Microbial Characteristics And Denitrification And Dephosphorization Mechanisms Of A~2/O Process In Tibetan Plateau Environment

The construction of urban sewage treatment plants in Tibet is in a period of rapid development,but the operation of the sewage treatment plant is not satisfactory at present.Typical plateau environmental factors such as high altitude,large temperature difference and strong ultraviolet(UV)radiation have seriously restricted the level of sewage treatment.In this study,domestic sewage was used as the treatment object,and the anaerobic anoxic aerobic process(A²/O)was used to carry out experimental research.The operating characteristics of different treatment units under different conditions such as temperature,dissolved oxygen(DO),hydraulic retention time(HRT),UV irradiation time were determined respectively.Illumina Mi Seq high-throughput sequencing technology were used to analyze the structure,abundance and metabolism of activated sludge microbial community,and the relationships between the influent water quality,process parameters and working conditions and the microbial community structure and abundance in activated sludge were discussed.The species and abundance changes of main functional proteins,functional genes and enzymes in the metabolism and transformation of pollutants were analyzed and the microbial mechanism of nitrogen and phosphorus removal in sewage at high altitude was studied at biochemical and molecular levels.Firstly,the influences of temperature,HRT,DO and UV on the operation of laboratory-scale sewage treatment were studied.The variation of COD,TP,TN and NH₃-N in anaerobic tank,anoxic tank and aerobic tank under different conditions were analyzed,and the operation characteristics of A²/O process in Tibet plateau environment were clarified.The results showed that the sludge settling ratio(SV₃₀)and mixed liquor suspended solids(MLSS)were relative low under different conditions,which reflected the lower activated sludge concentrations in A²/O process system under plateau environment.Under different temperature conditions,the removal rates of COD and TN were the highest at 15?,while TP and NH₃-N were the highest at 20?.Under different DO conditions,the removal rates of TN and NH₃-N were the highest at 2.0 mg/L,and COD and TP were the highest at 1.0 mg/L.The removal rates of TN and COD were the highest at 26.25 h of HRT,while NH₃-N and TP were the highest at 17.50 h.Under different UV irradiation time,TN and COD removal efficiency was the best at 10 min,TP was the best at 30 min,and NH₃-N was the best at 0 min.According to the effluent quality of the system,the optimal conditions of A²/O process were 20?of water temperature,2.0 mg/L of DO,17.50 h of HRT and 10min of UV irradiation.Secondly,the high-throughput analyses of 51 sets of activated sludge samples from the A²/O process system were completed.The number of microbial OTUs and the species at the phylum and genus levels were low.Verrucomicrobia belongs to the dominant bacteria phyla in the special habitat of the plateau,and the Chlamydiae belongs to the peculiar microbial phylum in the special habitat of plateau.The dominant bacteria on genus level are Dokdonella,Novosphingobium,norank?f??Saprospiraceae,etc.,which accounted for 59.52%.Among typical plateau environment factors,UV irradiation posed the most significant impact on the microbial community structure.The low concentration of activated sludge in the A²/O process system might be mainly related to filamentous bacteria such as Anaerolineae and Candidatus?Microthrix.The ratio is not high.On this basis,the correlation analysis between the dominant population of activated sludge microorganisms and the influent water quality,process parameters and working condition factors was carried out.The results showed that the concentrations of TP and COD in the influents were important factors affecting the microbial community.MLSS and SV₃₀ were important process parameters that affect the dominant microbial community,reflecting that the activated sludge biomass was more susceptible to the influence of the microbial community structure and abundance.UV irradiation time was the most important environmental factor affecting the microbial community.Due to the different distribution of dominant bacteria in different reactors,their responses to operating conditions were different.The results of correlation analysis between dominant bacteria showed that there was a symbiosis or antagonistic relationship in oxygen,energy,and phosphorus sources among the bacterial communities.Finally,the abundance and metabolic pathway of main functional proteins,functional genes and related enzymes in activated sludge were analyzed.The composition and abundance of main functional proteins in A²/O system were different under different conditions.The optimal conditions of functional protein abundance were 1.5 mg/L for DO,17.50 h for HRT,20?for water temperature and no UV irradiation.The types of main metabolic pathways were identical under different conditions,and the bacterial two-component regulation system showed strong adaptability to environmental changes.The abundance of cold adapted enzymes such as ribonuclease,RNA polymerase,iron superoxide dismutase and alkaline phosphatase was high,which reflected the adaptability of bacterial community to low temperature environment in plateau.Purine metabolism,ribosome,oxidative phosphorylation,pyrimidine metabolism,arginine and proline metabolism in dominant metabolic pathways of microorganisms were closely related to nitrogen and phosphorus removal of sewage.Eight genes involved in nitrogen metabolism and six genes involved in phosphorus metabolism were identified under different working conditions.The above metabolic functional genes are the dominant genes for nitrogen and phosphorus removal under plateau environmental factors,suggesting an adaptive mechanism of microbial species in the sewage treatment system in plateau.