Technological Parameters of Rotating Electrochemical and Electrobiological Disk Contactors Depending on the Effluent Quality Requirements

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

• Within the paper Paper claims current density increase improves effluent quality but also causes nitrite accumulation that violates discharge limits.

  One side

  Higher current density improves nitrogen removal: 'an increase in electric current density led to an increase in pollutant load removed at all HRTs tested' and 'increased current density and extended HRT decreased nitrogen concentrations in the effluent'

  “an increase in electric current density led to an increase in pollutant load removed at all HRTs tested”

  Other side

  Highest current density causes nitrite violation: 'At the highest electric current density tested, i.e., J = 10.0 A/m², the concentration of nitrites in the effluent from the RECDC exceeded 20.00 mg N/L regardless of HRT'

  “At the highest electric current density tested, i.e., J = 10.0 A/m², the concentration of nitrites in the effluent from the RECDC exceeded 20.00 mg N/L regardless of HRT”

  Authors’ reading. This is a real operating window constraint, not a logical contradiction. The paper explains it: 'current density that is too high contributes to a substantially exceeded saturation constant of hydrogen and to its excess remaining in the biofilm, which adversely affects denitrification.' Total nitrogen removal improves at J=10.0 A/m² but the pathway shifts toward incomplete reduction (NO3⁻ → NO2⁻ rather than NO2⁻ → N2), creating a trade-off between total removal and product selectivity.

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

• High No measurement of N2O or other gaseous nitrogen products; only NO3⁻, NO2⁻, and NH4⁺ quantified. Paper does not report whether denitrification produces N2 or N2O, which is critical for environmental impact and regulatory compliance.

  Paper states 'hydrogenotrophic and heterotrophic denitrification' occurs but provides no gas chromatography or N-balance closure to confirm N2 is the end product. At high current density (J=10.0 A/m²) where nitrite accumulates, N2O production risk is elevated but unmeasured.

  Try. Headspace gas analysis (GC-ECD or GC-MS) for N2O and N2 at J=0.63, 2.5, 5.0, 10.0 A/m² and HRT=24 h in both RECDC and REBDC; nitrogen balance closure (inlet N vs outlet N + gas N).

• High No systematic study of cathode potential or anode potential; only current density and HRT varied. Electrode potentials are not reported, making it impossible to assess whether electrochemical reduction is occurring at the cathode or if the system is operating in a regime where hydrogen evolution dominates.

  Paper states 'aluminum anode' and 'stainless steel cathodes' but provides no potentiostat data, no Tafel curves, no potential windows. At J=10.0 A/m², the cathode potential likely becomes very negative (favoring H2 evolution), which could explain nitrite accumulation (incomplete reduction) and high temperature rise.

  Try. Potentiostatic/potentiodynamic measurements at each J and HRT; report cathode and anode potentials vs Ag/AgCl; correlate potential with nitrite accumulation and H2 production rate.

• Medium C:N ratio fixed at 0.5 in REBDC; no variation of external carbon dose. Paper states 'Increasing the carbon dose has a direct impact on the effectiveness of denitrification' but does not test this range.

  Abstract recommends 'an external carbon source is recommended' for nitrate >30 mg N/L, but the paper only tests one C:N ratio (0.5). Optimal C:N for this wastewater type is not determined. Paper cites Hao et al. [34] showing C:N=1.5 improves performance but does not replicate.

  Try. REBDC at HRT=24 h, J=2.5 A/m² with C:N = 0.25, 0.5, 1.0, 1.5, 2.0; measure total nitrogen, nitrite, ammonia, and biofilm thickness.

• Medium No measurement of dissolved oxygen (DO) in the reactors. Rotating disk contactors are known to aerate the liquid; DO level affects both nitrification (in biofilm) and electrochemical reduction pathways.

  Paper mentions 'hydrogenotrophic bacteria' and 'heterotrophic denitrification' but does not report DO. At high rotation speed (implied by 'rotating disk'), oxygen transfer may be significant, potentially inhibiting anaerobic denitrification or promoting nitrification in the biofilm.

  Try. Measure DO at inlet and outlet of REBDC at each HRT and J; correlate with ammonia nitrogen and nitrite profiles; test under anaerobic vs aerobic conditions.

• Medium Catholyte pH not controlled or reported as a manipulated variable. Table 3 shows pH ranges 7.84–9.25 in RECDC and 8.22–8.85 in REBDC, but no systematic study of pH effect on denitrification or nitrite accumulation.

  Paper states 'optimal concentration of electron donors, beneficial pH, and optimal duration of the reaction' are required for hydrogenotrophic bacteria, but pH is not controlled. High pH (>8.5) at J=10.0 A/m² may inhibit denitrification or shift equilibrium toward NO2⁻ accumulation.

  Try. REBDC at J=2.5 A/m², HRT=24 h with pH buffered to 6.5, 7.0, 7.5, 8.0, 8.5; measure denitrification rate, nitrite, and biofilm composition.

• Medium No long-term stability or biofilm characterization. Experiments appear to be batch or short-term; no data on biofilm thickness, microbial community composition, or performance drift over weeks/months.

  Paper states 'Analytic control of processes began after adaptation of the reactors' and '20 samples were collected at each condition' but does not report adaptation time, steady-state confirmation, or biofilm growth kinetics. Biofilm thickness affects mass transfer and denitrification rate.

  Try. Continuous operation for 8–12 weeks at J=2.5 A/m², HRT=24 h; measure biofilm thickness weekly (optical coherence tomography or cross-section); 16S rRNA sequencing at weeks 2, 4, 8, 12; track effluent quality drift.