Effect of hydraulic retention time and substrate availability in denitrifying bioelectrochemical systems

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

• High N₂O formation and reduction mechanism under varying cathode potentials (−0.320 V was fixed); test range −0.1 to −0.5 V vs. Ag/AgCl to identify potential window minimizing N₂O slip.

  N₂O emissions remain 33–90% of removed NO₃⁻ across all HRTs; authors note this is higher than prior work but do not systematically vary potential to suppress N₂O formation.

  Try. Potentiostatic sweep at fixed HRT (0.46 h) from −0.1 to −0.5 V vs. Ag/AgCl; measure NO₃⁻, NO₂⁻, N₂O, and current density.

• High Effect of internal recirculation or improved hydrodynamics on N₂O reduction; authors hypothesize lack of recirculation drives high N₂O emissions but do not test it.

  Authors state: 'The difference in N₂O emissions could be explained through the different reactor design used' (lack of internal recirculation); no experimental validation provided.

  Try. Retrofit tubular reactor with internal recirculation loop; compare N₂O emissions and removal rates at same HRT (0.46 h) and potential (−0.320 V).

• Medium Influent NO₃⁻ concentration range above 112 mg N L⁻¹ to determine if inhibition at high HRT (3.4 h) continues or reverses; maximum tested was 112 mg N L⁻¹.

  At HRT 3.4 h, activity declined sharply above 40.6 mg N L⁻¹ (Fig. 5); mechanism unclear (FNA ruled out, no nitrite accumulation); testing higher concentrations may reveal saturation or true inhibition.

  Try. HRT 3.4 h, influent NO₃⁻ 112–200 mg N L⁻¹; measure NO₃⁻ removal rate, NO₂⁻, and pH to assess FNA formation.

• Medium Microbial community composition and gene expression (nir, nor, nos genes) across HRT range to link hydrodynamic changes to N₂O reduction enzyme activity.

  Authors conclude hydrodynamics dominate performance but do not characterize the microbial response; fast response (<0.25 d) suggests activity modulation, but mechanism is unexplored.

  Try. 16S rRNA and metatranscriptomics at HRT 10.89, 1.2, and 0.46 h; correlate denitrification gene expression with N₂O removal rate.

• Medium Series operation of compact reactors (as recommended for scaling) not experimentally tested; only single-reactor performance evaluated.

  Authors state: 'an appropriate approximation of scaling-up denitrifying BES would be the implementation of compact reactors connected in series' but provide no data on series performance, inter-reactor NO₃⁻/NO₂⁻ dynamics, or overall conversion.

  Try. Two or three tubular reactors in series at HRT 0.46 h per reactor; measure NO₃⁻, NO₂⁻, N₂O at each outlet; compare overall conversion and N₂O emissions to single reactor.

• Medium Dissolved oxygen and pH in cathode compartment not monitored; only bulk effluent pH and conductivity reported; local conditions may drive N₂O formation.

  Denitrification intermediates (NO₂⁻, N₂O) are sensitive to local pH and DO; no data on cathode microenvironment; could explain high N₂O emissions.

  Try. Insert pH and DO microsensors in cathode at different radial/axial positions during HRT 0.46 h operation; correlate local conditions with N₂O production.