Wastewater treatment plants present an unconventional yet promising source for hydrogen production, leveraging existing organic waste streams while contributing to circular economy principles. Two primary methods for extracting hydrogen from wastewater are electrolysis powered by renewable energy and microbial electrolysis cells (MECs). These approaches differ significantly from traditional dark fermentation, offering unique advantages and challenges in terms of efficiency, integration, and scalability.

Electrolysis in wastewater treatment involves using renewable electricity to split water molecules into hydrogen and oxygen. The process can utilize the treated effluent, which is relatively free of contaminants, as a feedstock. Renewable energy sources such as solar or wind power the electrolyzers, ensuring low-carbon hydrogen production. Alkaline and proton exchange membrane (PEM) electrolyzers are commonly employed due to their maturity and adaptability. The integration of electrolysis with wastewater plants is straightforward, as it does not interfere with existing treatment processes. However, the method requires high-purity water, necessitating additional filtration steps if untreated wastewater is used. Energy efficiency remains a concern, as electrolysis typically demands 50–55 kWh per kilogram of hydrogen produced, though advancements in electrode materials and system design are gradually reducing these requirements.

Microbial electrolysis cells (MECs) offer a biological alternative by harnessing electroactive bacteria to break down organic matter in wastewater, releasing protons and electrons that recombine to form hydrogen at the cathode. Unlike conventional electrolysis, MECs operate at lower voltages (0.2–0.8 V) since microbial activity provides part of the energy needed. This results in higher theoretical efficiency, with some systems achieving 70–80% conversion of organic content into hydrogen. MECs can process raw wastewater, eliminating the need for extensive pretreatment. Additionally, they simultaneously treat wastewater, reducing chemical oxygen demand (COD) by up to 90%, which aligns with the primary function of treatment plants.

Comparing MECs to dark fermentation (G6) highlights key differences. Dark fermentation relies on anaerobic bacteria to convert organic substrates into hydrogen and volatile fatty acids without external energy input. While simpler in design, it suffers from low yields, typically producing 1–2 mol H₂ per mole of glucose, with a significant portion of energy remaining in byproducts like acetate and butyrate. MECs, in contrast, can further oxidize these byproducts, extracting additional hydrogen and improving overall yield. However, MECs face challenges such as electrode fouling, where biofilm overgrowth or inorganic deposits impair performance. Scalability is another hurdle, as maintaining optimal microbial communities in large-scale reactors is complex.

Integration with existing wastewater infrastructure is feasible for both electrolysis and MECs. Wastewater plants already handle large volumes of organic waste, and retrofitting them with hydrogen-producing systems could offset operational costs. Sludge valorization is an added benefit, as anaerobic digestion of sludge can supply biogas to power electrolyzers or MECs, creating a synergistic loop. Some facilities have piloted combined systems where sludge-derived biogas is reformed into hydrogen, though this requires additional purification steps.

Despite the potential, several challenges hinder widespread adoption. Electrolysis faces high capital and operational costs, particularly for PEM systems, which rely on expensive catalysts like platinum. MECs, while cost-effective in theory, struggle with durability issues, as electrode materials degrade over time. Contaminants in wastewater, such as sulfides or heavy metals, can poison catalysts or inhibit microbial activity. System optimization is ongoing, with research focusing on robust electrode coatings, cheaper catalysts, and modular designs for easier scaling.

Regulatory and economic barriers also play a role. Wastewater treatment plants are primarily mandated for environmental compliance, not energy production. Incentives or policy frameworks would be necessary to prioritize hydrogen extraction. Public acceptance is another consideration, as safety concerns around hydrogen storage and handling must be addressed.

In summary, hydrogen production from wastewater treatment plants via electrolysis or MECs presents a dual opportunity for energy generation and waste valorization. While electrolysis offers simplicity and compatibility with renewables, MECs provide higher efficiency and integrated treatment benefits. Both outperform dark fermentation in yield but face distinct technical and economic hurdles. Advances in materials science, process engineering, and policy support will be critical to unlocking this potential and making wastewater-to-hydrogen a viable component of the green energy landscape.