Operation of a 2-Stage Bioelectrochemical System for Groundwater Denitrification

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

• Within the paper pH claimed as major player in denitrification, yet BES-2 succeeds at non-optimal pH

  One side

  pH reported as optimal around 7.5 for biocathodic denitrification [38,55]

  “Even though pH has been claimed as a major player in the biocathodic denitrification process, with optimal values around 7.5 [38,55]”

  Other side

  BES-2 inlet pH 8.6 ± 0.5 achieved nearly complete nitrite and N2O removal (0.1 mg/L each)

  “in this study no evidence of accumulation of intermediate N-forms due to pH was recorded: BES-2, whose influent pH was far from neutrality (8.6 ± 0.5)... succeeded in achieving almost a complete removal of nitrite and nitrous oxide”

  Authors’ reading. Authors acknowledge the contradiction but do not fully resolve it; suggest that pH may be less critical for intermediate reduction than for initial nitrate reduction, or that the second stage's lower load compensates for suboptimal pH.

• Within the paper N2O accumulation in this work contradicts prior single-stage experience

  One side

  Prior single-stage BES work showed nitrite as dominant intermediate, not N2O

  “whose presence was not relevant in our previous experience”

  Other side

  Current work: 'the majority of non-nitrate TN was originated from nitrous oxide, which concentration was far higher than that of nitrite during the whole experiment, and whose presence was not relevant in our previous experience'

  “the majority of non-nitrate TN was originated from nitrous oxide, which concentration was far higher than that of nitrite during the whole experiment”

  Authors’ reading. Authors attribute this to higher nitrate loading rate (75.6 vs. 67.0 mgNO3−-N L−1NCC d−1 in prior work), suggesting a load-dependent shift in the rate-limiting step of denitrification. The mechanism is not fully investigated.

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

• High N2O accumulation mechanism and threshold loading rate not characterized. Paper shows N2O dominates at 75.6 mgNO3−-N L−1NCC d−1 but does not test intermediate loading rates (e.g., 67–75 mgNO3−-N L−1NCC d−1) to identify the transition point.

  Authors state 'whose presence was not relevant in our previous experience' at lower load, indicating a threshold exists, but it is not mapped. This is critical for predicting when single-stage BES will fail.

  Try. Vary nitrate loading rate from 50 to 100 mgNO3−-N L−1NCC d−1 in single-stage BES at −0.303 V vs. SHE; measure N2O, NO2−, and NO3− in effluent to identify the loading rate at which N2O becomes dominant.

• High Cathode potential optimization for intermediate N-form reduction not explored. Both stages operated at identical potential (−0.303 V vs. SHE), yet literature (Pous et al. [65]) suggests nitrate and nitrite reduction occur at different potentials.

  Authors note in Discussion: 'Pous et al. [65] reported that nitrate and nitrite reduction happened at different potentials; the operation of the sequential system with different set potentials in the different steps may constitute an evolution of the BES-based denitrification described in this study.' This is explicitly identified as a gap.

  Try. Test BES-2 at potentials ranging from −0.2 to −0.5 V vs. SHE while maintaining BES-1 at −0.303 V; measure N2O and NO2− removal rates and coulombic efficiency to identify optimal potential for polishing stage.

• Medium HRT and current density trade-off in second stage not optimized. BES-2 HRT is 4.16 h with current density 14 A/m³NCC; paper does not test whether shorter HRT or higher current density would improve N2O/NO2− removal without increasing energy cost.

  BES-2 operates at low load (16.3 gNO3−-N/m³NCC d−1 removal vs. 117.3 in BES-1), suggesting it is oversized. Authors note 'the system may work at a higher nitrate load to fully profit from the presence of BES-2,' implying HRT could be reduced.

  Try. Reduce BES-2 HRT from 4.16 h to 2–3 h by increasing flow rate; measure N2O and NO2− removal efficiency and energy consumption to identify minimum HRT for complete polishing.

• Medium Microbial community composition and adaptation to high N2O loads not characterized. Paper does not identify which organisms reduce N2O or why they accumulate it at high loading.

  Authors mention 'Proteobacteria have been reported as abundant or dominant' and cite Vilar-Sanz et al. [36] showing monospecific biofilms cannot complete denitrification, but do not characterize their own biofilm or explain N2O accumulation mechanism.

  Try. Perform 16S rRNA gene sequencing and metatranscriptomics on BES-1 and BES-2 biofilms at 75.6 mgNO3−-N L−1NCC d−1 loading; correlate community composition with N2O and NO2− accumulation patterns.

• Medium Dissolved oxygen and anoxic conditions in cathodic chamber not monitored. Paper does not report DO levels, which could compete with nitrate as electron acceptor and explain intermediate accumulation.

  Autotrophic denitrification requires anoxia, but paper does not confirm DO was <0.5 mg/L or measure it during operation. High DO could suppress N2O reductase expression.

  Try. Monitor dissolved oxygen in BES-1 and BES-2 cathodic chambers continuously; correlate DO spikes with N2O and NO2− accumulation to test whether oxygen ingress is a contributor.

• Low Catholyte pH buffering capacity and long-term stability not tested. Anodic solution pH dropped to 5 every two weeks, requiring replacement; no data on whether this cycling affects biofilm stability or denitrification kinetics.

  Paper states 'Anodic solution was continuously recirculated in closed-loop mode, and substituted when its pH reached a value of 5 (usually every two weeks)' but does not measure biofilm response or test alternative buffering strategies.

  Try. Increase anodic buffer capacity (e.g., higher PBS concentration or alternative buffer) and measure whether pH cycling frequency decreases and whether denitrification performance improves.