Energy Consumption for Nutrient Removal from High-Nitrate and High-Phosphorus Wastewater in Aerobic and Anaerobic Bioelectrochemical Reactors

Nur Konflikte, die die Publikation selbst zeigt, jeweils am Text geprüft.

• Innerhalb der Publikation Nitrogen removal effectiveness increases with HRT extension, yet energy cost per kg N removed also increases—a paradox the authors acknowledge but do not fully explain mechanistically.

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  HRT extension improves removal effectiveness: 'Our previous research [27] demonstrated the highest nitrogen removal effectiveness (68.6%) at the highest electric current density and the longest hydraulic retention time. At all current densities tested, the effectiveness of nitrogen removal increased along with extended hydraulic retention time.'

  “the highest nitrogen removal effectiveness (68.6%) at the highest electric current density and the longest hydraulic retention time”

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  HRT extension increases energy cost per kg removed: 'HRT extension at a stable current density ensured both a technological effect, i.e., increased effectiveness of nitrogen removal, and a direct energetic effect, i.e., increased energy demand per unit of removed contaminant load.'

  “HRT extension at a stable current density ensured both a technological effect, i.e., increased effectiveness of nitrogen removal, and a direct energetic effect, i.e., increased energy demand per unit of removed contaminant load”

  Einordnung der Autoren. The authors observe the paradox but attribute it to the cumulative energy input (U·I·t in the numerator) growing faster than the incremental removal load in the denominator. This suggests that at longer HRT, the system reaches a plateau in removal rate while current continues to flow, wasting energy. The paper does not investigate whether this is due to kinetic limitation, biofilm saturation, or electrode potential constraints.

• Innerhalb der Publikation Anaerobic SBBR achieves 8–13× lower energy cost per kg N removed than aerobic REBDC, yet absolute denitrification efficiency is lower (36.5% vs 68.6%), implying the anaerobic system removes less N per unit time but uses far less energy per unit removed.

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  Anaerobic SBBR is more energy-efficient: 'In the SBBR, the values of the energy consumption during nutrient removal (Figure 5) ranged from 30 to 464 kWh/kg NUTRIENTSrem, whereas, in the REBDC, they reached 330 and 1380 kWh/kg NUTRIENTSrem.'

  “the values of the energy consumption during nutrient removal (Figure 5) ranged from 30 to 464 kWh/kg NUTRIENTSrem, whereas, in the REBDC, they reached 330 and 1380 kWh/kg NUTRIENTSrem”

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  Anaerobic SBBR has lower removal efficiency: 'The denitrification efficiency reached 36.5% at the lowest current density and 21.9% at the highest one.' vs. aerobic REBDC: 'the highest nitrogen removal effectiveness (68.6%) at the highest electric current density and the longest hydraulic retention time'

  “The denitrification efficiency reached 36.5% at the lowest current density and 21.9% at the highest one”

  Einordnung der Autoren. The authors do not reconcile this directly. The likely explanation is that the anaerobic SBBR operates at lower cell voltage or current density (0.63–5.0 A/m² vs 0.63–10.0 A/m² in REBDC), reducing energy input per unit time, and that the lower removal efficiency is acceptable if the goal is energy minimization rather than maximum treatment. The paper does not compare removal rates (kg/h) or discuss whether the anaerobic system could achieve higher efficiency with longer HRT.

Konkrete Lücken in dieser Publikation, jeweils mit einem Experiment, das sie schließen würde.

• Hoch No investigation of nitrite (NO₂⁻) or N₂O accumulation across the current density and HRT matrix, despite these being critical indicators of denitrification pathway selectivity and greenhouse gas risk in bioelectrochemical systems.

  The paper measures influent NO₂⁻ (0.092 mg N/L mean) and effluent NO₃⁻ and TN, but does not report effluent NO₂⁻ or N₂O at any condition. Given that electrochemical reduction can produce NO₂⁻ as an intermediate and that incomplete denitrification produces N₂O, the absence of these data prevents assessment of whether higher current densities or longer HRT favor complete denitrification or accumulation of intermediates.

  Versuch. Measure NO₂⁻ and N₂O (headspace gas chromatography or dissolved via membrane inlet MS) at each combination of J (0.63, 1.25, 2.5, 5.0, 10.0 A/m²) and HRT (4, 8, 12, 24 h) in REBDC and at J (0.63–5.0 A/m²), HRT 24 h in SBBR. Correlate with current efficiency and identify the operating window that minimizes NO₂⁻ and N₂O while maximizing N₂ production.

• Hoch No measurement of cathode and anode potentials (vs. Ag/AgCl or SHE) or cell voltage breakdown, making it impossible to identify which electrode limits performance or whether potential windows can be optimized for selective denitrification vs. phosphorus removal.

  The paper applies current density (J) and measures total cell voltage (U) but does not report individual electrode potentials. Since hydrogenotrophic denitrification requires a specific cathode potential window for H₂ production and denitrifier activity, and electrocoagulation requires a specific anode potential for Al dissolution, the absence of potential data prevents mechanistic understanding and optimization.

