Progress in Nitrogen Removal in Bioelectrochemical Systems

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

• Within the paper NaN₃ inhibitor increased nitrogen removal rate but simultaneously increased N₂O yield, suggesting NOB inhibition suppresses both nitrite oxidation and reduction pathways.

  One side

  Addition of NaN₃ increased total nitrogen removal by 50% to 0.075 ± 0.008 kg m⁻³ d⁻¹

  “With the addition of NaN3, for example, the total nitrogen removal was increased by 50% to the highest removal rate of 0.075 ± 0.008 kg m⁻³ net cathode chamber d−1”

  Other side

  Addition of NaN₃ increased N₂O yield by 5.6, indicating inhibitor-driven NOB suppression also limited nitrite reduction

  “However, a recent research revealed that the addition of NaN3 decreased the nitrate production, while it increased the N2O yield by 5.6, demonstrating the addition of an inhibitor limited the NOB activity and also, to a certain extent, the ”

  Authors’ reading. Authors acknowledge that NOB inhibition is a double-edged sword: it prevents NO₃⁻ formation but also impairs NO₂⁻ reduction, diverting electrons to N₂O. This suggests NOB and denitrifiers share overlapping enzymatic pathways or that NOB inhibition destabilizes the biofilm redox balance.

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

• High No systematic study of cathode potential (vs. SHE or Ag/AgCl) operating windows for nitrogen species reduction. Table 1 lists theoretical potentials (NO₃⁻/NO₂⁻ = +0.233 V, NO₂⁻/NO = +0.150 V, N₂O/N₂ = +1.155 V vs. Ag/AgCl) but paper does not report actual applied potentials or show how potential affects N₂O vs. N₂ selectivity.

  Section 2.1 and Table 1 present thermodynamic potentials but no experimental data on applied cathode potential; Section 3 mentions N₂O production in inner biofilm but does not link it to potential control. This is critical for preventing N₂O accumulation.

  Try. Vary applied cathode potential from −0.3 to −0.8 V vs. Ag/AgCl in a controlled MEC; measure N₂O, N₂, NO₂⁻, and NO₃⁻ in effluent and gas phase; correlate with biofilm norB gene expression.

• High Catholyte pH is mentioned as a factor affecting biofilm stability (Section 2.3: 'minimized impact of pH') but no data on optimal pH range or pH drift during operation. Nitrification and denitrification have different pH optima; no study reported here explores pH control strategy.

  Section 2.3 states three-chamber design minimizes pH impact but provides no pH values, drift rates, or buffering strategy. Equations (1)–(5) show H⁺ production/consumption but no pH management protocol is described.

  Try. Operate SND reactor at pH 6.5, 7.0, 7.5, 8.0 with and without buffer; measure NH₄⁺ removal rate, NO₂⁻/NO₃⁻ ratio, N₂O yield, and biofilm composition over 30 days.

• High N₂O accumulation is mentioned (Section 3: 'norB genes were more abundant within the inner layer' and 'N₂O was mainly produced within the inner portion') but no quantitative data on N₂O concentration, yield (mol N₂O per mol NO₃⁻ reduced), or operating conditions that minimize it.

  Section 3 identifies N₂O production in inner biofilm but provides no N₂O measurements, yields, or mitigation strategies. This is critical for environmental compliance (N₂O is a potent greenhouse gas).

  Try. Measure N₂O in gas and liquid phases using GC-MS; vary biofilm thickness, DO, and cathode potential; report N₂O yield (%) and identify threshold conditions for <5% N₂O.

• Medium Current density range for optimal nitrogen removal is not systematically explored. Table 2 and 3 report current densities of 4.10 A m⁻² (ref. 46) and applied voltages (0.70–0.80 V) but no study varies current density across a wide range (0.1–10 A m⁻²) to identify performance plateau or mass transfer limitation.

  Section 2.2 reports current density 4.10 A m⁻² for one system but no paper in the review systematically scans current density. This is essential for scale-up and energy optimization.

  Try. In a dual-chamber MEC, vary current density from 0.5 to 10 A m⁻² at fixed HRT = 24 h; measure NH₄⁺ removal rate, NO₂⁻/NO₃⁻ ratio, power consumption, and biofilm thickness.

• Medium Biofilm thickness is stated as critical (Section 3: 'thickness of the biofilm should be high enough to form an oxygen gradient') but no quantitative target thickness is provided, and no study reports actual biofilm thickness measurements or growth kinetics.

  Section 3 qualitatively describes need for thick biofilm but provides no thickness data, growth rates, or substrate design rules. This limits reproducibility and scale-up.

  Try. Measure biofilm thickness weekly using OCT or cross-section microscopy in carbon felt, membrane, and electrode materials; correlate thickness with DO penetration depth and nitrogen removal rate.

• Medium Anode biofilm composition and its role in electron supply is not characterized. Paper focuses on cathode biofilm but does not report anode microbial community, electron transfer rate, or how anode design affects cathode nitrogen removal performance.

  Section 2.4 mentions anammox sludge as anode catalyst (14.9% improvement) but no systematic study of anode biofilm optimization. Anode performance directly limits cathode electron availability.

  Try. Compare anode biofilms (acetate-fed, anammox sludge, mixed inoculum) in identical dual-chamber MEC; measure anode potential, current output, and cathode nitrogen removal rate; profile anode microbial community.