(Bio)electrochemical ammonia recovery: progress and perspectives

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

• Within the paper LN model predicts 100% removal efficiency at LN > 1.3, but observed data show ~60% recovery at LN > 1.3.

  One side

  Theory: 'at an LN > 1.3, 100% removal efficiency should be feasible'

  “Whereas in theory, at an LN > 1.3, 100% removal efficiency should be feasible, in practice, lower removal efficiencies are observed.”

  Other side

  Practice: 'in practice, lower removal efficiencies are observed' (~60% at LN > 1.3)

  “The two data points at an LN higher than 1.3 show recovery efficiencies around 60%.”

  Authors’ reading. Authors attribute gap to 'complications during the experiments, such as low stripping-absorption efficiency, or due to lower TAN concentrations in the wastewater compared to the other studies', indicating LN is necessary but not sufficient; other factors (membrane design, recovery device efficiency, wastewater composition) limit real-world performance.

• Within the paper TMCS is claimed to be more robust and less complex than stripping, yet both achieve similar energy demand and transport efficiency.

  One side

  Robustness claim: 'The integration of TMCS within (B)ES is less complex and therefore more robust than the integration of ammonia stripping'

  “The integration of TMCS within (B)ES is less complex and therefore more robust than the integration of ammonia stripping with (B)ES”

  Other side

  Performance similarity: TMCS (Kuntke et al. 2017) 7.3–15.6 kWh/kg-N, 55–58% ηN; stripping (Luther et al. 2015) 9.5–14.7 kWh/kg-N, 58–67% ηN

  “Several (B)ESs were studied with an integrated ammonia stripping process at the cathode, showing its potential to reduce the energy demand for TAN recovery compared to conventional stripping alone.”

  Authors’ reading. Authors distinguish on engineering grounds (integration complexity, operational stability) rather than thermodynamic efficiency; both methods are viable, choice depends on system design constraints and scale.

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

• High No systematic study of cathode potential (EpH7) vs TAN recovery efficiency or N2O/NO2⁻ accumulation. Table 1 lists conditional potentials (e.g., H₂ evolution −0.414 V, O₂ reduction +0.815 V) but no experiments vary cathode potential to map the operating window.

  Cathode potential determines which reduction reaction dominates (H₂ evolution vs O₂ reduction) and may trigger side reactions (N2O, NO2⁻ formation). Paper mentions 'toxic effect of the typical conditions' but does not isolate potential as a variable.

  Try. Vary cathode potential from −0.5 to +0.8 V vs NHE in MEC treating urine or digestate; measure TAN recovery, current efficiency, and monitor for NO2⁻/N2O via HPLC/GC at each potential.

• High No investigation of anode potential (EpH7) vs Coulombic efficiency or competing electron sinks (methane, sulfate reduction). Paper states CE is critical for RP but does not test how anode potential affects CE in high-TAN wastewaters.

  Text: 'electrons may be transferred to alternative terminal electron acceptors (e.g., methane, oxygen, metals, sulfate) in competing processes and do not end up at the anode'. High TAN may inhibit methanogenesis or favor sulfate reduction; anode potential could shift this balance.

  Try. Operate MEC on digestate at fixed current (20 A/m²) while varying anode potential from −0.2 to +0.4 V vs NHE; measure CE, methane production, and sulfide accumulation.

• High Catholyte pH is mentioned as critical (>8.5 for TMCS, high pH for stripping) but no systematic study of pH vs TAN transport number, N2O formation, or cathode fouling. Figure 4 assumes ηN = 60% but does not show how pH affects this.

  Text: 'high pH, TAN, and salt concentrations' are toxic to electroactive bacteria, and TMCS requires pH > 8.5 in feed. No data on whether high pH (>9) improves TAN selectivity or triggers side reactions.

  Try. Operate MEC on urine at LN = 1.2, current density 20 A/m², and vary catholyte pH from 7 to 10; measure TAN transport number, N2O/NO2⁻ accumulation, and cathode potential.

• Medium No study of dissolved oxygen (DO) in cathode compartment vs TAN recovery and N2O formation. Paper lists O₂ reduction (+0.815 V) as a cathode reaction but does not test whether aeration or anaerobic conditions affect selectivity.

  O₂ reduction is thermodynamically favorable vs H₂ evolution; if DO is present, it may compete with TAN reduction and reduce current efficiency. N2O is a concern in nitrogen cycling systems.

  Try. Operate MEC on digestate at 20 A/m², LN = 1.2, with cathode DO controlled at 0, 2, 5, and 8 mg/L; measure TAN recovery, current efficiency, and N2O/NO2⁻ via GC.

• Medium Recovery potential (RP) model assumes fixed ηN = 60% and LN ≈ 1.2 across all wastewaters (Figure 4), but Table 2 shows ηN ranges 13–141%. No sensitivity analysis of RP to these parameters or to HRT (hydraulic retention time).

  Figure 4 ranking (reject water > digestate > swine manure) is based on COD/TAN ratio alone; actual RP depends on achievable CE, CODr, and ηN, which vary by wastewater and system design. HRT is not mentioned.

  Try. For three wastewaters (reject water, digestate, urine), measure CE, CODr, and ηN at HRTs of 4, 8, and 16 h; recalculate RP and compare to Figure 4 predictions.

• High No data on N2O or NO2⁻ accumulation in any system. Paper mentions 'N2O emissions, a potent greenhouse gas, occur during these nitrogen removal processes' but does not test whether (B)ES for TAN recovery produces N2O or NO2⁻ under any operating condition.

  Cathode reduction of NO3⁻ or NO2⁻ (if present in feed) could produce N2O. High pH and high TAN may inhibit nitrification, but no data provided. This is a critical environmental metric.

  Try. Operate MEC on digestate spiked with 100 mg/L NO3⁻ at cathode potentials −0.3, −0.5, −0.7 V vs NHE; measure N2O, NO2⁻, and NH4⁺ via GC and HPLC.