Study On Transport And Transformation Of Nitrogen In Sewer Biofilm

The internal space of sewer was huge and the retention time of waste water can reach to several hours.what's more, there were aerobic, anoxic, anaerobic environment inside the pipe for its special structure, all of the above created a conducive condition for organic pollutants degradation objectively and the degradation of pollutants mainly depend on the sewer biofilm attached to the wall of pipe. This study focused on the transport and transformation mechanism of nitrogen in the drainage pipes, the hydraulic conditions and the substrate concentration of waste water were important factors affecting the biofilm structure and thereby affecting the transport and transformation of nitrogen. So a biofilm reactor of drainage pipe was built for this study, biofilms were cultured by running the simulated pipe at different conditions of shear stress and C/N ratio. The distribution of substance concentration including DO, NH4+, NO2-, NO3-, NO, N2 O inside the biofilm were tested through microelectrodes technology, and species diversity and functional diversity inside the biofilm was analyzed by metagenomic testing methods. Overall, the mechanism of transport and transformation of nitrogen was studied from the perspective of microbiology and micro-environment.The main achievements of this article are summarized as follows:(1) Biofilm was cultured about 45 days to reach maturity and the biofilm was at first gradually thickened until reached the maximum value,then gradually reduced and eventually to a stable level. The biofilm thickness were respectively(2.3 ± 0.1) mm,(1.9 ± 0.1) mm and(1.6 ± 0.1) mm under the conditions that shear stress were 1.0 Pa, 1.5 Pa and 2.0Pa. The thickness of the biofilm reduced with the increase of shear stress. The biofilm thickness were respectively(1.7 ± 0.1) mm,(1.9 ± 0.1) mm and(2.0 ± 0.1) mm when C/N were 2, 5, 10. The thickness of the biofilm increased with the increase of C/N.(2) The test results of microelectrode showed that the concentration of DO, NH4+ and NO3- inside the biofilm gradually decreased along the depth direction while the concentration of NO2-, NO and N2 O inside the biofilm was gradually increasing along the depth direction.The concentration of the substance within the biofilm showed obvious differences under three shear stress, three C/N ratio and three kinds of dissolved oxygen conditions.(3) The larger the shear stress, the more conducive for the DO spread to the biofilm, which resulted the oxygen in biofilm become greater and then determined the difference of the biofilm micro-environment and distribution of substances within biofilm. When the shear stresses were 1.0Pa and 1.5Pa, the aerobic / anoxic and nitrification / denitrification hierarchical structure were formed in the biofilms under the interaction of mass transfer, reaction and microorganism. Nitration reaction occurs mainly on the surface of the biofilm while denitrification occurs mainly in the bottom of the biofilm. Simultaneous nitrification and denitrification in biofilms was essentially due to the DO concentration gradient caused by the mass transfer resistance. However, when the shear stress was 2.0Pa, throughout the biofilm were aerobic condition which was not conducive to denitrification reactions.(4) There are rich species diversities in sewer biofilm, in which the bacteria accounted for the most(90.46%), In addition, eukaryotes, archaea and viruses took small counts. The dominant categories in bacteria were mainly Proteobacteria, Bacteroidetes, Verrucomicrobia e. What's more, Nitrospira and Planctomycetes were detected in the biofilms. Nitrospira was the main nitrite-oxidizing bacteria presented in the sewage treatment facilities and many bacteria of Planctomycetes belonged anammox bacteria.(5) There are rich functional genes in sewer biofilm. These genes not only maintained normal activities of microorganisms but also promoted microbial degradation of the pollutants. 722 functional genes involved in nitrogen metabolism were detected in the biofilms, the proportion of all genes was 0.67%. These genes took part in the process of nitrification, denitrification, nitrogen fixation and ammonification, and the functional genes related to denitrification took a dominant position.The paper provided a theoretical support for the further promotion of drainage pipes as sewage treatment reactor concept even further practical application through the research on the transport and transformation of nitrogen within the biofilm of drainage pipe, which had an important academic significance and application prospects.