Transformation of Nitrogen and Iron Species during Nitrogen Removal from Wastewater via Feammox by Adding Ferrihydrite

Fe(III) reduction coupled to anaerobic ammonium oxidation, namely, Feammox, plays an important role in the Fe/N cycle in natural environments. However, it has been rarely studied in wastewater treatment systems. To date, transformation of nitrogen and iron species of Feammox during anaerobic digestion remain unknown. In this study, ferrihydrite was supplemented (50 mM) in an anaerobic digester to remove ammonium through Feammox. Results showed that the ammonium removal efficiency after 63 days reached 69.49%, significantly higher than that of the control (35.63%). X-ray diffraction analysis showed that ferrihydrite was transformed into magnetite and akaganeite after the experiment. Further study found that Fe(II) could be oxidized by NOx- in the inoculants taken from the ferrihydrite-supplemented group; i.e., possible NOx--dependent Fe(II) oxidation could provide the possibility for further Feammox. Microbial analysis showed that iron-reducing bacteria and iron-oxidizing bacteria were both detected in two groups.