The effect of ammonia concentration on the treatment of bio electrochemical leachate using MFCs technology

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

• Within the paper Paper claims anammox is the primary nitrogen removal pathway but reports anammox bacteria comprise only 6% of anode community, while denitrifiers are 6.4% and electrogenic bacteria 18.2%.

  One side

  'Microbial fuel cells can efficiently and cost-effectively execute anammox, a green nitrogen removal process, in landfill leachate' (Abstract)

  “Microbial fuel cells can efficiently and cost-effectively execute anammox, a green nitrogen removal process, in landfill leachate.”

  Other side

  'anammox (6%), denitrifying (6.4%), and electrogenic bacteria (18.2%) making up the bulk of the population' (Abstract); 'In the anammox process, ammonium and nitrite are reacted directly to produce nitrogen gas N₂ (NH₄⁺-N, NO₂⁻–N → N₂)' (Introduction) but anammox is minority

  “anammox (6%), denitrifying (6.4%), and electrogenic bacteria (18.2%) making up the bulk of the population”

  Authors’ reading. Authors emphasize anammox as the key process in the abstract and introduction but the 16S rRNA data shows anammox bacteria are a minor component (6%). The high NO₂⁻ removal (93.17%) is likely driven by heterotrophic denitrifiers (6.4%) and possibly electrogenic nitrifiers, not anammox. The paper does not reconcile this discrepancy; it may reflect that anammox is thermodynamically favorable but kinetically limited, or that the paper overstates anammox's role based on its presence rather than its activity.

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

• High No comparison of nitrogen removal efficiency across different HRTs (tested only 4 h); unclear whether longer HRT (8–24 h) would improve TN removal or shift the NO₂⁻/NO₃⁻ balance.

  Paper states 'HRT was adjusted to 4 h' to accelerate biofilm development but provides no justification or sensitivity analysis. The 75.24% TN removal is lower than NH₄-N or NO₂-N removal alone, suggesting incomplete denitrification; longer HRT might allow more complete NO₃⁻ reduction.

  Try. Operate identical MFCs at HRT 4, 8, 12, 24 h; measure NH₄-N, NO₂-N, NO₃-N, TN removal and power density at each HRT to identify the trade-off between residence time and electricity generation.

• High No measurement of N₂O accumulation or emission; unclear whether the system produces N₂O as a byproduct of denitrification, which would reduce N₂ yield and create a greenhouse gas.

  Paper reports TN removal (75.24%) is lower than NH₄-N removal (87.11%), implying ~12% of nitrogen is unaccounted for; this could be NO₃⁻ accumulation or N₂O/NO emission. No gas chromatography or N₂O sensor data provided.

  Try. Measure headspace N₂O concentration via GC-ECD or real-time N₂O sensor during operation at 400–1600 mg/L NH₄-N; correlate N₂O yield with cathode potential, anode potential, and dissolved oxygen.

• High No systematic variation of cathode potential or anode potential; unclear how electrode potentials affect nitrification vs. denitrification balance and anammox activity.

  Paper operates MFC with fixed external resistance (1 kΩ) and does not report anode or cathode potentials. Anammox and denitrification have different redox requirements; varying cathode potential (e.g., via potentiostat) could shift the pathway.

  Try. Operate MFC at fixed cathode potentials (−0.2, −0.4, −0.6 V vs. Ag/AgCl) while measuring anode potential, power density, and nitrogen speciation (NH₄⁺, NO₂⁻, NO₃⁻, N₂O) to map the operating window for anammox vs. denitrification.

• Medium No measurement of dissolved oxygen (DO) in anode or cathode chambers; unclear whether the anode is truly anaerobic or if oxygen diffusion from the cathode affects nitrification and anammox.

  Paper states 'anode chamber was sparged with nitrogen gas to ensure anaerobic conditions' but does not report DO measurements. Anammox requires anoxia; if DO leaks into the anode, it would inhibit anammox and favor aerobic nitrification.

  Try. Measure DO in anode chamber via oxygen sensor during operation; if DO > 0.5 mg/L, test improved membrane sealing or anode chamber design to maintain strict anaerobiosis.

• Medium No variation of C/N ratio; unclear whether the 13,225 mg/L COD to 800 mg/L NH₄-N ratio (C/N ≈ 16.5 by mass) is optimal for anammox or whether lower C/N would favor anammox over heterotrophic denitrification.

  Paper uses a fixed leachate composition and does not test different C/N ratios. Anammox is autotrophic and does not require organic carbon; heterotrophic denitrifiers do. Lowering C/N might select for anammox.

  Try. Dilute leachate to achieve C/N ratios of 5, 10, 20, 40 (by mass); measure anammox and denitrifier abundance via qPCR and nitrogen removal pathways at each ratio.

• Medium No measurement of catholyte pH or its effect on nitrogen removal; unclear whether pH drift in the cathode (typically alkaline due to oxygen reduction) affects proton availability for nitrification or anammox.

  Paper states 'pH level was measured and adjusted once judged within the permissible range' but does not report pH values during operation or in the cathode chamber. Anammox and nitrification are pH-sensitive; alkaline pH favors free ammonia inhibition.

  Try. Monitor cathode and anode pH continuously; test pH control (e.g., buffer addition) to maintain anode pH 6.5–7.5 and measure impact on nitrogen removal and power density.