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82% confidence

This study demonstrates thermophilic single-chamber air-cathode MFCs for ammonia removal and recovery from high-strength leachate. Despite lower power density (105 vs 330 mW m⁻²), thermophilic operation achieved 83% ammonia removal in closed-circuit mode versus 48% for mesophilic, driven by higher unionized ammonia fraction and electromigration to the cathode. Water recovery reached 36.5 L m⁻² d⁻¹ at 55 °C without applied energy, and systems recovered within 24 h from inhibition by 7200 mg L⁻¹ ammonia. High-quality struvite was crystallized from recovered ammonia.

Key insights

  • Trade-off Thermophilic MFCs achieved 83% ammonia removal (closed-circuit) versus 48% for mesophilic at 2000 mg L⁻¹ NH₃-N input, despite generating only 875 mA m⁻² current versus 1900 mA m⁻² for mesophilic.

    So what. Operate thermophilic MFCs for ammonia-rich waste even if electrical output is lower; the higher unionized ammonia fraction at 55 °C compensates for lower electrochemical driving force.

  • Mechanism Water flux ratio (open-circuit to closed-circuit) was 39% for mesophilic but 85% for thermophilic MFCs, indicating electro-osmotic drag dominates water transport in mesophilic systems while evaporation and pressure differential dominate in thermophilic.

    So what. Do not assume electro-osmotic drag is the primary water transport mechanism at elevated temperature; design cathode sweeping and condensation systems accordingly for thermophilic operation.

  • Mechanism Cathode pH stabilized at 9.2 in both mesophilic and thermophilic closed-circuit modes, attributed to the NH₄⁺/NH₃ buffer couple (pKa ≈ 9.2), rather than reaching the 10.6–12.7 pH observed in prior sodium acetate-only systems.

    So what. In ammonia-rich systems, expect cathode pH to be buffered near 9.2; this reduces caustic overpotential and may improve power output compared to non-ammonia systems, but limits further pH rise for ammonia stripping.

  • Threshold Both meso-MFC and thermo-MFC recovered from 7200 mg L⁻¹ ammonia inhibition within 24 h, with ammonia concentration dropping 33% (thermo) and 22% (meso) in the first day, restoring current density to 1200–2300 mA m⁻².

    So what. Use MFCs as a buffer for thermophilic anaerobic digesters during high-ammonia transients (e.g., leachate recirculation); the system will self-recover without temperature reduction or chemical addition.

  • Performance Ammonia recovery efficiency was 60–70% (sum of condensate and acid-absorbed ammonia), with recovery water containing 172–230 mg L⁻¹ ammonia and 120–200 mg L⁻¹ COD due to volatile organic compound co-transport.

    So what. Account for 30–40% ammonia loss in sweeping gas and volatile organic compound contamination in recovered water; post-treatment or alternative condensation may be needed for fertilizer-grade product.

Opportunity areas6

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

  • High No measurement of cathode potential (vs Ag/AgCl or SHE) or anode potential during operation; only open-circuit voltage and polarization curves provided. Cannot determine if oxygen reduction kinetics or ammonia oxidation/reduction at cathode is rate-limiting.

    The paper states 'the oxygen reduction reaction occurs at the cathode and produces hydroxyl ions' but provides no electrochemical impedance spectroscopy, cyclic voltammetry, or potential-step data to confirm cathode kinetics or identify overpotential sources.

    Try. Measure cathode and anode potentials (vs reference electrode) during steady-state ammonia removal at 35 °C and 55 °C; perform electrochemical impedance spectroscopy to quantify charge-transfer resistance and diffusion limitations.

  • High No systematic variation of external resistance (only 200 Ω tested). Operating window for ammonia removal vs current density trade-off not explored.

    The paper uses a fixed 200 Ω resistor throughout but does not test how lower or higher resistance affects ammonia removal efficiency, water flux, or cathode pH. Prior work (Gajda et al.) showed resistance affects water transport; this paper does not explore the ammonia removal trade-off.

