Applicantion Of Autotrophic Nitrogen Removal Driven By Fe(Ⅱ)/Fe(Ⅲ) Cycle In Eutrophic Water

Deep nitrogen removal from eutrophic water with low nitrogen concentrations and C/N ratios needs to be addressed.Constructed wetlands as a comprehensive bioremediation technology are effective for nitrogen removal from natural water bodies,however,traditional nitrification-denitrification based biological nitrogen removal techniques are limited by low efficiency and insufficient carbon sources,and are unable to consistently provide the nitrite substrate required for Anammox processes.Iron,the fourth most abundant element on Earth,is typically present in the Earth’s crust in the redox state as Fe（II）and Fe（III）,and its redox reactions can drive global biogeochemical cycles of carbon,nitrogen,oxygen and sulphur.Two newly detected autotrophic biological nitrogen removal techniques,Fe（III）allochthonous reduction coupled with anaerobic ammonia oxidation（Feammox）and nitrate-dependent Fe²⁺oxidation（NDFO）,have been detected in natural habitats to achieve nitrogen removal in the biogeochemical cycle of iron and nitrogen.The continuous injection of iron sources into eutrophic waters is undesirable,and the formation of a dense oxide shell on the cell surface during long-term operation can lead to sludge mineralisation problems,making it difficult for individual Feammox or NDFO reactors to operate stably for long periods,limiting the application and dissemination of relevant microbial enrichment and related technologies.In this thesis,a new functional biocarrier modified with nanoscale iron powder was developed to drive two new iron-nourishing autotrophic bacteria（Feammox and NDFO）to achieve simultaneous removal of ammonia and nitrate from eutrophic water through iron recycling（Fe（II）/Fe（III）cycle）.Two additional microbial activity promoters,biochar and zeolite,were added separately and mixed with iron powder to investigate the differences in the performance of modified functional biocarriers in promoting nitrogen removal at different mixing ratios.The best modified functional carrier（BC1,Fe:BC=1:1）was then used as the functional substrate of the wetland,and four sets of tidal flow artificial wetlands in different modes were constructed.By adjusting the operating parameters and changing the water concentration,stable nitrogen removal was finally achieved,and the nitrogen removal rate and stability,the spatio-temporal nitrogen conversion pattern in a typical cycle and the functional microbial activity were investigated.The main research contents and conclusions are as follows:In this study,a new iron-based modified functional biocarrier was formed by loading nanoscale iron powder onto K3 filler,and an autotrophic biological nitrogen removal with Fe（II）/Fe（III）as the catalyst was constructed by two iron-autotrophic bacteria based on the collaboration of iron minerals.The experimental results showed that the carriers loaded with iron powder as the solid phase iron source achieved simultaneous removal of ammonia nitrogen and nitrate from eutrophic water with a total nitrogen removal rate of nearly 60%after a long-term domestication culture of 70 days,while the total nitrogen removal rate of the control group was less than 30%,demonstrating that the addition of iron promoted the removal of total nitrogen.Both valence and structure changed before and after application,verifying the involvement of microorganisms in the biochemical cycle of iron,indicating the presence of Feammox and NDFO processes.In addition,the SEM image changes show that the carrier catalyzes the attachment of microorganisms and the efficient use of iron after long-term incubation.The results of the allosteric activity assay showed that the activity of NDFO in the iron-based modified functional biocarrier system varied in the range of 1.019-5.903mg N/g VSS/d,while the activity of Feammox was relatively small,varying from 0.082-0.699mg N/g VSS/d.These results demonstrate the presence of functional bacteria in the system and their role in nitrogen removal.The above results demonstrate the presence of functional bacteria in the system and their role in nitrogen removal.Similarly,in situ experiments under different influent nitrogen conditions also showed the removal of nitrate and ammonia nitrogen,indicating autotrophic denitrification dominated by NDFO and Feammox.The carrier（BC1）was modified with a 1:1 mass ratio of biochar to iron powder to show optimal activity enhancement.In this study,the carrier was co-modified with zeolite/biochar powder and iron powder in different mass ratios to enhance the biological activity of functional bacteria.The final results showed that BC1 showed the best performance in terms of enhanced denitrification effect.The overall denitrification performance of the modified carriers in the biochar group was better than that of the zeolite group,because biochar as an electron shuttling agent could enhance the bioavailability of the solid-phase iron source and promote the substrate utilisation by the ferrotropic functional bacteria.The results of adsorption experiments showed that biochar could adsorb both ammonia and nitrate（saturation adsorption capacity of 1652mg/g for ammonia）,which increased the nitrogen concentration in the surface biofilm microenvironment and accelerated the reaction rate.The results of the allosteric activity experiments showed that the zeolite group was close to the biochar group in terms of ammonia nitrogen oxidation performance,which is the reason why the higher ammonia nitrogen adsorption performance of zeolite specifically enhanced the Feammox process.High throughput sequencing results showed that Proteobacteria showed the highest relative abundance in most samples（29.61%-50.52%）and were effectively enriched in the sludge of the iron-based modified carrier system,a similar situation occurred in Acidobacteriota.Some of the dominant microorganisms in the inoculated sludge at the genus level were gradually eliminated after a long period of domestication,while the typical NDFO bacterium Thermomonas became dominant in most of the iron-based modified carrier systems（3.40%-13.10%）,and Comamonadaceae,Citrifermentans,Ellin6067 and The results of the PCA and PCo A plots show a positive correlation between the trend of sample distribution and Fe content,i.e.Fe addition was the most important cause of the differences in microbial community succession.The main reason for the differences in microbial community succession was the addition of Fe.A tidal-flow constructed wetland using iron-based modified functional biocarriers as substrates was constructed to achieve autotrophic biological nitrogen removal from eutrophic water bodies.Four different types of tidal-flow constructed wetlands were constructed,distinguished by the mode of operation and the arrangement of functional substrates.Long-term results show that CWs-C（dosed and mixed fill）has the highest and relatively most consistent total nitrogen removal performance.The highest total nitrogen removal efficiency was 88.03%during the stable operation of StageⅢ,and the effluent total nitrogen was less than 1mg/L at influent ammonia and nitrate concentrations of 2.03mg/L and 4.18mg/L respectively,meeting the nitrogen requirements for level III in The Surface Water Environmental Quality Standard（GB3838-2002）,demonstrating that tidal-flow constructed wetlands based on modified functional carriers can be an effective means of biological nitrogen removal.This demonstrates that the tidal-flow constructed wetland based on modified functional substrate can be an effective biological nitrogen removal method to achieve energy-saving and efficient nitrogen removal from eutrophic water with low nitrogen concentration under low temperature environment.