The Microbial Mechanism Underling Municipal Sewage Treatment On Sub-plateau Region In Northwest China

The microbial community structure and metabolic function in activated sludge sampled from four municipal sewage treatment plants?MSTPs?on the sub-plateau regions in northwest China were investigated using Illumina MiSeq sequencing.The shifts of microbial community abundance,diversity and structure were analyzed against the different seasons,environmental factors?i.e.,altitude,dissolved oxygen,and temperature?,and process parameters.Furthermore,COG functional proteins,key genes and enzymes,major metabolic pathways were enriched to reveal the microbial mechanism under the processes of municipal sewage treatment.The main results are summarized below.1.The A²O and CAST processes run well on the sub-plateau regions in northwest China for efficiently removing the major pollutants,such as COD,NH₄⁺-N,and total phosphorus?TP?.The pollutant removal rates,activated sludge?AS?settling performance,and relative microbial biomass in summer are better than those in winter.The microbial community abundance index Sobs and Chao values in summer are higher than those in winter.The Shannon diversity index values in summer are significantly higher than that in winter.And there is a positive correlation between OTU number,microbial community abundance,diversity with nutrient removal rate in different MSTPs.Thus,it is a effective strategy of improving nutrient removal rate with high OTU number,microbial community abundance and diversity by adjusting NH₄⁺-N concentration,temperature and DO concentration in MSTPs.2.The dominant bacterial populations comprise Proteobacteria?26.1%-34.6%?,Actinobacteria?13.7%-27.8%?,Bacteroidetes?18.7%-21.0%?,Chloroflexi?7.7%-18.5%?,Firmicutes?4.0%-6.9%?,Saccharibacteria?TM7??1.5%-6.1%?in the sewage purification microecosystem.The dominant genus include Candidatus Microthrix,Trichococcus,Ornithinibacter,Terrimonas,Flavobacterium.There are no significant differences among the bacterial community compositions in the different MSTPs,seasons,and process units,suggesting a good stability of the bacterial community composition.Moreover,the dominant bacterial phylum and genera are shared with different AS samples in the plateau and plain region.This may be a reflection of ecological coherence in different geographical MSTPs.However,a significant difference of bactierail community abundance was observed across the different MSTPs,seasons,and process units.The result obtained from linear discriminant analysis?LDA?indicates that the populations of Chloroflexi,Proteobacteria,Caldilineales,and Caldilineae were enriched in the AS in summer,suggesting that these populations play an important role for pollutants removal in summer.The populations Candidatus Microthrix,A.Incertae Sedis,Acidimicrobiales,and Actinobacteria were enriched in the AS in winter,suggesting that these populations contribute more in pollutants removal in winter.Furthermore,the co-occurrence network analysis and the correlation network analysis indicate that these dominated populations have the highest coexistence degree,although there is a correlation between the low abundance populations with the other populations.The sewage purification microecosystem is composed of a lot of populations which is beneficial for the system diversity and stability.3.The result obtained using redundancy analysis?RDA?shows that the ordination of environmental variables that greatly affect the bacterial community structure and abundance are NH₄⁺-N concentration,temperature,dissolved oxygen?DO?,COD,sludge retention time,TP,altitude,and hydraulic retention time?HRT?.Furthermore,the analysis using non-metric multidimensional scaling?NMDS?demonstrates that temperature is the key variables for the formation and change of bacterial community structure in the AS.The similar result obtained from the linear regression analysis shows that the NH₄⁺-N concentration,temperature,and DO are the most important environmental factors affecting bacterial community structure and abundance in the AS.Additionally,the RDA result indicates that the microbial populations are influenced by the various factors.So it can be concluded that the distinctive AS bacterial inhabitants on sub-plateau are driving by the local climate conditions?temperature/altitude?,influent quality?NH₄⁺-N concentration?and operational parameters?DO?.4.The COG functional protein enrichment in the sewage shows that most of the functional proteins are closely related to microbial growth and reproduction.The higher richness of these functional proteins was observed in the AS at low temperature and higher altitude MSTPs.A total of 1794 enzymes was identified which belong to lyase,oxidoreductase,transferase,hydrolase,etc.The main KEGG pathways in the AS include active transport pathways,biosynthesis and decomposition of purine and pyrimidine,amino acid degradation,oxidative phosphorylation,glycolysis,butyric acid and pyruvate metabolism pathway,etc.These KEGG pathways contribute to nutrient transformation and adsorption and the removal of carbon,nitrogen and phosphorus in the sewage.Moreover,the functions of predicted genes,such as rpoE,paaF,echA,drrA,ABC.CD.P,are closely related to the bacteria proliferation,the transformation and adsorption of carbon,nitrogen and phosphorus,environmental signal transduction,etc.In conclusion,the unique microbial composition enriched in AS remains stable under different environmental variables and process parameters.However its richness changes a lot and parallels with the efficiency of nutrient removal.The sewage was purified cooperatively by multiple metabolic pathways,such as active transport pathways,biosynthesis and decomposition of purine and pyrimidine,amino acid degradation,oxidative phosphorylation,glycolysis,butyric acid and pyruvate metabolism pathway,which are catalyzed by lyase,oxidoreductase,transferase,hydrolase etc.