    Try. Test external resistance from 50 Ω to 1000 Ω at both 35 °C and 55 °C; measure ammonia removal, current, cathode pH, and water flux as functions of resistance.

  • Medium No measurement of dissolved oxygen (DO) in the anode chamber or control of anode headspace. Potential for oxygen intrusion and its effect on ammonia removal not quantified.

    The paper mentions 'oxygen intrusion' as a limitation of single-chamber air-cathode MFCs and notes 'coulombic efficiency is low because of oxygen intrusion', but does not measure DO or test anaerobic vs aerobic anode conditions.

    Try. Measure anode DO continuously during operation; compare ammonia removal and current density under N₂-purged vs air-exposed anode headspace.

  • Medium No analysis of nitrogen species other than ammonia (NH₃-N) and ammonium (NH₄⁺). No measurement of nitrite (NO₂⁻), nitrate (NO₃⁻), or N₂O production.

    The paper focuses on ammonia recovery but does not report whether any nitrification or denitrification occurs at the anode or cathode. Given the presence of exoelectrogens and oxygen at the cathode, nitrification is plausible but not measured.

    Try. Measure NO₂⁻, NO₃⁻, and N₂O in anode and cathode chambers over time; use ¹⁵N-labeled ammonia to trace nitrogen transformations.

  • Medium Struvite crystallization performed off-line from collected ammonia in phosphoric acid; no in-situ precipitation or continuous recovery tested.

    The paper demonstrates high-quality struvite can be formed (XRD match to pure struvite) but only from batch-collected ammonia. No data on struvite formation kinetics, purity, or yield as a function of ammonia concentration or pH.

    Try. Test in-situ struvite precipitation in the cathode chamber or a separate crystallizer fed with cathode effluent; measure crystal purity, yield, and recovery rate as functions of Mg²⁺ dosing and pH control.

  • Medium No measurement of hydraulic retention time (HRT) or residence time distribution. Leachate fed as batch; no continuous-flow operation tested.

    The paper operates MFCs in batch mode (120 mL medium, 7 d operation) but does not report HRT or test continuous feeding. Ammonia removal kinetics and steady-state performance under flow conditions unknown.

    Try. Operate MFCs in continuous-flow mode at HRTs of 1, 3, 7, and 14 d; measure ammonia removal, current, and water flux as functions of HRT and influent ammonia concentration.

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What they did

System
MFC
Substrate
real wastewater
Inoculum
mixed culture from thermophilic solid-state anaerobic fermentation leachate and mesophilic anaerobic wastewater

What worked

Power density330mW/m²best of 2
Current density1.90A/m²best of 2
COD removal59.0%best of 2

Abstract

Abstract In wastewater treatment, biological nitrogen removal is an important topic, and the optimal condition for it is a mesophilic environment. This study developed a thermophilic microbial fuel cells (thermo-MFCs) equipped with a hydrophobic membrane electrode to remove and recover ammonia and water from leachate. The results were compared with those of the mesophilic MFCs (meso-MFCs) and they show that the current and power densities for meso-MFCs are higher. The ammonia removal efficiencies of thermo-MFCs are 83% (closed circuit) and 60% (open circuit), higher than those of closed- and open-circuit meso-MFCs (48 and 38%, respectively). Water vapor, the main recovery water flux for the thermo-MFCs, provided 36.5 L m − 2 d − 1 using the closed-circuit mode without applied energy. Moreover, thermo-MFCs and meso-MFCs can be restored within 24 h even under inhibition by using 7200 mg L − 1 ammonia. The proposed process presents an economic and ecofriendly method to not only recover water and ammonia from leachate but also alleviate ammonia inhibition.

Keywords

Microbial fuel cellLeachateAmmoniaMesophileWastewaterThermophile

Identifiers

DOI
10.1186/s42834-020-0045-0
Journal
Sustainable Environment Research
Year
2020
Single-chamber air-cathode MFCRepresentative model
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Representative single-chamber air-cathode MFC — matched on the paper’s reported configuration, not its exact geometry.

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