Enhancing Stability of Microalgae Biocathode by a Partially Submerged Carbon Cloth Electrode for Bioenergy Production from Wastewater
Jiayin Ling, Yanbin Xu, Chuansheng Lu, Weikang Lai +5
AI summary
78% confidenceThis study demonstrates that a partially submerged (50%) carbon cloth cathode in a microalgae-assisted microbial fuel cell (MFC) improves voltage stability and power output by accessing both dissolved oxygen from algal photosynthesis and atmospheric oxygen. When DO dropped from 13.6–14.8 to 1.0–1.6 mg/L, the 50% submerged electrode maintained 256–239 mV versus 259–102 mV for fully submerged electrodes. The system achieved 297 ± 26 mV average working voltage, 99 ± 6 mW/m³ power density, and 250 ± 42 mg/L lipid production while removing 75 ± 1% total nitrogen from anaerobically digested swine wastewater, though nitrogen removal was not significantly affected by electrode submersion.
Key insights
Threshold Partial submersion (50%) of the cathode electrode reduced voltage variance from ±66 mV (100% submerged) to ±26 mV while maintaining higher average voltage (297 vs 268 mV), demonstrating that dual oxygen access stabilizes performance under variable DO conditions.
So what. Operators should target 50% submersion as an operating window to decouple electricity output from microalgae growth phase and light/dark cycles, enabling more predictable power generation in variable wastewater treatment systems.
Threshold Working voltage remained stable (256–239 mV) at 25% submersion despite 13.6× drop in DO (13.6–14.8 to 1.0–1.6 mg/L), whereas 100% submerged voltage collapsed to 102 mV, indicating a critical DO threshold below ~2 mg/L where fully submerged electrodes fail.
So what. Design cathode systems to operate above 2 mg/L DO if using fully submerged electrodes, or adopt partial submersion to extend stable operation into low-DO regimes (1–2 mg/L) common in late-batch or dark-period conditions.
Trade-off Nitrogen removal (TN 75 ± 1%, NH₃-N 63 ± 2%) was not significantly different across submersion levels (p > 0.05), but MFCs outperformed controls (53 ± 4% TN, 44 ± 5% NH₃-N), indicating that electrode geometry does not control N-removal kinetics in this system.
So what. Electrode submersion can be optimized for power stability without sacrificing nitrogen removal; focus design trade-offs on voltage/power rather than N-removal, which is driven by microalgae uptake and pH-dependent ammonia speciation.
Mechanism COD/TN ratio improved from 0.7–0.8 (inlet) to 1.7–3.0 (outlet) due to TN removal, approaching the 2.0–2.5 threshold needed for effective denitrification in downstream treatment, but the paper does not measure nitrite or N₂O accumulation.
So what. This MFC cathode can serve as a pre-treatment to raise COD/N for downstream denitrification, but operators should monitor for NO₂⁻ or N₂O slip in the cathode itself, which is not reported here.
Performance Internal resistance was lowest (6 ± 0.1 × 10² Ω) and most stable at 50% submersion versus 7 ± 1–2 × 10² Ω for other conditions, correlating with voltage stability but not significantly affecting power density (p > 0.05).
So what. Partial submersion reduces ohmic losses and variance, supporting the use of 50% submersion for stable, predictable voltage output in scaled systems.
Contradictions1
Tensions the paper itself points to. Each side quotes the passage it comes from.
Within the paper Electrode submersion significantly affects voltage stability but not nitrogen removal efficiency
One sideSubmersion proportion has significant effect on working voltage (p < 0.05, ANOVA F=23.927)
“ANOVA analysis also revealed that the effect of submerged proportion on working voltages is significant (p (Sig.) < 0.05, Table A1).”
Other sideSubmersion proportion has no significant effect on TN and NH₃-N removal (p > 0.05)
“This finding suggests that the effects for the submerged proportions of the cathode electrode were not significant for the removal efficiencies of nitrogen and COD.”
Authors’ reading. The authors explain that nitrogen removal is driven by microalgae uptake and pH-dependent ammonia speciation (free ammonia 0.8–20.2% of total), not by electrode oxygen availability. Voltage stability depends on dual oxygen access, which is decoupled from N-removal kinetics in this system.
Opportunity areas6
Specific gaps in this paper, with an experiment that would close each.
High No measurement of nitrite (NO₂⁻) or nitrous oxide (N₂O) accumulation in the cathode chamber despite achieving 75% TN removal and raising COD/TN ratio to 2.0–2.4, which are conditions favoring denitrification.
Section 3.4 states that 'Increasing the COD/N ratio could effectively improve the biological nitrogen removal by denitrification,' but the paper does not measure whether NO₂⁻ or N₂O are produced or accumulated in the cathode. This is critical for assessing true N-removal pathways and greenhouse gas emissions.
