Study On Enhanced Performance Of Biological Nitrogen Remvoal By Supported Nano-zero-valent Iron Composite Packing

The unsatisfactory nitrogen removal effect is the main problem of low C/N ratio wastewater treatment process.However,the autotrophic denitrification technology is the important ways to solve this problem,which has become a research hotspot in the field of biological nitrogen removal.Among them,iron-based denitrification technology is favored by scholars because of its advantages of environmental friendliness,cheap and easy acquisition,and non-toxic effects of reaction products,etc.However,the easy agglomeration and oxidation of reduced iron(Fe⁰/Fe²⁺)is the main obstacle to the successful realization of iron-based autotrophic denitrification process.For this problem,the green synthesis was used to prepare supported NZVI composite packing in this study.The nitrogen removal effect of supported NZVI composite packing was studied and optimized on treating low C/N ratio wastewater.The mechanism of enhanced biological nitrogen removal by supported NZVI composite packing was further revealed by the perspectives of chemometrics,physicochemical properties of sludge and microbial community structure,etc.The relevant research results not only provide new ideas for the study of autotrophic denitrification of wastewater,but realize the efficient denitrification of nitrogen removal in low C/N ratio wastewater,which is of great academic significance and practical value.The main research contents and results are as follows:（1）Preparation and nitrogen removal performance of supported NZVI composite packing.The supported NZVI composite packing was successfully synthesized by liquid phase reduction using chemically modified polyethylene（PE）as the supported material.The better preparation conditions were 50°C,p H=5.0,4.0 g/L Fe²⁺and 7.2 g/L tea polyphenol（TP）,at which the iron loading was 3.51±0.12 mg/g.The result of nitrogen removal effect showed that the nitrate（NO₃^-–N）removal rate of supported NZVI composite packing was 25.54±0.21%.When microorganisms were present,the NO₃^-–N removal rate was 79.88±0.17%.Combined with SEM、FTIR and XPS,the results showed that the surface of NZVI abundantly presented in oxygen-containing functional groups（C–O and O–H）by encapsulation of TP,which improved the antioxidant properties of NZVI.Meanwhile,the encapsulation of TP also could restrict the electron transfer process between inner NZVI and NO₃^-–N,which further limited the process of NO₃^-–N reduction by NZVI.However,the TP"shell"of TP-NZVI could be oxidatively degraded by microorganisms.The inner NZVI was exposed and restored its reduction activity,which was benefited to the electron transfer between NZVI and NO₃^-–N,and thus further improved the nitrogen removal efficiency of supported NZVI composite packing.（2）Effect of addition sites of the supported NZVI composite packing on the nitrogen removal performance.The A/O–MBR in treating simulated domestic wastewater was studied in the paper,and the supported NZVI composite packing was added into the anoxic tank,the aerobic tank,and the membrane effluent end of the process,respectively.The effect of the different addition sites on the nitrogen removal performance was investigated.The results showed that the supported NZVI composite packing could enhanced nitrogen removal under different adding sites.But the enhanced effect showed significant difference.When the supported NZVI composite packing was added to the aerobic tank,the enhancement effect was the best.The removal rate of TN was increased by 31.71%.Meanwhile,the oxidation rate of NZVI was the fastest by 0.41±0.11 g/（m²·d）.The antioxidant and anti-agglomeration properties of NZVI were improved by encapsulation of tea polyphenols and the support of packing,which extended its service life to 66 days in the aerobic tank.Simultaneously,with addition of the supported NZVI composite packing,the simultaneous nitrification denitrification process was achieved,and the carbon source consumption of denitrification process was reduced in the anoxic tank.Besides,the adsorption and chemical precipitation process of phosphorus was promoted in the system,and achieved synchronous nitrogen and phosphorus removal.