Enhancement Of Modified Basalt Fiber Bio-nest To Treat Low Ratio Of Carbon To Nitrogen Municipal Wastewater By Nano-iron Oxide

Due to the imperfect planning and insufficient construction of drainage network in China,rainwater and industrial wastewater are mixed into the municipal sewage drainage network,resulting in a serious imbalance in the C/N ratio.At present,the C/N ratio of municipal sewage in most areas of China is about 3.3,which is far lower than that in developed countries such as Europe and the United States.How to deal with low carbon and nitrogen ratio wastewater,improve the utilization efficiency of carbon source and achieve full nitrogen removal has become a difficult problem for municipal sewage treatment plants in China.Improving the biological community structure,weakening the electron transfer between denitrifying bacteria,strengthening the nitrate respiration of microorganisms and improving the denitrification potential of activated sludge may be an important way to solve the imbalance of C/N ratio in municipal wastewater.Existing studies have shown that the addition of iron source can effectively improve the nitrogen removal performance of activated sludge and promote the evolution of microbial communities in favor of biological nitrification and denitrification.In this study,iron oxide nanoparticles were added to the MBF biological nest reactor to investigate whether iron oxide nanoparticles could promote denitrification,prevent electron shunt,inhibit methanogenic bacteria,and achieve efficient utilization of carbon source.Secondly,considering the rich microenvironment of the biological nest,the effects of nano-iron oxide on heterotrophic nitrification-aerobic denitrifying bacteria were investigated under aerobic conditions,and the effects of nano-iron oxide on interspecies electron transfer of microorganisms were investigated under anoxic conditions;Under anaerobic conditions,the abundance of methanogenic bacteria in activated sludge and the expression of functional genes and electron transfer proteins in denitrifying bacteria under the influence of nano-iron oxide were investigated.Finally,the effect of MBF biological nest reactor with the addition of nano-iron oxide on the actual wastewater treatment was evaluated.(1)The feasibility study on the treatment of low C/N ratio wastewater by MBF biological nest enhanced by nano-iron oxide: The results of the experiment show that the removal effect of COD in the two groups of reactors is not obvious,but the removal effect of ammonia nitrogen and total nitrogen is greatly affected,and the reactor added with nano-iron oxide can improve the utilization efficiency of unit COD.Although the abundance of methanogenic bacteria did not decrease,the addition of nano-iron oxide reduced the abundance of common fermentative bacteria such as Bacteroides,Green Bay bacteria and Proteobacteria,which greatly increased the abundance of heterotrophic nitrification-aerobic denitrifying bacteria and nitrifying bacteria,and at the same time,conventional denitrifying bacteria such as Thauera became the dominant bacteria in the reactor.The experimental results show that it is feasible to treat low C/N ratio wastewater with MBF biological nest enhanced by nano-iron oxide.(2)Mechanism analysis of iron oxide nanoparticles enhancing MBF biological nest to treat wastewater with low C/N ratio: Under aerobic conditions,we screened out the dominant strains of heterotrophic nitrification-aerobic denitrification,annotated the nitrogen conversion related genes of the dominant strains by gene draft analysis,and observed that iron oxide nanoparticles could enter the cell membrane;Under the induction of nano-iron oxide,the expression level of functional genes related to nitrogen transformation was significantly improved.In the anoxic environment,the expression of functional genes related to electron transfer in activated sludge decreased,while the expression of functional genes related to nitrogen transformation increased.Nano-iron oxide weakened the expression of electron transfer proteins in the interspecific transmembrane electron transfer channel,which made more electrons participate in intracellular nitrate respiration and improved the effective utilization of carbon source.Under the anaerobic condition,the nano-iron oxide improves the relative abundance of denitrifying bacteria and the expression quantity of denitrification functional genes,particularly reduces the expression quantity of mtr ABC and dsm AB which are two groups of functional proteins forming a transmembrane electron transfer channel,weakens the electron transfer among biological species,and enhances the efficiency of biological denitrification.(3)Effect evaluation of MBF biological nest enhanced by nano-iron oxide on municipal wastewater with low C/N ratio: The experimental results show that nano-iron oxide can also improve the nitrogen removal efficiency of actual wastewater,the utilization rate of total nitrogen removal per unit of COD is increased by nearly58 %,and the reactor added with nano-iron oxide has higher degradation efficiency of organic nitrogen sources and denitrification efficiency.The concentrations of nitrate and total nitrogen in the effluent were lower.Metagenomic analysis showed that the expression level of functional genes related to nitrogen transformation in the experimental group was higher,and the ability of biological nitrogen removal was stronger.At the same time,the expression level of genes related to electron transfer in the experimental group was also lower than that in the control group.The results of actual wastewater experiments further strengthened the specific mechanism system of nano-iron oxide affecting biological nitrogen removal.