Nitrogen removal enhanced by its migration and transformation in a three-chamber microbial electrolysis cell

In this study, a three-chamber microbial electrolysis cell (MEC) was established for the simultaneous migration, transformation and removal of NH4+-N, NO2--N, and NO3--N under different applied voltages. The results showed that higher applied voltages accelerated the migration rate of NH4+-N while had little effect on NO2--N and NO3--N. The highest total nitrogen (TN) removal efficiency was achieved in the MEC system at 1.0 V (61.40 +/- 19.58 %). At the optimal voltage of 1.0 V, nitrogen removal performance at different carbon to nitrogen (C/N) ratios was also discussed. Under different C/N ratios, TN removal efficiencies maintained stable, which suggested that the migration of nitrogen species by applied voltages combined with nitrogen transformation was beneficial for nitrogen removal in MEC systems. Also, different denitrification pathways could be influenced by reasonably allocating carbon sources. Meanwhile, the whole system was enriched with autotrophic and heterotrophic denitrifiers (e.g. Proteobacteria, Patescibacteria, Thermomonas, and Ferruginibacter), implying that heterotrophic denitrification, hydrogenotrophic denitrification and electroactive autotrophic denitrification coexisted and collaborated in this MEC system for nitrogen removal. The "migration-transformation-removal" model of nitrogen was innovatively proposed and meanwhile this proof-of concept study provided a new insight into nitrogen removal in low C/N ratio wastewater.