The Construction And Performance Of Single-Chamber Air Cathode Microbial Fuel Cells For Denitrification

Microbial fuel cells?MFCs?are devices that use exoelectrogens as catalysts for simultaneous pollutant removal and electricity generation in wastewater treatment.Single-chamber MFCs have the potential for efficient nitrogen removal.For the critical problems of low efficiency and high energy consumption in traditional biological denitrification,single-chamber air cathode MFCs were constructed to treat synthetic wastewater containing different concentrations of nitrate and nitrite,with investigating the electricity generation and nitrogen removal in the MFCs.Simultaneous electricity generation and denitrification was achieved in single-chamber MFCs.The COD removed by denitrification for nitrate and nitrite was 4.33 g COD g^-1 NO₃^--N and 2.60 g COD g^-1 NO₂^--N,respectively.When the substrate concentration was higher than that required for the removal of nitrate and nitrite,nitrate and nitrite resulted in the decrease of the voltages of the MFCs at the denitrification stage,but did not affect the stable voltages and the maximum power densities of the MFCs after the denitrification stage.During the long-term operation,the maximum power densities of the MFCs after the denitrification stage(>25 W m^-3)were stable.The activity of certain non-exoelectrogenic bacteria was inhibited by nitrate and nitrite,resulting in the decrease of the COD removed by non-exoelectrogenic bacteria,thereby contributing to the increase of the coulombic efficiency of the MFCs after the denitrification stage.Both nitrate and nitrite were mainly reduced to nitrogen by denitrification in single-chamber MFCs.The time required for the development of stable denitrification systems in the MFCs extended as the initial nitrate and nitrite concentration increased.The pre-enrichment of exoelectrogens at the anode could accelerate the development of stable denitrification systems in the MFCs.The average nitrate removal rates of the MFCs inoculated and operated under closed-circuit?closed-circuit MFCs?and open-circuit?open-circuit MFCs?conditions increased as the initial nitrate concentration increased.The maximum average nitrate removal rate of the closed-circuit MFCs was up to 12.2±0.6 kg NO₃^--N m^-3 d^-1 at an initial nitrate concentration of 2000 mg NO₃^--N L^-1,which was higher than that of the open-circuit MFCs(7.0±0.2kg NO₃^--N m^-3 d^-1)and those of many traditional biological denitrification systems.As the initial nitrite concentration increased,the average nitrite removal rates of the closed-circuit and open-circuit MFCs first increased and then decreased,with a maximum value of 9.0±0.1 and 5.7±0.1 kg NO₂^--N m^-3 d^-1 at an initial nitrite concentration of 500mg NO₂^--N L^-1,respectively.When the initial nitrite concentration was up to 1000 mg NO₂^--N L^-1,the inhibition of free nitric acid on the activity of denitrifying bacteria led to the continuous decline of the denitrification activity of the MFCs.The denitrification rate of the closed-circuit MFCs was higher than that of the open-circuit MFCs,which was attributed to the bioelectrochemical nitrite reduction by the anodic biofilms of the closed-circuit MFCs.When the MFCs were fed with nitrate,Geobacter and Thauera were the dominant genus of exoelectrogens and denitrifying bacteria at the anodes of the closed-circuit MFCs,respectively.Azoarcus and Thauera were the dominant genera of denitrifying bacteria at the anodes of the open-circuit MFCs.When the MFCs were fed with nitrite,Thauera was the dominant genus of denitrifying bacteria at the anodes of the closed-circuit MFCs.When the initial nitrite concentration was no higher than 300 mg NO₂^--N L^-1 and as high as 500 mg NO₂^--N L^-1,Geobacter and Geoalkalibacter were the dominant genus of exoelectrogens at the anodes of the closed-circuit MFCs,respectively.Azoarcus,Thauera and Paracoccus were the dominant genera of denitrifying bacteria at the anodes of the open-circuit MFCs.As the initial nitrate and nitrite concentration increased,both the relative abundance and biomass of Thauera significantly increased,while the relative abundance of Geobacter significantly decreased,the biomass of Geobacter changed little.The high denitrification rate of the closed-circuit MFCs was attributed to the enrichment of Thauera at the anode and the high relative abundance of nirS from Thauera.The high relative abundance of electrically conductive pili from Geobacter,Geoalkalibacter and Thauera indicated the possibility of syntrophic growth between Geobacter or Geoalkalibacter and Thauera,thereby contributing to the enrichment of Thauera at the anode,the fast development of stable denitrification systems and the high denitrification rate of the closed-circuit MFCs.