Effect Of Sponge Iron Dosing On The Microbial Community Structure In Batch Biological Sponge Iron Reactor

Due to the advantages of simple operation and low energy consumption,biological denitrification has become a common method for treating nitrogen-containing wastewater in wastewater treatment plants.As the main body of biological denitrification,microbial community plays an important role in the process of denitrification.It has been shown that the addition of zero-valent iron can influence the structural composition of microbial community,promote the enrichment of nitrogen-related functional bacteria and improve the nitrogen removal effect.And sponge iron is special zero-valent iron.Compared with traditional zero-valent iron,it has high iron content,large specific surface area,light specific gravity,strong adsorption capacity,and easy to occur iron-carbon microelectrolysis.The previous research of our group found that the batch biological sponge iron reactor（BBSIR）formed by injecting sponge iron into activated sludge to form a biological sponge iron system（BSIS）into SBR has a high nitrogen removal effect.However,there are few studies related to the structure and functional genes of BBSIR microbial community,and the mechanism of nitrogen removal is still unclear.Therefore,in this study,the pollutant removal effect of BBSIR was investigated under different sponge iron dosing amounts with simulated domestic wastewater as the influent and ordinary activated sludge system as the control group.The changes of microbial communities and functional genes inside each reactor mixture and carrier were also analyzed by high-throughput sequencing technology.And the mechanism of nitrogen metabolism of BBSIR was analyzed,which is important for the practical application of BBSIR technology.The specific conclusions are as follows:（1）After stable operation of each reactor,the removal rates of COD_Cr and NH₄⁺-N were above 98%,and the NO₂^--N effluent concentrations were all lower.Compared with the control group,the average TN removal rates in BBSIR increased by 8.18%,21.19%and16.46%,respectively,and the best nitrogen removal effect was achieved in BBSIR at the dosage of 90 g/L.The degradation rate constants of NH₄⁺-N in BBSIR（0.2574 1/h-0.3008 1/h）were greater than those of the control group（0.2127 1/h）during the cycle period,and the NO₃^--N accumulation concentrations were smaller than those of the control group.The injection of sponge iron promoted the occurrence of simultaneous nitrification denitrification in the BBSIR.（2）The analysis of the microbial community structure of the mixed liquor at different dosing levels showed that the microbial community composition in the control and BBSIR sludge samples were found to be less similar,and the microbial community structure in the mixed liquor sludge samples at 45 g/L dosing level and 90 g/L dosing level were more similar.The addition of iron sponge changed the microbial community structure at the gate level,in which Proteobacteria（50.57%-62.62%）had the highest relative abundance and dominated in both MP1-MP4 samples.The casting of sponges was unfavorable to the enrichment of nitrifying bacteria and favorable to the enrichment of denitrifying bacteria,resulting in a significant increase in the relative abundance of iron bacteria（Rhodoferax,Brevundimonas）and autotrophic denitrifying bacteria（Hydrogenophaga,Arenimonas）in BBSIR.Among them,the highest total relative abundance of denitrifying bacteria（47.46%）was found at the sponge iron dosing level of 90 g/L.In addition,Hydrogenophaga,Saccharimonadales,Sphingopyxis,Ferruginibacter and Rhodoferax were found to be positively correlated with the removal of TN.SWB02 and Nannocystis were negatively correlated with the removal of TN and COD_Cr.（3）The analysis of microbial community structure inside the carrier at different dosing levels showed that the highest diversity and richness of sludge samples were found at 135 g/L dosing level.Planctomycetes containing more anaerobic bacteria（0.21%-071%）were significantly enriched inside the BBSIR carriers.90 g/L sponge iron dosing was most favorable for the enrichment of denitrifying bacteria inside the BBSIR carriers（total relative abundance of 64.54%）.Compared with the mixture,the total relative abundance of aerobic denitrifying bacteria inside the carrier was significantly lower,the total relative abundance of anoxic denitrifying bacteria was significantly higher,and nitrate-dependent divalent iron-oxidizing bacteria（Thiobacillus）were present.In addition,Arenimonas,Saccharimonadales,Acinetobacter and Flavobacterium were found to be positively correlated with COD_Cr,NH₄⁺-N and TN treatment effects.（4）Functional gene prediction showed that metabolism（31855.92-34345.57）was the most enriched KEGG primary metabolic pathway category.The addition of sponge iron increased the membrane transport function of microorganisms and promoted the enrichment of nitrification genes and denitrification genes.The highest total abundance of denitrification genes（1030.05）was observed at a dosing level of 90 g/L.Nitrogen was removed mainly by redox reactions at the beginning of the operation of BBSIR,and with the increase of the operation cycle,nitrogen was removed efficiently by biochemical action.