| Extracellular polymer?EPS?is a polymer formed by polysaccharides,proteins,DNA and other substances.The main source of EPS is the polymer produced by microbial metabolism and autolysis,which plays an important role in supporting the structure of sludge floc.EPS was composed of two types of tightly packed extracellular polymers?TBEPS?and loose extracellular polymers?LBEPS?.The main influencing factors included:temperature,water quality,sludge age and heavy metal ions.Free nitrite?FNA?is a function of NO-2-N concentration,temperature,and pH.When the concentration of FNA changes,the metabolic function of microorganisms will be significantly affected,resulting in changes in the content of EPS and the main components?polysaccharide PS,protein PN and DNA?.To delve into FNA on nitrification process?NO2--N?NO3--N?EPS content structure and microbial flora,this subject adopts the sequencing batch type activated sludge SBR reactor,the simulated wastewater,water FNA concentration was studied respectively:0.001 mg/L,0.004 mg/L,0.008 mg/L and 0.012 mg/L,and the concentration of EPS in activated sludge flocs main ingredients?PS?,PN and DNA,and explore different FNA concentration on activated sludge microbial flora structure of the system.Under the conditions of different FNA concentrations,the nitrification effect of SBR activated sludge system was good,and the nitrite nitrogen removal rate reached more than97%.Among them,the content of TBEPS in activated sludge system was significantly higher than that in LBEPS.At the end of nitrification,the content of EPS and TBPS increased with the increase of FNA concentration?0.001mg/L?0.012mg/L?,that is,the increase of FNA concentration had a promoting effect on the content of EPS and TBEPS.The trend of increasing?0.001 mg/L?0.004mg/L?followed by decreasing?0.008mg/L?0.012 mg/L?of LBEPS indicated that the increase of FNA concentration to a certain value had an inhibitory effect on the content of LBEPS after the concentration was 0.004mg/L in this experiment.With the increase of the concentration of FNA?0.001mg/L?0.008mg/L?,the contents of the main components?PS,protein PN and DNA?of EPS all showed an increase,indicating that the increase of the concentration of FNA had a promoting effect on the main components of EPS.However,when FNA increased to?0.008mg/L?0.012 mg/L?,PS and PN content both decreased,and the increase of PS?3.24[mg·?gSS?-1]?1.92[mg·?gSS?-1]?was the most obvious,indicating that the concentration of FNA over 0.008mg/L had a significant promoting effect on PS content in EPS.With the increase of FNA concentration?0.001mg/L?0.012mg/L?,the content of PS and DNA in LBEPS showed a decreasing trend,while the content of PN showed an obvious accumulation.As the concentration of FNA increased?0.001mg/L?0.008mg/L?,both the content of PS and DNA in tb-eps increaseThe number of OTU in activated sludge system increased with the increase of free nitrite FNA concentration,while the number of horizontal endemic bacteria in activated sludge system decreased gradually.The trend of a slight increase in the number of unique OTU species in the microbial activated sludge samples after a sharp decline?144?23?32?79?indicated that when the concentration of FNA increased?0.001mg/L?0.008mg/L?',it was obviously lethal to the flora of the activated sludge system.The richness of vertical comparison of microbial flora structure is usually measured by Chao1 index and ACE index,while the horizontal comparison uniformity of different activated sludge system communities is Shannon index and Simpson index.Chao1 index,Simpson index and ACE index were0.001mg/L,0.012 mg/L,0.008mg/L and 0.004mg/L respectively.Shannon index was 4.61>3.11>3.02>2.75,that is,FNA concentration was 0.001mg/L,0.008mg/L,0.012 mg/L and0.004mg/L respectively.According to the change of richness and evenness index,FNA has obvious influence on community structure of activated sludge system.As Nitrospira belongs to nitrite oxidizing bacteria group of Nitrospira,the relative abundance of Nitrospira was much lower than that of other systems under the condition of FNA concentration of 0.001,but the nitrification effect was better than that of FNA concentration of 0.004mg/L,0.008mg/L and 0.012mg/L.The results showed that the concentration of FNA would affect the nitrification effect and nitrification rate.Since sodium nitrite was used as the only nitrogen source in this experiment,this condition was not conducive to the growth of other functional bacteria,so the nitrifying bacteria were the absolute dominant bacteria at the five classification levels,and the relative abundance of other bacteria was relatively small.The results of PCA analysis under different FNA conditions showed that the microbial community structure of the system was similar and close when the FNA concentration was 0.004mg/L and 0.008mg/L.When the FNA concentration was0.008mg/L and 0.012mg/L,the microbial community structure of the system had a great similarity.The results showed that the increase of FNA concentration to a certain value at0.008mg/L had a significant effect on the microbial community structure.The results of this experiment show that the structure of the microbial community of the system is quite different under the conditions of four different FNA concentrations.The results of the PICRUSt can predict the metabolic function of the microflora by comparing the well-known microbial reference gene database with the existing 16SrRNA gene sequencing data.Carbohydrate metabolism had the highest relative peak value among the12 metabolic functions predicted under four different FNA concentrations.Compared with other systems,the two metabolic functions?energy metabolism and amino acid metabolism?under the condition of FNA0.001mg/L showed some differences.Amino acid metabolism was the strongest of the four activated sludge systems under the FNA condition.It indicated that the increase of FNA concentration did not inhibit amino acid metabolism,carbohydrate metabolism and energy metabolism of the flora. |
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