Study On The Effect Of Inoculation Sludge And Iron Source On The Enrichment And Culture Of Iron Ammonia Oxidizing Bacteria

In recent years,China’s sewage discharge has increased year by year,and China’s urban sewage has the characteristics of low COD and low C/N ratio.At present,although the traditional denitrification process is widely used,for wastewater with low C/N ratio,the traditional denitrification process requires an additional carbon source to provide an electron donor for denitrification,which is easy to cause secondary pollution.The development of new nitrogen removal technology based on anaerobic ammonia oxidation process is also limited due to environmental sensitivity（such as p H,temperature,salinity,organic carbon）and difficult access to electron acceptors（NO₂^-）.The iron-ammonia oxidation process（Feammox）with Fe（III）as the electron acceptor to oxidize NH₄⁺-N is an autotrophic denitrification process,which has become a hot spot in scholars’research due to its significant advantages such as no organic carbon source,low cost,low sludge yield,and no greenhouse gas production.At present,some scholars use anaerobic ammonia to oxidize sludge and ordinary activated sludge to initiate iron ammonia oxidation,but there is no study on the difference between different inoculated sludge on the initiation process of iron ammonia oxidation.Iron is a reaction substance for iron ammonia oxidation,and different forms of iron sources will have different effects on microorganisms.Therefore,the effects of different inoculated sludge and iron sources on the enrichment process of iron-ammonia-oxidizing bacteria were studied,and the mechanism of denitrification of long-term operating reactor under optimal conditions was investigated.In this paper,the effects of different inoculated sludge（anaerobic ammonia oxidizing sludge,biological iron sludge,activated sludge）and different Fe（III.）iron sources（iron oxide scale,green ore,Fe（OH）₃）on the enrichment of iron ammonia oxidizing bacteria were studied.Using the best inoculated sludge and the best Fe（III.）iron source,the Feammox reactor was started to study its nitrogen removal performance and explore its nitrogen removal mechanism.The main findings are as follows:（1）The effect of different inoculated sludge on the enrichment of iron-ammonia oxidizing bacteria was investigated.The results showed that iron-ammonia oxidation occurred in the reactors A1（inoculated with anaerobic ammonia-oxidizing sludge）,A2（inoculated with biological iron sludge）and A3（inoculated with ordinary activated sludge）.In terms of ammonia and total nitrogen removal,the anaerobic ammonia oxidation sludge was more suitable as the inoculated sludge for initiating ferric ammonia oxidation.The differences in denitrification performance of each reactor were due to the different dominant bacterial groups in different inoculated sludge.A1 reactor had the best denitrification performance due to a large number of anaerobic ammonia oxidizing bacteria,A2 reactor had better ammonia nitrogen removal performance due to a large number of iron reducing bacteria,but the accumulation of NO₃^--N was more serious,and A3 reactor had the lowest total nitrogen removal rate.The products also differed when different inoculated sludge was used to initiate iron-ammonia oxidation.The accumulation of NO₂^--N and NO₃^--N could be detected in the A1 reactor,but the accumulation of NO₃^--N was dominant in the A2 and A3 reactors.（2）Using anaerobic ammonia oxidation sludge as inoculated sludge,the effects of three different Fe（III）iron sources,Fe（OH）₃,iron oxide and green ore,on the enrichment of Feammox bacteria were investigated.The results showed that the addition of Fe（III）iron source helped to improve the denitrification efficiency of Feammox,and the denitrification effect of the three reactors with additional iron source was higher than that of the blank control group.After stable operation,the ammonia nitrogen removal rate of each reactor was:F2>F1>F3>F0,and the total nitrogen removal rate was:F2>F1>F3>F0.In terms of the removal efficiency of ammonia nitrogen and total nitrogen,iron oxide was a more readily available iron source for microorganisms.Due to the different structure and iron content of Fe（III）iron source,it leads to differences in iron dissolution,which in turn affects the denitrification performance of the reactor.Fe（OH）₃ is non-crystalline iron,with larger specific surface area and easy to dissolve,so the concentration of iron ions are high at the beginning of the reaction;and the redox potential of Fe（OH）₃ is higher than other iron oxides,so it is easier to produce Fe²⁺.The crystal structure of cyanite is stable and not easily used by microorganisms,and the iron content in cyanite is low,resulting in the lowest iron content in the A3 reactor.Iron oxide may be a source of iron that is more easily utilized by microorganisms because the concentration of iron ions in the F2system with the addition of iron oxide was higher than that in the F1 system in the late stage of the reaction,which may be because iron oxide is more easily utilized by microorganisms and Fe²⁺is produced by the Feammox reaction;or it may be that iron oxide has a higher iron content.The products of Feammox reaction in this study were mainly N₂,NO₂^--N,and NO₃^--N,but the content of NO₂^--N was lower.（3）The Feammox reactor was started under the condition that anaerobic ammonia oxidizing sludge was used as inoculated sludge and iron oxide scale was used as trivalent iron source to study the enrichment of iron ammonia oxidizing bacteria during long-term operation.The results showed that the iron ammonia oxidizing bacteria were successfully enriched in the reactor,and the ammonia nitrogen removal rate was up to 56.47%,and the total nitrogen removal rate was up to 43.06%.In this experiment,NO₂^--N and NO₃^--N as the main products of iron ammonia oxidation reaction,are detected in the reactor.The decrease in the rate of removal in the later stages of the reaction may be due to the appearance of crusts on the cell surface.The alkaline p H of the effluent also verifies the presence of iron ammonia oxidation reaction,and p H is also positively correlated with the activity of iron ammonia oxidation.Fluctuations in Fe3+in solution mean that iron cycles between Fe²⁺and Fe³⁺are likely to form during enrichment.Through microbial diversity analysis,the richness and uniformity of microbial communities decreased after enrichment and culture,indicating that the dominant bacteria were enriched during the reaction.Ignavibacteriales PHOS-HE36,Desulfuromonas,MSB-3CB are microorganisms with iron ammonia oxidation function,and the increase in relative abundance indicates that iron ammonia oxidizing bacteria are enriched.The emergence of BRC1,OPB56,Ellin6067 and Limnobacter dominant genera indicated that the microbial flora in the enrichment process underwent great changes.The appearance of Desulfuromonas,which can participate in iron reduction,and Thiobacillus,which can participate in iron oxidation,indicates that there are other biochemical reactions involved in iron during the initiation of iron ammonia oxidation.