Biological Denitrification in Microbial Fuel Cells

Microbial fuel cells (MFCs) that remove carbon as well as nitrogen compounds out of wastewater are of special interest for practice. We developed a MFC in which microorganisms in the cathode performed a complete denitrification by using electrons supplied by microorganisms oxidizing acetate in the anode. The MFC with a cation exchange membrane was designed as a tubular reactor with an internal cathode and was able to remove up to 0.146 kg NO₃^--N m^-3 net cathodic compartment (NCC) d^-1 (0.080 kg NO₃^--N m^-3 total cathodic compartment d^-1 (TCC)) at a current of 58 A m^-3 NCC (32 A m^-3 TCC) and a cell voltage of 0.075 V. The highest power output in the denitrification system was 8 W m^-3 NCC (4 W m^-3 TCC) with a cell voltage of 0.214 V and a current of 35 A m^-3 NCC. The denitrification rate and the power production was limited by the cathodic microorganisms, which only denitrified significantly at a cathodic electrode potential below 0 V versus standard hydrogen electrode (SHE). This is, to our knowledge, the first study in which a MFC has both a biological anode and cathode performing simultaneous removal of an organic substrate, power production, and complete denitrification without relying on H₂-formation or external added power.