Features:Through the changes of the main pollutants（N and P）in each operation unit and the change of sludge activity,the performance of nitrogen and phosphorus removal by oxidation ditch A²/O is evaluated and analyzed.The main conclusions of the study are as follows:（1）Through the analysis of the water quality of the inflow and outflow of the oxidation ditch A²/O process of Xi’an No.10 Wastewater Treatment Plant,the average removal rates of BOD₅,COD,NH₄⁺-N,TN,TP,and SS are 94.7%,93.7%,and SS,respectively.98.8%,78.5%,95.3%,96.8%.All indicators of the effluent have reached the level A standard of GB18918-2002 stably,and the pollutant removal standard is up to the standard and the operation is stable.The concentration of influent COD,TN and NH₄⁺-N has a significant positive correlation with the concentration of effluent COD,TN and NH₄⁺-N;the concentration of influent NH₄⁺-N,TN,TP and SS has a significant positive correlation effect on the concentration of effluent COD,The influent COD,TP and SS are positively correlated with the effluent NH₄⁺-N and TN.All indicators have no significant correlation with the effluent TP.The reason is that the aerobic pool outlet is equipped with chemical phosphorus removal facilities to ensure that the TP removal meets the standard.（2）The distribution of nitrogen and phosphorus along the oxidation ditch A²/O process of the No·10 Wastewater Treatment Plant in Xi’an City was measured,which showed that the oxidation ditch A²/O process has a good treatment effect on nitrogen and phosphorus in urban sewage.The PO₄^3--P concentration at the end of the anaerobic tank is increased to 2.39 mg/L compared to the PO₄^3--P concentration（1.36 mg/L）after mixing the raw water and the sludge backflow.The phosphorus release effect is better.Later,the concentration of PO₄^3--P in the water decreased to 1.71mg/L,indicating that there is denitrifying phosphorus in the system.The NO₃^--N concentration at the end of the hypoxic zone was 3.64 mg/L,indicating that some NO₃^--N was not completely denitrified.The NH₃-N concentration at the end of the aerobic zone is reduced to4.08mg/L relative to the NH₃-N concentration at the end of the anaerobic tank（8.59mg/L）,and the nitrification effect is good.（3）The maximum phosphorus release rate and maximum phosphorus absorption rate of activated sludge in the oxidation ditch A²/O process are 3.519mg PO₄^3--P/（g MLVSS·h）and 2.66 mg PO₄^3--P/（g MLVSS·h）;the acetic acid absorption rate is 15.952 mg HAC/g MLVSS·H,the amount of phosphorus released per unit of acetic acid is 0.221.The AOB activity of the sludge（calculated as NH₃-N）is 1.48（mg/g VSS·h）,and the NOB activity（calculated as NO₂^--N）is 2.88（mg/g VSS·h）,and the nitrification activity of the sludge is normal Level.（4）The average vector velocity of the anaerobic tank and the anoxic tank are0.156m/s and 0.123m/s,respectively.Although both are less than 0.2m/s,the sludge non-siltation velocity has not been found since the sewage treatment plant was put into operation.The precipitation of sludge in the anaerobic tank and anoxic tank indicates that the anaerobic tank and the anoxic tank can still operate normally under the current agitation and propulsion intensity.The average vector velocity of the aerobic tank is0.272m/s,which can meet the requirement of non-silt velocity（that is,mixed propulsion）.The dilution ratioα_Aof section A oxidation ditch is 13.9,and the dilution ratioα_Oof section O oxidation ditch is 7.02,both of which are smaller than the dilution ratio of conventional oxidation ditch.However,the raw water quality and quantity of the plant are stable,and the sewage in the service area is mainly domestic sewage.The process operation is stable and the discharged water quality meets the standard,indicating that the current dilution ratio can meet the normal operation of the oxidation ditch A²/O process.The reflux ratioβof the mixed solution is 710%,which is much higher than that of the conventional A²/O process.Combined with the measurement data analysis along the way,the effluent of the anoxic tank still contains 3.64mg/L NO₃^--N,indicating denitrification in the anoxic tank The process is not complete,which affects the denitrification efficiency.