Study On The Effect Of Iron And Carbon Source On Denitrification Performance Of Iron-based Denitrification

Iron-based denitrification technology is a new wastewater denitrification technology that replaces organic matter with zero-valent iron or divalent iron as the electron donor of denitrification process.This technology has the advantages of environmental friendliness,economic efficiency and multi-effective products,which can effectively solve the problem of denitrification of low C/N wastewater in China.The Fe（III）mineral precipitates generated in the iron-based denitrification process tend to adsorb to the cell surface or accumulate in the periplasm and crust,thus inhibiting microbial activities and physiological metabolism and even causing cell death,thus weakening the performance of the reactor and failing to achieve long-term effective and stable operation.Opening up new Fe（II）sources may be another novel direction for future research in the field of iron-based denitrification.The main component of sponge iron（s-Fe⁰）is zero-valent iron（ZVI）,and its inherent chemical properties and physical structure make it possess a large number of microelectrolytic corrosion system units.While providing electron donors for Fe-type denitrification reactions,sponge iron can also regulate p H and create a more favorable anaerobic environment for the growth and metabolism of denitrifying microorganisms.Therefore,sponge iron may be a suitable electron donor material for iron-based denitrification reaction.In this study,activated sludge from long-term laboratory operation was inoculated in a reactor and sponge iron was injected for the removal of nitrite from low carbon to nitrogen ratio wastewater.The concentrations of ammonia,nitrate and nitrite in the influent and effluent were measured,the removal efficiency was calculated,and the changes in COD,TFe,Fe²⁺concentrations and p H were observed.In this project,we tested the optimal preservation of iron autotrophic denitrification sludge,explored the effect of sponge iron and carbon source on the denitrification effect of iron autotrophic denitrification,evaluated the removal effect of this system with nitrite as the target pollutant,and analyzed the specific role and contribution of both in the iron autotrophic denitrification system,which can provide a basis for improving the removal of nitrogen in water bodies,and the iron reduction coupled denitrification enhanced activated sludge method in engineering and practical It can provide technical support for improving the nitrogen removal in water bodies and the application of iron-reduction coupled denitrification-enhanced activated sludge method in engineering and practice,and provide a more cost-effective solution to the increasingly urgent water environment situation.Specific results of the study are as follows:（1）Effect of different sludge preservation methods on the recovery of iron-type denitrification reactor performance:the reactor with direct sludge freezing preservation did not observe stable nitrogen effluent for up to 70 d of operation cycles and declared reactor startup failure,proving that the low-temperature starvation environment is not conducive to the preservation of iron-type denitrification sludge.The other three reactors did not differ significantly in start-up time and all reached stabilization in about 24 reaction cycles.The average TN removal rates during the stabilization period were as follows:frozen preservation after replacement with 80mg/L nitrite（59.59%）>ambient preservation after replacement with 300 mg/L nitrite（54.07%）>direct ambient preservation（47.62%）>direct frozen preservation（unsuccessful startup）.Under the experimental conditions,cryopreservation after injection of target contaminants was the most effective way of sludge preservation.（2）Effect of iron and carbon source on the start-up and performance of iron-type denitrification reactor:Both sponge iron and carbon source can effectively promote the removal of nitrite and inhibit the conversion of nitrite to nitrate in the reaction system.The addition of sponge iron provided electrons for the iron-based denitrification reaction while ensuring a more stringent anaerobic environment in the reaction system,which provided more favorable growth conditions for denitrifying bacteria.The carbon source,on the other hand,plays an important role in maintaining microbial growth and activity,and enriches the denitrification pathway of the reaction system based on increasing microbial diversity,which can serve as a mixed electron donor for Fe-type denitrification-related microorganisms in addition to additionally enriching some specialized heterotrophic denitrifying bacteria for conventional denitrification.In addition,more active microorganisms would also enhance the MIC process to promote more Fe²⁺leaching as additional electron donor.The simultaneous dosing of iron and carbon source accelerated the reactor start-up,while the dosing of sponge iron or carbon source alone had no significant effect.The start-up times to reach the stabilization period were as follows:control group（43 days）=sponge iron group（43 days）=carbon source group（43 days）>sponge iron+carbon source group（31 days）.The average TN removal rate in the stabilization period was:sponge iron+carbon source group（47.84%）>carbon source group（29.50%）>sponge iron group（14.58%）>control group（8.21%）,respectively.The average removal rate of total nitrogen in the stabilization period in the sponge iron+carbon source group was higher than the sum of the sponge iron and carbon source groups,which indicated that there might be a synergistic effect of sponge iron and carbon source in the system.（3）The effect of iron dosing on the start-up and performance of iron-based denitrification reactor:the start-up time to reach the stabilization period at each dosing amount for C/N=1 was 0 g/L（43 days）>30 g/L（31 days）=30 g/L（31 days）=30 g/L（31 days）,respectively,and unlike the previous case,the dosing of sponge iron in the presence of carbon source significantly promoted the reaction system to reach the stabilization period of The start-up rate was not significantly different at different dosage levels.The average TN removal rate in the stabilization period under each dosage was 30 g/L（52.66%）>50 g/L（52.15%）>40 g/L（47.84%）>0 g/L（29.50%）,and the amount of sponge iron dosage under the experimental conditions did not significantly affect the TN removal effect of the reaction system.（4）The effect of carbon to nitrogen ratio on the start-up and performance of iron-based denitrification reactor:The start-up time to reach the stabilization period at each carbon to nitrogen ratio was C/N=0（43 days）>C/N=0.5（31 days）>C/N=1（31days）>C/N=1.5（25 days）when s-Fe⁰was injected at 40 g/L.In the presence of sponge iron,the injection of carbon source obviously promoted the start-up of the reaction system to reach the stabilization period This promotion effect was more obvious with the increase of carbon to nitrogen ratio.The average TN removal rate in the stabilization period under each carbon to nitrogen ratio was:C/N=1.5（63.92%）>C/N=1（47.84%）>C/N=0.5（28.62%）>C/N=0（14.58%）,and the TN removal rate of the reaction system increased significantly with the increase of carbon to nitrogen ratio.The optimal parameters for TN removal in the iron autotrophic denitrification system were determined based on the s-Fe⁰dosage of 30 g/L and C/N=1.5.The degree of influence of the two factors on the nitrogen removal in the iron autotrophic denitrification system was ranked as C/N>s-Fe⁰dosage.