Carbon Aerogel Coated Air-cathode Bioelectrochemical System for Simultaneous Nitrogen and Phosphorus Removal

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

• High No measurement of N₂O or N₂ gas production; only liquid-phase nitrogen species tracked. CANDO process hypothesis (partial nitrification → N₂O → N₂) cannot be validated.

  Authors claim 'CANDO is the most probable process which occurred in this BES system' but provide no gas chromatography, isotope labeling, or headspace analysis. Nitrite accumulation and incomplete denitrification suggest N₂O may be a byproduct.

  Try. Measure headspace N₂O and N₂ by GC-MS or membrane inlet MS; operate at controlled cathode potentials (-0.2 to -0.6 V vs. SHE) to favor different reduction endpoints (NO₃⁻ → NO₂⁻ vs. NO₂⁻ → N₂O vs. N₂O → N₂).

• High No systematic variation of cathode potential or current density; only external resistance (100 Ω) and polarization curves reported. Operating window for nitrogen reduction vs. oxygen reduction not mapped.

  Electrochemical denitrification kinetics depend critically on cathode potential. Paper shows LSV peaks at +17 and -13 mV vs. Ag/AgCl but does not correlate these to nitrogen species reduction. High DO (12 mg/L) in aeration chamber suggests oxygen reduction dominates.

  Try. Potentiostatic or galvanostatic operation at cathode potentials from -0.1 to -0.8 V vs. SHE; measure N-NO₃⁻, N-NO₂⁻, N₂O, and N₂ at each potential and current density (0.5–5 mA/m²).

• Medium No investigation of C/N ratio effects on nitrogen removal. Influent COD (677 mg/L) and TN (63.8 mg/L N-NH₄⁺ + 6.66 mg/L N-NO₃⁻ ≈ 70.5 mg/L) give C/N ≈ 9.6, but no variation tested.

  Text states 'DO's concentration, pH, carbon/nitrogen ratio, and electricity generation in anode are the main factors that can affect nitrogen removal in BESs' but RSM only varied temperature, pH, and HRT. C/N ratio is known to limit denitrification kinetics.

  Try. RSM or factorial design with C/N ratio (5, 10, 15, 20) as a fourth variable; measure TN removal, power density, and N₂O accumulation.

• High Aeration chamber operated at high DO (12 mg/L) throughout; no test of anoxic or low-DO operation to suppress nitrification and favor denitrification.

  Authors observe that 'Denitrification was less likely to occur because the aeration chamber's DO was at a high level (12 mg/L)' and that nitrification dominates post-aeration. Yet they do not test anoxic aeration chamber or intermittent aeration.

  Try. Operate aeration chamber under anoxic conditions (N₂ sparging) or with intermittent aeration (e.g., 2 h aeration / 2 h anoxic); measure TN removal, N₂O, and power density.

• Medium No long-term stability or fouling assessment. Six-month operation reported but no analysis of biofilm composition, electrode degradation, or performance drift over time.

  Text mentions 'the development of biofilm on the cathode and its effect on the catalytic activity of carbon' as a cause of current decline but provides no SEM time-series, impedance spectroscopy, or microbial community profiling.

  Try. Monthly SEM, electrochemical impedance spectroscopy (EIS), and 16S rRNA gene sequencing of anode and cathode biofilms; track voltage, current, and nitrogen removal efficiency over 12 months.

• Medium pH buffering not addressed; anode pH drifted from 7.6 to 6.8 per cycle, potentially inhibiting exoelectrogens. No buffer system or pH control tested.

  Text notes 'Low pH in the anode chamber had an inhibitory effect on exoelectrogenic bacteria activities and resulted in decreasing the electricity production during the last days of each batch cycle.' Yet no mitigation (buffer, pH control, or longer HRT) was tested.

  Try. Add bicarbonate buffer (50–100 mM) to anode chamber or operate at longer HRT (60–80 h) to reduce pH drift; measure voltage stability, current density, and TN removal.