Study On Start-up And Microbial Flora Of SNAD Process In Micro-oxygen Up-flow Membrane Bioreactor For Wastewater Treatment With Low C/N Ratio

Urban domestic sewage shows the trend of low C/N ratio,so it greatly reduces the efficiency of traditional biochemical denitrification and carbon removal.If the wastewater can meet the discharge standard,it usually needs to invest a lot of resources and high cost.SNAD process is a new type of biological denitrification technology,which is carried out simultaneously by partial nitrification,anaerobic ammonia oxidation and denitrification under the condition of micro oxygen.It is an economic and environmental protection denitrification and carbon removal process.Membrane bioreactor realizes the complete separation of HRT and sludge age,and then maintains the high concentration of microorganisms,so it is conducive to the cultivation and enrichment of autotrophic bacteria such as anammox bacteria(AnAOB)and ammonium oxidation bacteria(AOB),which grow slowly.In this experiment,the simulated industrial wastewater with C/N=0.5 and total nitrogen loading rate(NLR)=0.9Kg-N/(m3·d)was used as the research object to study the effect of SNAD start-up and long-term operation in the micro oxygen up-flow membrane bioreactor on nitrogen and carbon removal,as well as the change of microbial community structure.(1)During 180 days of start-up operation,the average ammonia nitrogen conversion rate was 86.5%,the average NO2--N and NO3--N concentration in effluent was 7.1 mg/L and 13.2 mg/L,respectively,the total nitrogen removal rate(NRR)was 0.7 Kg-N/(m3·d),and the average total nitrogen(TN)and COD removal efficiency was 75.2%,77%.(2)The calculation of the nitrogen and carbon removal path of the SNAD process shows that about 70.9%of the TN was removed by anammox,and heterotrophic denitrification was responsible for the removal of 19.0%TN when the role of nitrite oxidizing bacteria(NOB)can be neglected.(3)In the denitrification stage,COD consumed by NO2--N accounts for 97%of the total removal,while COD consumed by NO3--N only accounts for 3%of the total removal.Considering the role of NOB this experiment uses([NH4+-N]inf-[NH4+-N]eff-[NO2--N]eff)/[NO3--N]eff ratio to evaluate the role of AOB,AnAOB and NOB in SNAD system.The higher the activity of AOB and AnAOB,the higher the ratio.(4)During the 366-day long-term operation,the ammonia nitrogen removal rate was 68.5%in the stable period,the average total nitrogen removal rate was 57.5%,and the average COD removal rate was 75%.(5)During the long-term operation,with the increase of operation time,the content and activity of AnAOB in the reactor increased gradually,the average particle size of sludge increased gradually,the particle size d(0.5)of sludge increased from 121.489?m to 160.039?m,and the MLVSS/MLSS ratio decreased from 0.65 to 0.53.(6)Through high-throughput sequencing analysis,it was found that the AnAOB at genus level were Candidatus jettenia,the AOB were Nitrosomonas and phaeoxylibacter,denitrifying bacteria were Denitratisoma and norank_F_Anaerolineaceae?norank_C_Ardenticatenia,Citrobacter,etc.In SNAD system,the mixed liquor is divided into granular sludge and flocculent sludge,among which AnAOB strain accounts for 44.17%and 0.79%,AOB strain accounts for 0.16%and 3.05%respectively,and denitrifying bacteria strain accounts for 25.93%and 18.57%respectively.The aerobic bacteria in the flocculent sludge preferentially consume the dissolved oxygen in the reactor,thus creating an anoxic environment for the anaerobic bacteria and facultative anaerobic bacteria in the granular sludge,and realizing the common existence of facultative anaerobic bacteria and aerobic bacteria in the SNAD process of the micro oxygen up-flow membrane bioreactor.