Bacterial communities in a bioelectrochemical denitrification system: The effects of supplemental electron acceptors

• Nitrogen removal was enhanced with biocathode in comparison with abiotic cathode.
• Simultaneous reduction of NO3 and NO2 was achieved by bioelectrochemical reduction.
• Electron transport phenomenon of biocathode with different N ions was revealed.
• Firmicutes and Proteobacteria were mostly enriched with NO3 and NO2, respectively.
• Proteobacteria and Bacteroidetes were enriched with a mixture of NO3 and NO2 ions.

Electrochemical treatment of nitrate (NO3−), nitrite (NO2−) and mixtures of nitrate and nitrite was evaluated with microbial catalysts on a cathode in three different bioelectrochemical denitrification systems (BEDS). The removal rates and removal percentage of nitrogen (N) compounds varied during biotic and abiotic operations. The biotic cathode using NO3−-N as an electron acceptor showed enhanced removal percentages (88%) compared to the operation with NO2−-N (85%). The simultaneous reduction of NO3−-N and NO2−-N occurred in the operation with a mixture of N compounds. The bacterial diversity from the initial inoculum (return sludge) changed at the end of bioelectrochemical denitrification operation after 55 days. The microbial community composition was different depending on the type of electron acceptor. BEDS operation with NO3−-N and NO2−-N was enriched with Proteobacteria and Firmicutes respectively. BEDS with a mixture of N electron acceptors showed enrichment with Proteobacteria. There was no clear, distinct microbial community between the cathode biofilm and suspended biomass.