Microbial electrolysis cells (MECs) extend the logic of microbial fuel cells: microbes break down organics in wastewater, and with a small electrical input, the system produces hydrogen at the cathode. In principle, MECs turn wastewater treatment from a cost into a source of green-ish hydrogen. In practice, they are still early-stage. This brief explains how MECs work, where the technology stands, and how their economics compare with conventional electrolysis.
What You'll Learn
- 1. MEC Technology Basics
- 2. Energy Balance & Hydrogen Yield
- 3. Integration with Wastewater Treatment
- 4. Economics vs Conventional Electrolysis
- 5. Use Cases: Niche vs Scale
- 6. Devil's Advocate: Technical & Scale-Up Risks
- 7. Outlook to 2030: MECs in the Hydrogen Landscape
- 8. FAQ: Questions from Utilities & Innovators
1. MEC Technology Basics
MECs are electrochemical systems where:
- At the anode, electroactive microbes oxidise organic matter in wastewater, releasing electrons and protons.
- At the cathode, with a small applied voltage, protons are reduced to hydrogen gas.
Energy-Assisted Treatment
Part of the electrical energy needed for electrolysis is effectively supplied by the chemical energy in wastewater organics.
Dual Benefit
MECs provide wastewater treatment and hydrogen production in a single system.
Still Early
Most MEC deployments remain at lab or pilot scale; materials and reactor designs are evolving quickly.
2. Energy Balance & Hydrogen Yield
The theoretical energy input for MECs can be lower than stand-alone electrolysis because microbes provide part of the driving force. In practice:
- Applied cell voltages ~0.2–1.0 V are common in experimental systems.
- Hydrogen yields per kg COD removed depend on reactor design, overpotentials and losses.
Illustrative Energy Comparison (Indicative, per kg H2)
| Route | Electricity Input | Comment |
|---|---|---|
| Conventional electrolysis (PEM) | ~ 50–55 kWh/kg H2 | From deionised water, no treatment benefit. |
| MEC (experimental) | ~ 30–45 kWh/kg H2 (effective) | Part of energy comes from wastewater COD; values are still highly variable. |
Illustrative Electricity Requirement per kg H2
Indicative comparison of electricity input for MEC vs conventional electrolysis under optimistic but plausible assumptions.
3. Integration with Wastewater Treatment
MECs are often envisioned as a polishing step or side-stream integrated into existing WWTPs:
- Treating high-COD side streams (e.g. industrial discharges, digester liquors).
- Coupling with anaerobic processes to maximise electron recovery.
- Potential co-location with hydrogen users (e.g. onsite mobility, industry).
From a broader waste-to-energy planning perspective, utilities usually benchmark MEC pilots against more mature options already covered on Energy Solutions, including sewage sludge energy recovery configurations, upgrading biogas to biomethane via membrane and water-scrubbing routes, and landfill gas recovery projects targeting RNG pipeline injection.
4. Economics vs Conventional Electrolysis
Today, MECs are far from cost-competitive with mature electrolysis for bulk hydrogen, but they may be interesting in niches where:
- Wastewater treatment costs are high, and MECs can offset part of OPEX.
- There is local demand for small volumes of hydrogen.
- Innovation budgets support pilots and demonstrations.
Relative Cost & Maturity
Qualitative positioning of MECs vs conventional electrolysis in terms of cost and technology maturity.
Comparative Benchmark: MEC vs PEM vs SMR+CCS (Illustrative)
| Route | Indicative TRL (1 119) | Full H2 Cost Today (order, ac/kg) | Electricity Input (kWh/kg H2) | Notes |
|---|---|---|---|---|
| SMR + CCS | 8 119 | ~ 2 113 (region-dependent) | n/a (mainly natural gas input) | Lower-cost low-carbon benchmark where gas and CO2 storage are available. |
| PEM electrolysis (renewable power) | 7 118 | ~ 4 118+ at current power prices | ~ 50 1155 | Mainstream route for green H2 roll-out this decade. |
| MEC (wastewater) | 3 114 | > 8 1112 (pilot-scale, high uncertainty) | ~ 30 1145 (effective) | Electricity partly offset by wastewater organics; still far from commercial cost levels. |
Values are order-of-magnitude indicators for framing strategy, not bankable numbers. Local power prices, utilisation factors and financing conditions drive actual project costs.
5. Use Cases: Niche vs Scale
Plausible early use cases include:
- On-site hydrogen for lab or fleet pilots at WWTPs or industrial parks.
- Integration in research campuses and innovation districts.
- Longer-term, partial contribution to decentralised hydrogen supply in regions with strong water-energy nexus strategies.
Case Study – Pilot MEC at a Municipal WWTP
A city may deploy a 5–20 kW scale MEC pilot treating a side stream:
- Hydrogen used for on-site fuel cell demo or mobility pilot.
- Data gathered on treatment performance, energy consumption and O&M.
- Project framed as innovation and learning rather than pure financial IRR play.
6. Devil's Advocate: Technical & Scale-Up Risks
Major challenges include:
- Materials and durability: Electrodes, membranes and catalysts must withstand harsh wastewater conditions.
- Biofilm control: Maintaining healthy electroactive communities over time at scale.
- System complexity: MECs add another layer of complexity to already complex WWTPs.
7. Outlook to 2030: MECs in the Hydrogen Landscape
By 2030, MECs are likely to remain a specialised, niche technology but could play useful roles in:
- Demonstrating integrated water–hydrogen systems.
- Serving small, local hydrogen loads as part of innovation districts.
- Informing the design of future hybrid electrochemical–biological systems.
For most utilities and cities, the realistic deployment arc looks like:
- 2025: Lab rigs and a handful of 1–15 kW pilots, primarily research-driven.
- ~2030: 10–150 kW MEC units embedded in WWTPs and campuses to test integration, control and O&M at slightly larger scale.
- 2035+ (optionality): Potential 0.5–1.5 MW demonstration systems if technology, materials and policy signals justify stepping up.
For now, MECs should be viewed as R&D and pilot opportunities rather than mainstream hydrogen supply options.