Try. Measure NO₃⁻, NO₂⁻, and N₂O in the cathode chamber at 2-day intervals across all submersion levels; use gas chromatography for N₂O and ion chromatography for NO₃⁻/NO₂⁻. Determine if denitrification or nitrification-denitrification is occurring.
High No systematic variation of cathode potential or current density to assess their effect on nitrogen speciation or removal pathway selection.
The paper operates at fixed 1 kΩ external resistance (generating 239–344 mV working voltage) but does not test how cathode potential affects N-removal mechanisms. Denitrification and nitrification are potential-dependent processes; the paper does not explore this trade-off.
Try. Vary external resistance (100 Ω to 10 kΩ) to span cathode potentials from −0.2 to +0.3 V vs. Ag/AgCl. Measure NO₃⁻, NO₂⁻, NH₄⁺, and N₂O at each potential. Identify the potential window for complete denitrification vs. NO₂⁻ accumulation.
Medium No measurement of dissolved oxygen concentration in the anode chamber or assessment of whether partial submersion of the cathode affects anode-side anaerobiosis.
The paper seals the anode with silicone and parafilm, but does not verify that partial cathode submersion (which increases air exposure) does not compromise anode anaerobiosis. Oxygen leakage could affect anode biofilm composition and acetate oxidation kinetics.
Try. Measure DO in the anode chamber at the start and end of each batch for all submersion levels using a sealed DO probe. Verify that anode remains <0.5 mg/L DO regardless of cathode design.
Medium No exploration of the optimal submersion proportion as a function of DO setpoint or light intensity, which vary with microalgae growth phase and day/night cycles.
The paper tests fixed submersion levels (25%, 50%, 75%, 100%) but does not investigate whether the optimal submersion should be adjusted dynamically based on real-time DO or light conditions. This limits practical applicability to variable wastewater treatment scenarios.
Try. Implement a feedback control system that adjusts submersion proportion (e.g., via a motorized electrode holder) based on measured DO. Compare voltage stability and average power output to fixed-submersion controls across a 12-hour light/dark cycle.
Medium No characterization of the microbial community (bacteria, archaea) in the cathode chamber, despite operating under non-sterile conditions and achieving high lipid content (41 ± 6%).
Section 3.3 notes 'negligible presences of bacteria cells' and attributes high lipid content to low contamination, but provides no 16S rRNA or metagenomic data. This limits understanding of whether heterotrophic bacteria compete with microalgae for oxygen or nitrogen, and whether they contribute to denitrification.
Try. Perform 16S rRNA gene sequencing on cathode samples at days 0, 4, 8 of each batch. Quantify bacterial vs. algal biomass. Correlate community composition with nitrogen removal pathway and N₂O production.
Low No measurement of pH dynamics in the cathode chamber beyond the statement that pH was adjusted to 7.0 every 2 days; no assessment of how pH variation affects ammonia speciation and nitrogen removal.
Section 3.4 states that pH increased from 7.0 to 8.3–8.5 between adjustments, and that free ammonia ranged from 0.8–20.2% of total ammonia in this pH range. However, the paper does not measure the kinetics of pH change or its effect on N-removal rate or pathway.
Try. Measure pH continuously (every 2 hours) using an in-situ pH probe. Correlate pH trajectory with ammonia removal rate and nitrogen speciation. Determine if pH control (e.g., via CO₂ sparging) improves N-removal consistency.
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What they did
- System
- MFC
- Substrate
- real wastewater
What worked
Abstract
The electricity output from microbial fuel cell (MFC) with a microalgae assisted cathode is usually higher than that with an air cathode. The output of electricity from a photosynthetic microalgae MFC was positively correlated with the dissolved oxygen (DO) level in the microalgae assisted biocathode. However, DO is highly affected by the photosynthesis of microalgae, leading to the low stability in the electricity output that easily varies with the change in microalgae growth. In this study, to improve the electricity output stability of the MFC, a partially submerged carbon cloth cathode electrode was first investigated to use oxygen from both microalgae and air, with synthetic piggery wastewater used as the anolyte and anaerobically digested swine wastewater as the catholyte. When the DO levels dropped from 13.6–14.8 to 1.0–1.6 mg/L, the working voltages in the MFCs with partially submerged electrodes remained high (256–239 mV), whereas that for the conventional completely submerged electrodes dropped from 259 to 102 mV. The working voltages (average, 297 ± 26 mV) of the MFCs with the 50% submerged electrodes were significantly (p < 0.05) higher than with other partially or completely submerged electrodes. The associated maximum lipid production from wastewater was 250 ± 42 mg/L with lipid content of 41 ± 6% dry biomass. Although the partially submerged electrode had no significant effects on lipid production or nitrogen removal in wastewater, there was significant improvement in the stability of the electricity generated under variable conditions.
Keywords
Identifiers
- Journal
- Energies
- Year
- 2019
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