Simultaneous Anaerobic Ammonium Oxidation and Electricity Generation in Microbial Fuel Cell: Performance and Electrochemical Characteristics

Specific gaps in this paper, with an experiment that would close each.

• High No systematic variation of HRT (only 4 h tested); nitrogen removal rate and power output may be optimizable by testing HRT from 2–12 h to find the window where kinetics and residence time balance.

  Section 2.2 states 'The hydraulic retention time (HRT) was set to 4 h' with no justification or sensitivity analysis. Maximum removal rate of 3.01 kgN/(m³·d) may not be the true optimum.

  Try. Vary HRT from 2 to 12 h at fixed inlet NH4+-N 200 mg/L; measure nitrogen removal efficiency, volumetric rate, and power density to identify optimal residence time.

• High No investigation of cathode potential or cathode reaction kinetics; cathode potential remained stable at ~865 mV vs. SHE but no optimization attempted.

  Section 3.2 and Figure 4 show 'the cathode potential remained nearly stable (about 865 mV vs. SHE)' and 'the cathodic polarization was not significant', but no exploration of whether different cathode potentials (e.g., 700–1000 mV) could improve overall voltage or power.

  Try. Use potentiostat to set cathode potential at 700, 800, 900, 1000 mV vs. SHE; measure anode potential, output voltage, and power density at fixed inlet NH4+-N 200 mg/L.

• High No testing of external electron mediators (e.g., anthraquinone, phenazine) to reduce anode charge transfer resistance, despite authors identifying it as the primary bottleneck.

  Section 3.3 concludes 'adding artificial electron mediators in the electrolyte' as a potential solution but provides no experimental data. Charge transfer resistance is 59–68% of anode resistance.

  Try. Add 0.1–1 mM anthraquinone-2,6-disulfonate (AQDS) or other mediators to anode chamber; measure anode charge transfer resistance via EIS and power density at inlet NH4+-N 200 mg/L.

• Medium No measurement of N2O or other nitrogen oxide intermediates; only NH4+-N, NO2−-N, NO3−-N, and TN reported, leaving incomplete nitrogen balance and potential N2O emissions unquantified.

  Section 3.1 reports stoichiometry matches anammox reaction (Equation 1) but does not measure N2O, which is a known byproduct of anammox under certain conditions and a potent greenhouse gas.

  Try. Use gas chromatography or membrane inlet mass spectrometry to quantify N2O in anode off-gas at inlet NH4+-N 100, 200, 250 mg/L; correlate with anode potential and current density.

• Medium No variation of NH4+-N to NO2−-N ratio; ratio fixed at 1:1.32 (stoichiometric for anammox) but no exploration of sub- or super-stoichiometric ratios and their effect on power or byproduct formation.

  Section 2.2 states 'The ratio of NH4+-N to NO2−-N was fixed at 1:1.32' with no justification. Deviations from stoichiometry might reveal kinetic limitations or reveal whether NO2− or NH4+ is rate-limiting.

  Try. Test NH4+-N:NO2−-N ratios of 1:1.0, 1:1.32, 1:1.5, 1:2.0 at fixed total inlet N = 200 mg/L; measure removal efficiency, power density, and effluent NO2−-N and NH4+-N to identify limiting substrate.

• Medium No investigation of anode potential control or optimization; anode potential allowed to drift passively with current, not actively set to maximize power or minimize charge transfer resistance.

  Polarization curves (Figure 6B) show anode potential varies from ~600 mV to ~900 mV vs. SHE across current densities, but no potentiostatic control attempted to find optimal anode potential for anammox bacteria.

  Try. Use potentiostat to set anode potential at 400, 500, 600, 700 mV vs. SHE; measure nitrogen removal rate, power output, and anode charge transfer resistance via EIS at each potential.