Nitrogen and carbon removal from anaerobic digester effluents with low carbon to nitrogen ratios under feammox conditions

[Display omitted] •AD effluent was treated under feammox conditions in a multistage bioreactor (MSFB).•The feammox was most effective at C/N 2.5, enabling high NH4+ and COD removal.•The higher the C/N, the less efficient the feammox, and the lower the NH4+ removal.•At C/N 3.3 – 5.6, the conventional iron reduction was more effective than feammox.•Iron-reducers Pseudomonas, Geobacter, Acinetobacter were the key players in MSFB. Removal of nitrogen and carbon from anaerobic digester (AD) effluents is challenging for currently available technologies. Herein, effective treatment for real AD effluents was achieved via the feammox process by using a Multistage Feammox Bioreactor (MSFB). The reactor achieved the best performance with AD effluent of a low carbon to nitrogen (C/N) ratio of 2.5. A 6-day retention time reached removal efficiencies for NH4+ and COD at 99 % and 97 %, respectively, with a thorough conversion of NH4+ to N2. Accordingly, the MSFB achieved removal rates for N and C of 14 and 34 mg L−1 d−1, respectively. The C/N ratio of 2.5 is regarded to be the critical point above which the feammox is shifted to conventional iron reduction with organic carbon. Iron-reducing bacteria of the γ- Proteobacteria (Pseudomonas and Acinetobacter), and δ- Proteobacteria (Geobacter) were dominant in the MSFB and were supposed to drive the feammox process.