  Versuch. Install reference electrodes (Ag/AgCl, +0.197 V vs. SHE) in both REBDC and SBBR. At each J and HRT, measure and report cathode potential (Ecath), anode potential (Eanode), and cell voltage (U = Eanode − Ecath). Correlate Ecath with H₂ production rate (gas chromatography) and denitrification efficiency; correlate Eanode with Al dissolution rate (ICP-OES) and phosphorus removal. Identify the potential windows that maximize current efficiency for each process.

• Mittel No systematic variation of influent C/N ratio or external carbon source type; sodium acetate was added but its concentration and the resulting C/N ratio are not reported, limiting reproducibility and generalizability to other wastewater types.

  The paper states 'sodium acetate was added to wastewater as a source of carbon to ensure the growth of the biofilm biomass' but does not specify the acetate concentration, the resulting C/N ratio, or whether this ratio was optimized. The influent TN is 486 mg N/L and COD (with acetate) is 260 mg O₂/L, implying a very high N:C ratio. This may explain why nitrogen removal is incomplete and why the anaerobic SBBR achieves only 36.5% denitrification efficiency.

  Versuch. Test influent C/N ratios of 0.5, 1.0, 2.0, and 4.0 (by varying acetate concentration) at fixed J = 1.25 A/m² and HRT = 24 h in both REBDC and SBBR. Measure TN removal efficiency, EEITNrem, and effluent NO₂⁻ and N₂O. Identify the C/N ratio that maximizes denitrification efficiency and minimizes energy cost per kg N removed.

• Mittel No measurement of dissolved oxygen (DO) in the REBDC bulk liquid or biofilm profile, despite the paper claiming 'aerobic conditions' in the tank and 'transitional and anaerobic layers' in the biofilm. DO is critical for nitrification and oxygen-dependent denitrification pathways.

  The paper assumes aerobic conditions in REBDC based on disc submergence (40%) and aeration, but does not measure DO. If DO is low or depleted, nitrification may be incomplete, and the measured TN removal may be dominated by electrochemical reduction rather than biological denitrification, affecting the interpretation of energy efficiency and the applicability to other systems.

  Versuch. Install DO probes (optical or electrochemical) in the REBDC bulk liquid and use microelectrodes to profile DO in the biofilm at disc surface, mid-depth, and base. Measure at J = 0.63, 2.5, and 10.0 A/m² and HRT = 4, 12, 24 h. Correlate DO profile with TN removal pathway (nitrification vs. electrochemical reduction vs. heterotrophic denitrification) using 15N-labeled substrates or molecular methods.

• Mittel No investigation of the effect of influent pH or catholyte pH on nitrogen and phosphorus removal, despite pH being a critical variable for electrochemical reactions, H₂ production, and denitrifier activity.

  The paper reports influent pH of 6.19 ± 0.11 but does not measure effluent pH or pH in the cathode and anode compartments. Electrochemical H₂ production is pH-dependent (more favorable at low pH), and denitrification is sensitive to pH (optimal ~7–7.5). Phosphorus precipitation (electrocoagulation) is also pH-dependent. Without pH data, the mechanisms and optimization potential are unclear.

  Versuch. Measure influent and effluent pH at each J and HRT condition. In REBDC, measure pH in the bulk liquid and at the cathode surface (using pH microelectrodes). In SBBR, measure pH in the bulk liquid and in the anode and cathode compartments (if separated). Vary influent pH from 5.5 to 8.0 at fixed J = 1.25 A/m² and HRT = 24 h. Measure TN removal, EEITNrem, H₂ production rate, and effluent NO₂⁻ and N₂O. Identify the pH window that maximizes energy efficiency.

• Mittel No comparison of the two reactor types at identical HRT and current density ranges; SBBR was tested only at 24 h HRT and 0.63–5.0 A/m², while REBDC was tested at 4–24 h HRT and 0.63–10.0 A/m², preventing direct assessment of whether the anaerobic advantage holds across the full operating window.

  The paper justifies limiting SBBR testing to 24 h HRT and 5.0 A/m² based on REBDC results (temperature rise at 10.0 A/m²), but this creates an incomplete comparison. It is unclear whether the anaerobic advantage persists at shorter HRT (4, 8, 12 h) or at higher current density (10.0 A/m²), or whether the SBBR would show similar energy penalties at longer HRT.

  Versuch. Conduct a full factorial test of SBBR at J = 0.63, 1.25, 2.5, 5.0, 10.0 A/m² and HRT = 4, 8, 12, 24 h (with temperature control to prevent >50 °C). Compare EEINUTRIENTSrem, EEITNrem, EEITPrem, and removal efficiencies directly to REBDC results. Identify the operating window where anaerobic SBBR is superior and where aerobic REBDC may be preferable (e.g., if higher removal efficiency is needed despite higher energy cost).