Enhancing Nitrate Removal from Waters with Low Organic Carbon Concentration Using a Bioelectrochemical System—A Pilot-Scale Study

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

• High N2 production was not measured; only NO3− removal and N2O suppression were quantified. Cannot confirm that electrons were used for complete denitrification to N2 vs. other pathways (e.g., DNRA, anammox).

  Section 3.3 states 'most of the NO3− was completely denitrified to nitrogen gas' but this is inferred from low N2O, not measured. NO2− accumulation (0.82 mg N L⁻¹) and NH4+ presence suggest incomplete denitrification or competing pathways.

  Try. Install N2 headspace analyzer (e.g., membrane inlet mass spectrometry or gas chromatography with thermal conductivity detector) and measure N2 production rate alongside NO3− removal to close the nitrogen balance.

• High Cathode potential was fixed at −550 mV vs. SCE after day 7; no systematic variation of potential was tested to identify the optimal window for NO3− removal vs. NO2− accumulation vs. power efficiency.

  Section 3.1 notes that −550 mV was 'proven to work in previous articles' but no justification or sensitivity analysis is provided. NO2− accumulation suggests this potential may not be optimal for complete denitrification.

  Try. Vary cathode potential from −400 to −700 mV vs. SCE in 50 mV steps; measure NO3− removal rate, NO2− accumulation, current density, and N2O flux at each potential to identify the Pareto frontier.

• High Inoculum source (municipal wastewater treatment plant) was not characterized for electroactive bacteria abundance or denitrifier diversity. Authors acknowledge 'lack of electroactive bacteria in the inoculum' as a possible cause of low removal rates but provide no microbial data.

  Section 3.1 states 'This could have been caused by the lack of electroactive bacteria in the inoculum and by the short experiment period, as it takes time for a bacterial community to develop; however, this cannot be said in certainty as no microbial analyses were conducted during the experiment.'

  Try. Perform 16S rRNA gene sequencing and quantitative PCR of denitrifier (nirK, nirS, nosZ) and electroactive (Geobacter, Shewanella) genes at days 0, 14, 36, and 45 to track community assembly and identify rate-limiting taxa.

• Medium Hydraulic retention time (HRT) was not explicitly controlled or varied. Reactors were operated as batch (no continuous flow), making it impossible to assess the effect of HRT on removal rate, NO2− accumulation, or N2O suppression.

  Section 2.3 states 'Both reactors were operated as patch reactors' with substrate additions when NO3− dropped below 5 mg N L⁻¹. No flow rate, residence time, or space velocity is reported. Batch operation limits applicability to continuous groundwater treatment.

  Try. Operate the MES in semi-continuous or continuous mode with controlled HRT (e.g., 2, 4, 8, 16 h) and measure steady-state NO3− removal rate, NO2− accumulation, and current density as functions of HRT and inlet NO3− concentration.

• Medium pH was not independently controlled in the cathode; it drifted widely and required frequent manual adjustment with HCl/NaOH. The effect of pH on autotrophic denitrification kinetics and NO2− accumulation was not isolated.

  Section 3.2 states 'The pH was strongly influenced by electric manipulation…the pH had to be constantly regulated by adding HCl and NaOH to the cathode and anode chamber, respectively.' Figure 5A shows pH variation; optimal range is 7.0–7.5 but control was not tight.

  Try. Use a pH-stat or buffer system to maintain cathode pH at 6.8, 7.2, 7.6, and 8.0; measure NO3− removal rate, NO2− accumulation, and current density at each pH to quantify the pH optimum for autotrophic denitrification.

• Medium C/N ratio and DOC concentration were not systematically varied. DOC was measured (15.2 vs. 19.9 mg C L⁻¹ in MES vs. control) but the paper does not test whether residual DOC was limiting heterotrophic denitrification or whether further DOC reduction would improve autotrophic performance.

  Section 3.2 states 'the DOC was similar enough in both reactors to not have any significant effect on the discrepancy of nitrate removal rate' but does not test DOC-limited regimes or the transition from heterotrophic to autotrophic dominance.

  Try. Operate MES with inlet DOC concentrations of 0, 5, 10, 20, and 50 mg C L⁻¹ at fixed NO3− (50 mg N L⁻¹) and cathode potential (−550 mV); measure the C/N ratio threshold below which autotrophic denitrification becomes dominant and NO2− accumulation occurs.