Simultaneous carbon and nitrogen removal accompanied by energy recovery from wastewater in a coupled microbial fuel cells system

For over a decade, microbial fuel cell (MFC) has received much attention as a pioneering technology for wastewater treatment, owing to its ability to generate electricity from organic matter. However, most wastewater has the presence of nitrogen and the requirement to remove nitrogen as a contaminant has caused limitations in energy recovery from organic carbon. Despite the advantages of integrating biological nitrogen removal (BNR) into MFC and the promising results in organic carbon removal, achieving high nitrogen removal efficiency while optimizing energy recovery from organic matter remains a challenge. It is difficult to select an external resistance (ER) value that satisfies both goals. The chambers of a stacked MFC system can be sequencing-batch operated to achieve specific goals. System can configured accordingly with suitable conditions for each chamber to achieve the goal of energy recovery from organic carbon and nitrogen removal.In this study, BNR was integrated into a coupled MFC with four chambers sequencing-batch operated reactor. With ER close to its internal resistance, the first MFC (N-MFC) was responsible for power generation from organic carbon and ammonium oxidation in the input wastewater. The ER of the second MFC (D-MFC) was set at a small value (10 Ω) to have a high current density, facilitating nitrogen removal in presence of minimum carbon as required for the denitrification. The study evaluated the removal efficiency of carbon and nitrogen accompanied by power generation by applying three different sequencing-batch operation modes to four chambers of a coupled MFC. In the first mode, wastewater transferred from the N-MFC anode chamber to the cathode chamber, then to the D-MFC anode chamber, and lastly to the D-MFC cathode chamber. In the second and the third modes, the wastewater was fed into anode chamber of the N-MFC. The D-MFC received the N-MFC output (in order from anode chamber to anode chamber, from cathode chamber to cathode chamber). The output of the cathode chamber of the D-MFC was the effluent of the coupled MFC system, while the output of the anode chamber of the D-MFC came back to the cathode chamber of the N-MFC. The distinction between the second and third modes is the different dissolved oxygen (DO) in the cathode chamber of N-MFC to control the nitrification process. The study provided a solution for optimizing electrical energy recovered from organic matter in parallel with efficient nitrogen treatment and better understanding of integrating BNR process into MFC technology. Depending on each operation mode, the following specific objectives are proposed: (i) to investigate the effect of DO, and initial ammonium concentration for ammonium oxidizing and power generation from N-MFC; (ii) to assess the ability of ammonium diffusion through cation exchange membrane (CEM) in N-MFC; (iii) to investigate the effect of the COD/N ratio for nitrogen removal in D-MFC; (iv) to evaluate the coulombic efficiency of the system; (v) to evaluate the ratio of autotrophic denitrification in cathodic chamber of D-MFC.The following is a summary of the results and findings of this study.(i) In the N-MFC, the second operational mode produced more power than the first operational mode by addressing the flaws that impeded electricity production in the first operational mode. Overall, power generation decreases as DO at the cathode decreases, while variations in nitrogen input showed no great influence on power generation of the N-MFC. Ammonium was completely oxidized to nitrate as the major product with very small amounts of nitrite detected under high DO at the cathode chamber. In the third operational mode with low DO at the cathode chamber of the N-MFC, nitrite was the main product of the ammonium oxidation process. (ii) The input COD of the anode chamber and the DO concentration in the cathode chamber affect the electricity generation of the N-MFC, directly influencing the diffusion of cations (such as ammonium) from the anode chamber to the cathode chamber in order to balance the charge. In addition, the ER of N-MFC in the second operational mode (50 Ω) is lower than that in the third operational mode (100 Ω), resulted in more favorable current production and more ammonium diffusion for charge balance.(iii) The nitrogen removal efficiency at the D-MFC increased when the COD/N ratio of wastewater entering the D-MFC increased. The first operational mode, which used both the anode and cathode chambers for denitrification, enhanced nitrogen removal efficiency. In the second and third operational modes, the denitrification process only occurred in the cathode chamber of D-MFC. The nitrogen removal efficiency at the cathode chamber of the D-MFC was higher in the third operational mode than in the second operational mode for the same COD/N ratio input to the D-MFC, which is the result of a higher reduction rate of nitrite than nitrate.(iv) In the first operational mode, an increase in the COD input of the anode chamber resulted in a decrease in the anodic coulombic efficiency. The anodic coulombic efficiencies of N-MFC in the second operational mode was higher than that in the first operational mode. The anodic coulombic efficiencies of N-MFC in the third operational mode were relatively low because of low DO in the cathode chamber.(v) In all operational mode, the high ratio of autotrophic denitrification in the cathode chamber of the D-MFC demonstrated that autotrophic denitrification was the primary process assisting in nitrogen removal.In conclusion, this study showed that a properly configured and operated coupled MFC can effectively remove carbon and nitrogen with energy recovery from wastewater. By taking advantage of ammonium diffusion across the CEM to improve the operating method, the second operational mode outperformed the first operational mode with respect to power generation and coulombic efficiency. The suitable setup of the system allowed the N-MFC to oxidize over 75% of organic matter input and isolate nitrogen input simultaneously, giving favorable environmental conditions for generating the energy from wastewater primarily in the N-MFC. The nitrogen isolation efficiency of the N-MFC depends on input organic matter of the anode chamber, the DO concentration in the cathode chamber, and the ER. In the second and the third operational mode, the main mechanism for nitrogen removal at the D-MFC was autotrophic denitrification. When comparing the second and the third operational modes, the power generation was higher in the second mode, which followed a conventional nitrification/denitrification system. However, the third mode with shortcut nitrification-denitrification was more energy-efficient and better in nitrogen removal.