（3）Enhanced biological nitrogen removal of low C/N ratio wastewater by supported NZVI composite packing.The effect of low C/N ratio on the enhanced biological nitrogen removal of supported NZVI composite packing was investigated under the optimization of the adding sites.Due to add the supported NZVI composite packing,the nitrogen removal was stable under low C/N.Compered with the control,the TN removal rate of A/O–MBR was increased by 56.67%under the influent C/N ratio of 1.5.The degradation of tea polyphenol shells was promoted by heterotrophic bacteria in the aerobic tank under low C/N ratio,which accelerated the release of NZVI.The stronger oxidant was generated between NZVI and oxidants(O₂,SO₄^2-,etc.),which could mineralize extracellular polymers（EPS）to increase the ATP and DHA content and microbial activity.The effect of low C/N ratio on the efficiency of heterotrophic denitrification could be mitigated by the NZVI.Moreover,both NZVI and its oxidation products(H₂/Fe²⁺)could be used as electron donors for denitrification process.The activity of AOM,NAP and NIR could be increased by Fe²⁺/Fe³⁺.But,the activity of NXR was inhibited.Combined with the kinetic parameters of heterotrophic bacteria and nitrifying bacteria,the supported NZVI composite packing could not only strengthen the heterotrophic denitrification process,but inhibit the yield of sludge in the anoxic tank.Simultaneously,the fully autotrophic denitrification process of simultaneous shortcut nitrification-NZVI-mediated autotrophic denitrification was realized in the aerobic tank,which was the intrinsic mechanism for stable apparent denitrification efficacy of A/O-MBR at low C/N ratio.（4）Biofilm characteristics of supported NZVI composite packing in treatment of low C/N ratio wastewater.In order to in-depth understand the mechanism of enhanced biological denitrification by NZVI,the changes of biofilm physicochemical properties and microbial community structure on the filler surface were emphatically studied.The results showed that the biofilm on supported NZVI composite packing surface was denser,and the extracellular protein content was decreased,the polysaccharide content was increased,and the presence of numerous electronegative functional groups in the structure led to increase inorganic fraction significant in the biofilm.Due to presence of NZVI and its oxidation products(Fe²⁺/H₂),the many autotrophic microorganisms were found in the biofilm（Hydrogenophaga（1.52%）,Paracoccus（1.46%）,Azospira（1.11%））,which enriched the microbial structure in the biofilm.The TN concentration of the A/O–MBR effluent was 11.83±2.04 mg/L under the influent C/N ratio of 1.5,which complied with the criterion of Integrated Wastewater Discharge Standard（GB 18918–2002,Class A of the First Grade Standard）in China.In addition,after the oxidation of NZVI mainly was transformed to polymer iron complex/coordination compound,which existed in the biofilm and could hardly lead to the mineralization of the biofilm.（5）Bioaugmentation of supported NZVI composite packing source bacterium agent on biological nitrogen removal in low C/N ratio wastewater.An strain iron autotrophic denitrifying bacteria,named 7–6,was isolated from the biofilm on the supported NZVI composite packing.The strain was spherical and Gram-negative.The strain 7–6 was classified as Paracoccus sp.by determination of the 16Sr RNA gene sequence.Batch tests results demonstrated that strain7–6 could simultaneously use both Fe²⁺and Fe⁰ as electron donors for autotrophic denitrification.The continuous experiments results showed that the maximum amount of biofilm（14.68±1.07 mg/g packing）was obtained within 18 days by addition of 7–6,and the operation of the A/O–MBR reactor was realized stable within 21 days.The biofilm amount was maintained at 13.48±0.71 mg/g packing,which was 1.13 times higher than that of the control（11.97±0.45 mg/g packing）.Meanwhile,the concentrations of COD,NH₄⁺–N,TN,and total phosphorus in the effluent of A/O–MBR–2 were 23.11±4.06,0.87±0.22,9.83±2.16,and0.58±0.14 mg/L,which were increased by 7.56%,10.31%,11.44%and 9.37%than the control.Therefore,the addition of strain 7–6 not only shortened the start-up time of the low C/N ratio wastewater treatment process,but further improved the enhanced biological treatment efficiency in low C/N ratio wastewater by the supported NZVI composite packing.