Bioelectrochemical systems convert organic matter directly into current or hydrogen using bacteria that respire onto an electrode. The science is genuinely elegant. The arithmetic on electrode area is what decides whether it can ever treat municipal sewage — and it is not close.
How bacteria respire onto an electrode
Electroactive organisms — principally Geobacter sulfurreducens and Shewanella oneidensis — can use a solid electrode as their terminal electron acceptor in place of oxygen or nitrate. Three transfer mechanisms operate:
- Direct contact. Outer-membrane c-type cytochromes touch the anode.
- Conductive pili. Protein filaments with metal-like conductivity bridge the gap from cell to electrode, allowing organisms not in contact to respire.
- Mediated transfer. Soluble shuttles — flavins, phenazines — ferry electrons across.
Anode potential sets the metabolic state. Around −0.2 V against Ag/AgCl, Geobacter reaches maximum current density at roughly 2–5 A/m². Push the potential more positive and oxidative stress reduces viability — more driving force does not mean more current.
Microbial fuel cells
Bacteria oxidise organics at the anode; electrons travel an external circuit to a cathode where oxygen is reduced.
Anode: C6H12O6 + 6H2O → 6CO2 + 24H+ + 24e−
Cathode: O2 + 4H+ + 4e− → 2H2O
Cell voltage is the thermodynamic difference less three losses:
Ecell = Ecathode − Eanode − ηact − ηohmic − ηconcActivation, ohmic and concentration overpotentials respectively
| Quantity | Domestic wastewater, COD ≈300 mg/L |
|---|---|
| Open circuit voltage | ≈0.75 V |
| Operating voltage at 1 A/m² | ≈0.35 V |
| Power density | ≈0.35 W per m² of anode |
| Coulombic efficiency | 20–40% |
Losing more than half the open circuit voltage at a current density of 1 A/m² is the fundamental constraint. Coulombic efficiency is low for the same reason any anaerobic process loses substrate: methanogens and fermenters take a share of the electrons before the electrode does.
Microbial electrolysis cells
An MEC adds a modest applied voltage to drive hydrogen evolution at the cathode:
2H+ + 2e− → H2E° = −0.41 V vs SHE at pH 7
Thermodynamically the requirement is only about 0.14 V, against roughly 1.23 V for water electrolysis — the bacteria have already done most of the work. Overpotentials in practice push the requirement to 0.6–0.8 V.
Yield follows from the electrons recovered:
YH2 = (CE · nsubstrate) / (n · F) · ηcathode
For acetate at eight electrons per molecule and 80 per cent coulombic efficiency, roughly 0.35 mol hydrogen per mol acetate. The comparison that matters is against conventional water electrolysis: 0.6–0.8 V is a genuine saving on 1.23 V, but only competitive if the treatment value is counted alongside the hydrogen.
The scale-up arithmetic
This is where enthusiasm meets Faraday. The electron flux implied by a municipal organic load fixes the electrode area required, and the number is not negotiable by better engineering.
- Organic load: 10,000 × 100 g COD/cap/d = 1,000 kg COD/d.
- Charge equivalent: 8 g COD per mole of electrons, so 1,000 kg COD ≈ 1.25 × 105 mol e−/d ≈ 1.21 × 1010 C/d.
- Current at 100% coulombic efficiency: ≈140,000 A.
- Anode area at 0.5 A/m²: ≈280,000 m².
- At a realistic 30% coulombic efficiency: ≈42,000 A, still ≈84,000 m².
That is between 8 and 28 hectares of anode surface for a small works. Even allowing three-dimensional electrodes with high specific surface area, the material quantity and the cost of the current collection are prohibitive.
| Barrier | Detail | Direction of travel |
|---|---|---|
| Electrode cost | Graphite felt and carbon cloth are expensive per square metre — and the area required is enormous | Stainless mesh with activated carbon coating |
| Membrane cost | Perfluorinated ion-exchange membranes are prohibitive at municipal scale | Bipolar or membrane-less designs |
| Footprint | Anode area implies a plant far larger than conventional activated sludge for the same load | Three-dimensional and high-surface-area electrodes |
| Coulombic efficiency | 20–40% on domestic wastewater; methanogens take the rest | Selective inhibition, shorter retention |
Where it makes sense, and where it does not
That said, the technology is not without application. It performs where the constraint is not throughput:
- Remote sensing and instrumentation power. Milliwatts from ambient organic matter, where no other supply exists.
- High-strength industrial effluent above roughly 5,000 mg/L COD, where the electron flux per unit volume is an order of magnitude higher than domestic sewage.
- Landfill leachate, for the same reason.
- Hydrogen from concentrated streams, where the 0.6–0.8 V requirement genuinely undercuts water electrolysis and the treatment credit can be counted.
For UK application before 2030, expect industrial pilot demonstrations rather than municipal deployment. The comparison to make is against anaerobic digestion, which recovers energy from the same electrons through a route that is already economic, and against the wider net zero options that compete for the same capital.
Frequently asked questions
Why can microbial fuel cells not treat municipal sewage?
Electron flux. A 10,000 PE organic load corresponds to roughly 140,000 amperes at full coulombic efficiency, and at an achievable 0.5 A/m² that is about 280,000 m² of anode — some 84,000 m² even at a realistic 30 per cent efficiency. That is tens of hectares of electrode for a small works. The limit follows from Faraday’s constant, not from immature engineering.
Are MECs competitive with water electrolysis for hydrogen?
On voltage alone they look attractive: 0.6–0.8 V against about 1.23 V, because the bacteria have already done most of the thermodynamic work. Whether that translates into competitiveness depends on counting the treatment value alongside the hydrogen, and on the same electrode-area constraint that limits fuel cells.
Why is coulombic efficiency only 20-40%?
Because the electrode is competing for electrons with methanogens and fermenters that are already present in the mixed culture. They consume substrate that never reaches the anode. Raising efficiency means suppressing those organisms, which is the same control problem faced in any anaerobic process.
Is there any realistic UK application?
Yes, but not municipal. High-strength industrial effluent above roughly 5,000 mg/L COD, landfill leachate, and remote low-power instrumentation are the credible niches — all cases where volumetric electron flux is high or where the alternative power supply is absent. Municipal deployment is not economically justified at current energy prices.