Sponge iron strengthens the activity of anammox biofilm under low nitrogen conditions in a two-stage fixed-bed biofilm reactor

Strengthening the activity competitiveness of anaerobic ammonium oxidation (anammox) bacteria (AnAOB) under low nitrogen conditions is indispensable for mainstream anammox application. This study demonstrates that sponge iron addition (42.8 g/L) effectively increased apparent AnAOB activity and extracellular polymeric substance (EPS) production of low load anammox biofilms cultivated under low (influent of 60 mg N/L) and even ultra -low (influent of 10 mg N/L) nitrogen conditions. In -situ batch tests showed that after sponge iron addition the specific AnAOB activity in the low and ultra -low nitrogen systems further increased to 1.18 and 0.47 mmol/g VSS/h, respectively, with an apparent growth rate for AnAOB of 0.011 +/- 0.001 d-1 and 0.004 +/- 0.001 d-1, respectively. The averaged EPS concentration of anammox biofilm in both low (from 35.84 to 71.05 mg/g VSS) and ultra -low (from 44.14 to 57.59 mg/g VSS) nitrogen systems increased significantly, while a higher EPS protein/polysaccharide ratio, which was positively correlated with AnAOB activity, was observed in the low nitrogen system (3.54 +/- 0.34) than that in the ultra -low nitrogen system (1.82 +/- 0.10). In addition, Candidatus Brocadia was detected as dominant AnAOB in the anammox biofilm under the low (12.2 %) and ultra -low (24.7 %) nitrogen condition. Notably, the genus Streptomyces (26.3 %), capable for funge-like codenitrification, increased unexpectedly in the low nitrogen system, but not affecting the nitrogen removal performance. Therefore, using sponge iron to strengthen AnAOB activity under low nitrogen conditions is feasible, providing support for mainstream anammox applications.