Study On Advanced Nitrogen Removal From A Municipal Wastewater Treatment Plant Through Partial Denitrification And Anammox Process

With the ever-increasing requirements for nutrient control,nitrogen contents in effluent from municipal wastewater treatment plants（WWTPs）need to be further reduced.Anaerobic ammonium oxidation bacteria（Anammox）,ammonium is oxidized using nitrite as the electron acceptor to dinitrogen gas under an anoxic condition.In theory,the Anammox process requires a low level of aeration and no additional carbon sources compared to the traditional nitrification-denitrification process.Therefore,the application of anammox in mainstream sewage treatment has received growing interest.However,it is difficult to achieve stable NO₂^--N accumulation when enriching and cultivating Anammox bacteria in WWTPs,which has become a major difficulty in engineering applications.Therefore,using partial denitrification to provide NO₂^--N for the growth of Anammox bacteria that has become a current research hotspot.The main goal of this study was to test the feasibility of applying the PD/A in moving-bed biofilm reactor（MBBR）for secondary effluent treatment from start-up to stable operation.The biodiversity and microstructure of biofilm were analyzed by high-throughput sequencing and fluorescent in situ hybridization（FISH）techniques.The potential capabilities of Anammox and partial denitrification were evaluated by activity tests.The results laid the foundation for the secondary effluent nitrogen polishing with the PD/A process in MBBR.The main conclusions are as follows:（1）The long-term operation results showed that the MBBR treatment load can reach838.63mg N·（m²·d）^-1,and the effluent NH₄⁺-N and NO₃^--N concentrations are 2.02 mg·L^-1 and 1.13 mg·L^-1,respectively.and effluent TN is less than 5 mg·L^-1.The average removal rates of NH₄⁺-N,NO₃^--N and TN were（88.88±7.02）%,（91.80±8.20）%,and（83.86±5.43）%,respectively.（2）The contribution of anammox and denitrification to TN removal shows that full denitrification dominated in MBBR during phase I.As the cultivation progresses,the proportion of full denitrification decreased gradually.The contribution of anammox to TN removal increased stepwise from 6%in phase I to 95%in phase III.The accumulation of anammox and partial denitrification bacteria in biofilm stimulated the PD/A process,which caused anammox to dominate the TN removal in MBBR.（3）The activities of anammox bacteria in biofilm raised from 47.15 mg N/（m²·d）to736.75 mg N/（m²·d）on 10^th day to 150^th day and then maintained in a constant of728.84±7.90 mg N/（m²·d）.Regarding the capacity of partial denitrification,the activity increased from 389.74 mg N/（m²·d）to 1206.58 mg N/（m²·d）and then maintained in a constant of 1192.83±13.75 mg N/（m²·d）with the average nitrite accumulation ratio of88%.（4）Carbon balance analysis shows that the COD required for partial denitrification was 13.89 mg·L^-1,the COD required for synthetic cells was 13.75 mg·L^-1,and the COD of the effluent was 21.80 mg·L^-1,which was consistent with the influent COD,and the actual carbon to nitrogen ratio was 2.43.（5）Through batch experiments,it can be seen that the best C/N of the MBBR in this experiment was between 3 and 4,it can be sure that partial denitrification capacity is stable and there was sufficient NO₂^--N provides the substrate for the Anammox.（6）Fluorescence in situ hybridization shows that Anammox has a certain proportion,and the microorganisms form a spatial distribution of Anammox bacteria inside and heterotrophic bacteria outside on the biofilm.High-throughput sequencing showed that after enrichment culture,the proportion of Ca.Brocadia increased from 0.62%to 5.61%,the proportion of Thaurea increased from 0.004%to 3.74%.