Anaerobic Ammonium Removal Driven By Fe(Ⅱ)/Fe(Ⅲ) Cycle

Feammox,i.e.,Fe（Ⅲ）reduction coupled to anaerobic ammonium oxidation to Fe（Ⅱ）and N₂/NO_x^-,has been reported to play an important role in the nitrogen cycle in natural environments such as riparian zone,marine,wetland,tropical forest and paddy soil.However,Feammox has been rarely investigated in the wastewater treatment because the process is slow and requires a large amount of Fe（Ⅲ）.To explore the possible application of Feammox in wastewater treatment,Fe（Ⅲ）was added to the anaerobic digester to investigate its effect on nitrogen removal,and AQDS was used to improve the efficiency of Feammox.Then,the transformation of nitrogen/iron species,the possible reactions and their potential effects on nitrogen removal during the long-term Feammox process were investigated.In addition,for the purpose of iron cycle,intermittent NO_x^-supplement or air aeration into the Feammox system were applied to oxidize Fe（Ⅱ）generated from Feammox to Fe（Ⅲ）,then the produced Fe（Ⅲ）participated in the next round of Feammox,again and again,leading to continuous nitrogen removal through the Fe（Ⅱ）/Fe（Ⅲ）cycle.Through the studies,it was expected to reveal the route,strengthening measures and operating mechanism of Feammox in the anaerobic reactor,and to form a novel in-situ removal of ammonium during anaerobic digestion driven by iron cycle.The main results are as follows:（1）To investigate the effects of supplementing Fe（Ⅲ）on nitrogen removal,Fe₂O₃ and Fe（OH）₃ were supplemented respectively into anaerobic digestion of sludge.The results showed that NO₂’ was generated in the Fe（Ⅲ）-added reactors.20.1%and 11.3%of total nitrogen were removed in Fe（OH）₃-added and Fe₂O₃-added reactor after 40 days.The amount of theoretical nitrogen removal based on the produced Fe（Ⅱ）amount was much lower than the actual nitrogen removal,which meant that the Fe（Ⅱ）/Fe（Ⅲ）cycle may result in additional nitrogen removal.Additionally,AQDS was used to promote Feammox.During 118 days,AQDS increased the efficiency of nitrogen removal by 18.3 percent.AH2QDS,the reduced state of AQDS,was detected in the AQDS-added reactor,suggesting that AQDS/AH2QDS acted as electron shuttle between ammonium and Fe（Ⅲ）.（2）To investigate transformation of nitrogen and iron species of Feammox during anaerobic digestion,ferrihydrite（similar to Fe（OH）₃）was supplemented into an anaerobic digester.Results showed that the nitrogen removal efficiency after 63 days reached 69.5%.X-ray diffraction（XRD）analysis showed that magnetite（Fe₃O₄）and akageneite（FeOOH）were detected in the ferrihydrite-added reactor after the experiment,which may be caused by Fe（Ⅱ）oxidation by NO_x^-produced from Feammox.Further study showed that Fe（Ⅱ）could be oxidized by NO_x^-in the inoculants taken from the ferrihydrite-supplemented group,and produced Fe（Ⅲ）could provide the possibility for further Feammox.Microbial analysis showed that iron-reducing bacteria and iron-oxidizing bacteria were both detected in two groups,further demonstrating that iron cycle occurred.（3）The NO_x^-and Fe（Ⅱ）produced from Feammox subsequently react to regenerate Fe（Ⅲ）that potentially stimulates next round of Feammox.However,because NO_x^-is not the dominant product during Feammox（N₂ is the dominant product）,thus Feammox couldn’t be lasting if only relying on the produced NO_x^-from Feammox.Intermittent NO_x^-addition could supplement the deficiency of NO_x^-from Feammox,and more Fe（Ⅲ）regeneration and successive nitrogen removal were likely achieved.The results showed that periodically adding NO₃^-caused regeneration and consumption of Fe（Ⅲ）.Consequently,nitrogen removal efficiency of the digester with an initial total nitrogen of 1036.7 mg/L reached 90.1%after 98-day operation.Similar results were obtained by replacing NO₃^-with NO₂^-:intermittent NO₂^-addition into the Feammox system also caused the Fe（Ⅱ）/Fe（Ⅲ）cycle and resulted in the removal of 96.0%of the NH₄⁺-N.Isotope experiment between 14NO₂^-and 15NH₄⁺ demonstrated that the production rates of 30N₂ and 29N₂ in the 14NO₂^-added group were 12 and 106 folds higher than those in the control（without 14NO₂^-）,respectively,confirming that 14NO₂^-induced Fe（Ⅱ）oxidation to participate in Feammox for 15NH₄⁺ removal.Anammox bacteria was not detected in the NO_x^--added reactors,and iron-reducing bacteria were enriched,suggesting that anammox could be excluded.In addition,it was found that the Fe（Ⅱ）oxidation after NO₃^-addition was caused by the NO₂^-generated from the denitrification of NO₃^-.The results indicated that Fe（Ⅱ）oxidation induced by NO₂^-and Fe（Ⅲ）reduction induced by NH₄⁺（Feammox）（Fe（Ⅱ）/Fe（Ⅲ）cycle）drove a new in-situ removal process of ammonium.This process was driven by the Fe cycle without the participation of anammox bacteria,and NO₂^-was the terminal electron acceptor and NH₄⁺was the electron donor.（4）In addition to NO_x^-,O₂ could also oxidize Fe（Ⅱ）.Therefore,by controlling the aeration time,aeration rate and pH,O₂ pulse into the Feammox system could likely rapidly oxidize Fe（Ⅱ）,and forward the successive Feammox and NH₄⁺ removal.The results showed that after aeration,dissolved oxygen（DO）increased from 0 to 0.4 mg/L,and Fe（Ⅱ）content decreased rapidly.After 1 day of aeration,DO could not be detected,while Fe（Ⅱ）content increased,indicating that Fe（Ⅲ）generated by O₂ oxidation was reduced again.XRD showed that the generated Fe（Ⅲ）was Fe（OH）₃.On the 90th day,the content of NH₄⁺-N in the aeration reactor was only 10.2 mg/L,while it remained around 288.3 mg/L in the control group.Moreover,anammox and nitrifying microorganisms were undetected,while the iron-reducing bacteria were enriched in the aeration reactor,indicating that anammox and nitrification/denitrification could be excluded in the system.Actually,the pathway of nitrogen removal was driven by Fe（Ⅱ）/Fe（Ⅲ）cycle,with O₂ as the terminal electron acceptor for NH₄⁺ oxidation.