Accomplishing a N-E-W (nutrient-energy-water) synergy in a bioelectrochemical nitritation-anammox process

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

• Within the paper Aeration improves power and reduces ohmic resistance, but inhibits anammox and causes nitrite accumulation.

  One side

  NiAmoxMDC at 10 ml/min aeration: 1,007 mW/m³ power, 50.1 Ω ohmic resistance, 92% NH₄⁺-N removal in cycles 1–2

  “The maximum power density produced by NiAmoxMDC (1,007 mW/m³) was higher than that of AnAmoxMDC (444 mW/m³)”

  Other side

  Same system cycles 3–4: NO₂⁻-N accumulation to 10.3 mg/L, NH₄⁺-N removal drops to 78.6%, anammox inhibited

  “accumulation of NO₂⁻-N and decrease in the removal rates of NH₄⁺-N in third and fourth cycles confirmed the inhibition effect on the anammox process due to continued exposure of anammox bacteria to aerobic condition”

  Authors’ reading. Authors attribute this to oxygen toxicity to anammox bacteria over time; the solution is to reduce aeration to 3 ml/min, which restores 75% TN removal but likely reduces power. This is a fundamental trade-off, not a true contradiction, but the paper does not quantify power at 3 ml/min aeration.

• Within the paper Coulombic efficiency is low despite high COD removal, suggesting most organic matter is not converted to electricity.

  One side

  COD removal 72.1% in NiAmoxMDC

  “COD removal efficiency was significantly higher in NiAmoxMDC (ANOVA, p < 0.0001)”

  Other side

  Coulombic efficiency only 25.6% in NiAmoxMDC

  “the CE of NiAmoxMDC was only 25 ± 6.4%, indicating that a substantial portion of COD utilization was not associated with electron release”

  Authors’ reading. Authors explain this is due to glucose fermentation and methanogenesis competing with electrochemical oxidation. This is expected for glucose but limits the energy-positive claim; the paper does not test non-fermentable substrates.

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

• High Aeration rate and dissolved oxygen setpoint were not systematically varied; only two points tested (10 ml/min and 3 ml/min). Optimal DO window for balancing nitritation, anammox activity, and power generation is unknown.

  Paper shows 10 ml/min causes anammox inhibition by cycle 3–4 (NO₂⁻-N rises to 10.3 mg/L), but 3 ml/min restores 75% TN removal. No intermediate points (5, 7 ml/min) or DO targets (0.5, 1.0, 2.0 mg/L) tested. Power density at 3 ml/min not reported.

  Try. Vary aeration rate (2–10 ml/min) or set DO setpoint (0.5–3.0 mg/L) in fed-batch cycles; measure power density, NH₄⁺-N removal, NO₂⁻-N accumulation, and anammox activity (e.g., N₂ production rate) at each point.

• High Substrate choice (glucose) is highly fermentable and suppresses coulombic efficiency to 25.6%. No comparison with acetate or other less fermentable substrates.

  Text states: 'glucose was used as an organic substrate...fermentable substrate, which implies its possible consumption by diverse competing metabolisms such as fermentation and methanogens that do not contribute towards columbic efficiency.' Supplementary Table S3 compares to other nitritation-anammox systems but not to MES systems using acetate.

  Try. Run parallel NiAmoxMDC fed-batch cycles with acetate (10–20 mM) at same aeration (3 ml/min); measure power density, CE, COD removal, and nitrogen removal to determine if substrate choice is the limiting factor.

• Medium Hydraulic retention time (HRT) and batch cycle duration not specified; fed-batch mode means HRT varies. No kinetic data on nitritation or anammox rates.

  Paper states 'operated in fed-batch mode' but does not report cycle duration, volume exchanged per cycle, or HRT. Supplementary Table S2 shows removal rates per cycle but not per hour. Nitritation-anammox kinetics (e.g., specific nitritation rate, anammox rate) not measured.

  Try. Report cycle duration and HRT; measure time-course of NH₄⁺-N, NO₂⁻-N, NO₃⁻-N, and N₂ production within a single cycle to quantify nitritation and anammox kinetics at 3 ml/min aeration.

• Medium N₂ production (the desired end product of anammox) was not directly measured. Nitrogen balance relies on mass balance (input NH₄⁺ + NO₂⁻ minus output NH₄⁺ + NO₂⁻ + NO₃⁻).

  Text states anammox oxidizes ammonium to nitrogen gas, but no N₂ measurement (e.g., gas chromatography, ¹⁵N tracing) provided. Nitrogen recovery is inferred from disappearance of NH₄⁺ and NO₂⁻, but some may be assimilated into biomass or lost as N₂O.

  Try. Measure N₂ production by headspace gas chromatography or ¹⁵N-labeled batch experiments; quantify N₂O production to confirm anammox selectivity and rule out competing denitrification pathways.

• Medium Cathode potential (vs. SHE or Ag/AgCl) during operation not reported; only redox peak from cyclic voltammetry (−140 mV vs Ag/AgCl) provided. Operating cathode potential window unknown.

  CV shows redox peak but not the steady-state cathode potential during fed-batch operation. Polarization curves (Fig. 2B) show voltage but not electrode potentials. No data on how cathode potential affects nitritation vs. anammox selectivity.

  Try. Measure cathode potential (vs. Ag/AgCl reference) during fed-batch cycles at 3 ml/min aeration; correlate with NH₄⁺-N removal, NO₂⁻-N accumulation, and power output to define optimal operating window.

• Low Desalination performance limited by low ion exchange membrane (IEM) surface area ratio (0.8:1 cm²/cm³) and volumetric ratio (1.5 v/v). Scaling factors for salt removal not explored.

  Text states: 'At a higher volumetric ratio of 100 (v/v)...93% salt removal was reported' vs. 53.9% in this study at 1.5 v/v. Authors acknowledge this but do not test higher ratios or larger IEM areas.

  Try. Increase volumetric ratio to 5–10 (v/v) and IEM surface area ratio to ≥2 (cm²/cm³); measure salt removal and power density to determine if desalination can be improved without sacrificing nitrogen removal.