Effect of nitrate on power generation and nitrogen removal and bacterial community in Microbial fuel cell (MFC)

Tensions the paper itself points to. Each side quotes the passage it comes from.

• Within the paper High NO3-N removal rate coexists with severe NO2-N accumulation, suggesting the removal pathway is incomplete rather than proceeding to N2.

  One side

  NO3-N removal rate of 97.48% at 414 mg/L

  “The removal rate of NO3-N was 97.48%”

  Other side

  NO2-N accumulation of 188.83 mg/L at 414 mg/L, indicating incomplete reduction

  “the lack of anode electron supply resulted in the incomplete reduction of nitrate nitrogen and accumulation of nitrite-nitrogen (188.83 mg/L)”

  Authors’ reading. Authors explicitly attribute NO2-N accumulation to insufficient anode electron supply, not to a failure of the NO3-N removal process itself; the system removes NO3-N but cannot complete the reduction cascade to N2 or NH3.

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

• High No measurement of N2O or N2 gas production; only NO3-N and NO2-N in solution were tracked, leaving the final denitrification product unknown.

  Section 3.3 states 'No ammonia nitrogen was detected' and NO2-N accumulated to 188.83 mg/L at 414 mg/L, but the paper does not report whether NO2-N was reduced to N2O, N2, or remained as NO2-N. This is critical for assessing true denitrification efficiency and N2O emission risk.

  Try. Install gas chromatography or membrane inlet mass spectrometry to measure N2O and N2 production rates across the 138–552 mg/L NO3-N range; correlate with NO2-N accumulation and Thauera abundance.

• High No applied cathode potential control; the system operated at open circuit or fixed external resistance, not at controlled electrode potential.

  Section 3.3 attributes NO2-N accumulation to 'lack of anode electron supply,' but the paper does not test whether applying a more negative cathode potential (e.g., −0.4 to −0.6 V vs. Ag/AgCl) would drive NO2-N reduction to N2. This is a direct lever for improving incomplete denitrification.

  Try. Operate M3 (414 mg/L NO3-N) at fixed cathode potentials from −0.2 to −0.8 V vs. Ag/AgCl; measure NO2-N accumulation, N2O/N2 production, and power output to identify the potential window that minimizes NO2-N while maintaining power.

• Medium No variation of HRT; all experiments used 10 d HRT, which may not be optimal for complete denitrification or for balancing power generation with nitrogen removal.

  Section 2.2 states 'hydraulic retention time per cycle was 10 d,' but no shorter or longer HRT was tested. At 414 mg/L NO3-N, NO2-N accumulated to 188.83 mg/L by end of cycle, suggesting the 10 d window may be insufficient for complete reduction or may allow NO2-N to accumulate before being further reduced.

  Try. Test HRT of 5, 10, 15, and 20 days at 414 mg/L NO3-N; measure NO2-N accumulation profile over time and final N2O/N2 production to identify the HRT that minimizes NO2-N and maximizes complete denitrification.

• Medium No measurement of dissolved oxygen (DO) in the cathode; the paper notes that oxygen entry during sampling caused voltage spikes but does not quantify DO or test anaerobic vs. aerobic cathode operation.

  Section 3.1.1 states 'Oxygen enters the cathode chamber during sampling, and oxygen acts as an electron acceptor in the cathode chamber for reduction reaction,' but the paper does not systematically test how DO concentration affects NO3-N vs. O2 reduction competition or NO2-N accumulation.

  Try. Maintain cathode DO at 0, 1, 2, and 5 mg/L using N2 sparging or air injection; measure NO3-N removal, NO2-N accumulation, and power density to identify the DO window that optimizes denitrification without promoting aerobic heterotrophy.

• Medium No variation of C/N ratio or anode COD concentration; the paper uses a fixed anolyte composition (11.67 g/L citric acid) and attributes NO2-N accumulation to 'lack of anode electron supply,' but does not test whether increasing anode COD improves cathode denitrification.

  Section 3.3 states 'the lack of anode electron supply resulted in the incomplete reduction of nitrate nitrogen and accumulation of nitrite-nitrogen,' but the paper does not experimentally increase anode COD to test this hypothesis.

  Try. At 414 mg/L NO3-N, vary anode citric acid from 5.84 to 23.34 g/L (0.5× to 2× baseline); measure anode CE, cathode CE, NO2-N accumulation, and power density to quantify the C/N ratio window that minimizes NO2-N.

• Low No long-term stability test; the paper reports 10 d cycles but does not show whether performance is stable over multiple cycles or whether biofilm composition and NO2-N accumulation change over weeks or months.

  Section 3.1.1 shows voltage and NO2-N profiles within a single 10 d cycle, but does not report whether the system reaches steady state or whether repeated cycles show drift in performance or microbial community.

  Try. Operate M3 (414 mg/L NO3-N) for 10–20 consecutive 10 d cycles; measure power density, NO3-N removal, NO2-N accumulation, and cathode microbial community composition at cycles 1, 5, 10, 15, and 20 to assess long-term stability and community succession.