Improvement of biohydrogen production in microbial electrolysis cell (MEC) system by tackling methanogenesis

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

• Within the paper Cycle 2 shows clear H₂ advantage for denitrifier reactor, but cycle 1 shows no predicted difference despite same conditions.

  One side

  Cycle 1 (69 hrs): reactor A 29.06% H₂, reactor B 24.63% H₂ — 'the predicted production difference was not yet seen in this cycle'

  “However, the predicted production difference was not yet seen in this cycle, which may be caused by either long reactor operation time that made H₂ evolution rate at a slower interval ot observed or simply that the methanogens have yet to b”

  Other side

  Cycle 2 (39 hrs): reactor A 10.85% H₂, reactor B 5.60% H₂ — 'H₂ production is increased by 100%'

  “This result proved the hypothesis in which the presence of the denitrifer inside the reactor is beneficial, in which in this case the H₂ production is increased by 100%.”

  Authors’ reading. Authors attribute cycle 1 lack of difference to either slow H₂ evolution rate at long intervals or delayed methanogen activation; by cycle 2, methanogens became active and denitrifier inhibition became measurable. Shorter sampling interval (39 vs 69 hrs) also improved detection.

• Within the paper Cycle 3 shows near-complete H₂ production failure in both reactors despite fresh medium, contradicting the expectation that nutrient replenishment would sustain production.

  One side

  Cycle 2: reactor A 10.85% H₂, reactor B 5.60% H₂ — both producing

  “Analysis of the headspace taken after 39 hrs of reactor operation is shown in Fig. 5 above. Chromatogram result for reactor C still did not yield any peak of hydrogen as predicted. As consequent analysis proved the premise that the voltage ”

  Other side

  Cycle 3: reactor A 2.95% H₂, reactor B 0% H₂ — production collapsed despite fresh medium added

  “Analysis of the headspace taken after 46 hrs of reactor operation is shown in Fig. 6 above. Only reactor A showed peak for hydrogen at around 0.6, with reactor A's headspace composed of 2.95% H2. H2 peak was not observed with reactor B. It ”

  Authors’ reading. Authors attribute cycle 3 collapse to lag phase entry by denitrifiers or exoelectrogens due to nutrient depletion or accumulation of inhibitory metabolites, despite fresh medium addition. This suggests medium replacement alone is insufficient; the microbial community may require recovery time or removal of accumulated inhibitors (e.g., nitrite, N₂O).

• Within the paper Cycle 4 shows dramatic H₂ recovery in reactor A (78.60%) after cycle 3 collapse (2.95%), but the mechanism is unclear and not fully explained.

  One side

  Cycle 3: reactor A 2.95% H₂ after 46 hrs — production near zero

  “Analysis of the headspace taken after 46 hrs of reactor operation is shown in Fig. 6 above. Only reactor A showed peak for hydrogen at around 0.6, with reactor A's headspace composed of 2.95% H2.”

  Other side

  Cycle 4: reactor A 78.60% H₂ after 71 hrs — highest yield of all cycles

  “Analysis of the headspace taken after 71 hrs of reactor operation is shown in Fig. 7 above. Although reactor B consistently yielded no peak for H2 akin to the previous cycle, hydrogen peak was observed yet again in reactor A. The headspace ”

  Authors’ reading. Authors acknowledge the mechanism is unclear ('needs to be analysed further') and speculate that reduced microbial biomass combined with fresh medium enabled recovery. The 78.60% H₂ spike may reflect a transient condition (e.g., rapid H₂ accumulation before methanogenic consumption resumes) rather than sustained high production.

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

• High No measurement of nitrogen species (NO₃⁻, NO₂⁻, N₂O, N₂) in liquid or gas phases to verify denitrification pathway completion or identify N₂O accumulation.

  Paper claims denitrifiers produce 'nitrogen oxides' that inhibit methanogens, but provides no analytical evidence of which N-species are produced, their concentrations, or whether complete denitrification to N₂ occurs. Section 3.2 states 'the direct mechanism behind these phenomena is yet to be verified'.

  Try. Measure NO₃⁻, NO₂⁻, N₂O, and N₂ in reactor A (denitrifier) vs B (control) at each cycle using ion chromatography (liquid) and GC-ECD or GC-MS (gas headspace). Quantify N-balance to confirm complete denitrification.

• High No measurement of cathode or anode potential (vs Ag/AgCl or SHE) to characterize electrochemical conditions and assess whether denitrification is electrochemically driven or purely biological.

  Only applied potential (0.7 V) is reported; actual cathode and anode potentials are unknown. This prevents assessment of whether denitrification is coupled to cathodic reduction or occurs independently. Denitrifiers may be exoelectrogens themselves.

  Try. Install reference electrode (Ag/AgCl) and measure cathode and anode potentials vs SHE at each cycle. Correlate potential windows with H₂ production and N-species accumulation.

• Medium No measurement of dissolved oxygen (DO) in reactors to confirm anaerobic conditions and assess whether residual O₂ affects denitrifier activity or methanogen inhibition.

  Anaerobic procedure uses N₂ flushing and methylene blue color change as proxy for anoxia, but no quantitative DO measurement. Denitrifiers require anoxia; residual O₂ could shift metabolism and reduce N-oxide production.

  Try. Measure dissolved O₂ in reactors A and B at start and end of each cycle using O₂ electrode or optode. Confirm DO < 0.5 mg/L throughout operation.

• Medium No measurement of pH in reactors; catholyte pH is critical for denitrification kinetics and methanogen inhibition but is not reported.

  Medium formulation is described but pH is not measured or controlled. Denitrification and H₂ production both affect pH; pH drift could explain cycle 3 collapse and cycle 4 recovery.

  Try. Measure pH in reactors A and B at each cycle start and end. If pH drifts >0.5 units, add pH control (buffer or acid/base dosing) and repeat cycles 3–4.

• Medium No variation of applied potential or current density to identify optimal operating window for simultaneous H₂ production and denitrification-mediated methanogen inhibition.

  Only 0.7 V and 1 A tested; no exploration of trade-offs between H₂ yield, denitrification rate, and N₂O accumulation. Higher potentials may increase H₂ production but suppress denitrification; lower potentials may favor denitrification but reduce H₂.

  Try. Test applied potentials 0.5, 0.7, 0.9, 1.1 V at constant 1 A in reactors A and B. Measure H₂ yield, N-species, and methanogenic activity (CH₄ production) at each potential.

• High No measurement of methane (CH₄) production in reactors to quantify methanogenic suppression and confirm that H₂ improvement is due to reduced methanogenesis rather than increased H₂ evolution.

  Paper claims denitrifiers inhibit methanogens, but CH₄ is not measured. H₂ improvement could reflect increased H₂ production, decreased methanogenic consumption, or both. GC analysis includes only H₂ detection.

  Try. Modify GC method to detect both H₂ and CH₄ (or use separate FID detector). Measure CH₄ in reactors A and B at each cycle. Calculate net H₂ yield as (H₂ produced) − (H₂ consumed by methanogens).