Study On Denitrification Performance And SMP Production Of Denitrification Biofilters With Different Carbon Sources

Effluent from municipal wastewater treatment plants is an important source of excessive nitrogen in water bodies.Therefore,it is necessary to take cost-effective measures for advanced treatment of secondary effluent.Denitrification biofilters（DNBFs）are widely used as a reliable and practical technology for nitrogen removal due to their small footprint,high efficiency,and strong shock resistance,but it needs to add organic carbon to improve their denitrification performance.The properties and concentrations of the carbon sources not only play vital roles in the operation of the DNBF but also affect the production of soluble microbial products（SMPs）.The production of SMPs can lead to degradation of water quality and affect biological treatment efficiency.Therefore,this study investigated the effects of different carbon sources and carbon to nitrogen ratios（C/N）on denitrification performance and SMP production in DNBF.The biological mechanism of denitrification under the different carbon sources was also analyzed.In addition,the relationships between SMP components and denitrification efficiencies were deeply explored.The characteristics of production and utilization of SMPs under different C/N conditions were also revealed.It provides an important theoretical basis for the improvement of the denitrification efficiency and control of the SMP production in DNBFs.The specific research achievements are as follows:Firstly,this study compared the denitrification performance,SMP variation,and microbial community composition and activity characteristics in DNBFs under different carbon source conditions.Three biofilters,named D₁,D₂,and D₃,were fed with sufficient supplies of sodium acetate,methanol,and glucose,respectively.It was found that under the condition of influent NO₃^--N=15 mg/L and COD=120 mg/L,the denitrification efficiencies and denitrification rates of the three DNBFs followed the order D₂>D₁>D₃.The final effluent SMP concentrations were 26.24±3.55 mg/L,24.95±2.67 mg/L,and 28.00±2.44 mg/L in bioreactors D₁,D₂,and D₃,respectively.Therefore,glucose was the most unsuitable carbon source for denitrification among the three carbon sources from the perspective of denitrification efficiency and SMP generation.Three-dimensional fluorescence and Fourier transform infrared spectroscopy analysis showed that protein was the main component of SMPs.External carbon sources were also involved in the initial denitrification in bioreactors D₁ and D₂,while denitrifying bacteria were induced to utilize SMPs in bioreactor D₃.High-throughput sequencing analysis showed that the relative abundance of these denitrifying bacteria followed the order D₁>D₂>D₃.The activities of denitrase and dehydrogenase followed the order D₁>D₂>D₃.This confirmed that the low abundance of denitrifying bacteria and low metabolic activity of microorganisms in bioreactor D₃ led to a poor denitrifying performance.Moreover,the multivariate statistical analysis showed that the relative content of fluorescent soluble microbial byproduct-like substances in SMPs was negatively correlated with denitrification efficiency,which was related to the influence of fluorescent soluble microbial by-products on the growth of functional bacteria.Therefore,because the SMPs also significantly related to nitrate removal in the DNBFs,their different components exhibited differing relationships with denitrification.Secondly,this study further analyzed the denitrification performance,SMP composition,and microbial characteristics of DNBF under different C/N conditions when sodium acetate was used as a carbon source.The results showed that when the influent NO₃^--N concentration was 25 mg/L and C/N was 6,the removal rate of total nitrogen（TN）was the highest,and the percentage of effluent SMP to COD concentration was the smallest.The SMP composition was analyzed by three-dimensional fluorescence spectroscopy,Fourier transform infrared spectroscopy,gas chromatography-mass spectrometry,and X-ray photoelectron spectroscopy.The results showed that low C/N conditions promoted the secretion of polysaccharides.With the increase of C/N,the concentration of dissolved organic carbon in SMPs gradually increased,while the concentration of dissolved organic nitrogen and bioavailable dissolved organic nitrogen decreased gradually.The main substance of low molecular weight SMPs（30～580 Da）was alkane.The comparison of microbial activity,community structure,and functional genes under different C/N conditions showed that when C/N was 6 and 8,the microbial enzyme activities（denitrase and dehydrogenase）were enhanced,and the electron transport system activity was improved,which is conducive to the enrichment of denitrifying bacteria and nitrite reductase functional genes.This was the main reason for the efficient denitrification of DNBF under high C/N conditions.Microorganisms have important effects on denitrification performance and SMP production in DNBF.Statistical analysis showed that SMP formation under different C/N conditions was significantly correlated with microbial activity and microbial community,and the correlation between microbial activity and SMP production was greater than that of the microbial community.In addition,Comamonadaceae＿uncultured,Lentimicrobiaceae＿norank,and Geobacter were the main microbial taxa affecting SMP production.Finally,the feasibility of SMPs as electron donors for denitrification under different C/N conditions was investigated.It was found that when C/N were 2 or 4,the absolute contribution rate of SMPs as electron donors to TN removal was higher than 10.34%,with proteins as major electron donors in SMPs.When C/N was 6,the absolute contribution of SMPs to TN removal was lower than 1.84%,with proteins and fulvic acids as major electron donors in SMPs.When C/N was 8,the microorganisms mainly used the influent organic substrates as electron donors.This indicated that SMP as an electron donor had a higher contribution to denitrification under low C/N conditions.This study emphasized the significant influence of different carbon sources on SMP formation and denitrification performance in DNBFs.It confirmed the feasibility of controlling SMP production by regulating microbial community structure and activity.It was also found that SMPs can act as electron donors for denitrification under low C/N conditions.The results provide new ideas for improving the denitrification performance and reducing the SMP generation during advanced wastewater treatment.