From wastewater to green hydrogen: Closing the loop in Denmark

Esbjerg has long been defined by its relationship with water. First as a fishing harbour, then as the operational base for Denmark's North Sea oil and gas industry, the town has built its identity on the resources the sea provides. Now, it is at the centre of something different: more than 2 gigawatt water electrolysis that could become one of the largest of its kind in Europe.

The scale is significant. But so is the engineering challenge behind it.

Water quality is not a footnote - it’s a fundamental input

Water electrolysis depends on more than renewable electricity. It requires Ultrapure Water (UPW) as a process input. At scales above 2 gigawatts, the volumes involved are substantial, and in a region like Esbjerg, drawing from scarce groundwater reserves is technically straightforward and well understood. However, legal approval is extremely difficult due to competing drinking water interests, existing unsustainable withdrawals, and high local demand. Adding to the challenge, there are no permitted discharge options for non-usable water within the protected area.

The approach being developed here takes a different starting point: Treated municipal wastewater. Using advanced membrane-based treatment, the facility is designed to produce Pure Water at over 85% efficiency from a source that would otherwise be discharged. This reduces pressure on local freshwater supply while making productive use of an existing resource, reducing the pollution of the protected area compared to today.

That efficiency figure is worth noting - not as a headline, but as an engineering benchmark. Achieving it consistently, at scale, and within the constraints of a municipal wastewater input is a non-trivial process challenge.

The reject stream problem

What makes this project technically distinctive, and genuinely complex, is what happens to the water that does not make it through the purification process.

Membrane-based treatment concentrates what it removes. The result is a reject stream carrying elevated levels of nitrogen, arsenic, phosphate, and PFAS - contaminants that cannot simply be discharged. The facility sits adjacent to a protected coastal environment, and both local and national regulations require that this reject stream is treated before release.

This is not a straightforward problem. Dedicated Reject Water Treatment (RWT) systems for large-scale pure water facilities are rare globally. The engineering work involves designing a treatment train capable of handling a chemically complex and variable reject stream, while meeting strict environmental discharge standards.

The technical work combines membrane filtration, advanced oxidation, and targeted chemical treatment. Getting the chemistry right across a range of operating conditions is as important as the physical design of the system.

Stakeholder alignment is part of the engineering

Projects at this scale do not succeed on technical merit alone. Delivering water for 2GW of water electrolysis in an active coastal municipality involves utilities, regulators, local government, environmental bodies, and community stakeholders. Each with legitimate interests and different thresholds for risk and uncertainty.

Part of the engineering challenge is making the technical case legible to non-technical decision-makers: Explaining why a reject water treatment system that meets or exceeds regulatory requirements is an integral part of the facility design, not an add-on. It also means working within permitting frameworks that were not written with facilities of this type in mind.

Getting stakeholder alignment early, and maintaining it as design evolves, is not soft work. It is a core project delivery discipline.

What this means for the wider PtX sector

The Power-to-X sector is expanding rapidly, driven by EU energy policy and national hydrogen strategies. Denmark is among the leading markets, but the water question is a challenge shared across geographies. As facilities scale up, the combined demands for high-quality process water and responsible wastewater management will become a constraint that project developers cannot afford to treat as secondary. This approach could help other water-scarce regions meet their energy demands while conserving water.  

The Esbjerg project demonstrates that it is technically feasible to use treated municipal wastewater as a primary input, manage the resulting reject stream to a high environmental standard, and do so at a scale relevant to industrial hydrogen production. It also illustrates that this kind of integrated water design needs to be built into the project from the outset - not resolved at the permitting stage.

For NIRAS, this work sits at the intersection of our water and green fuels expertise, where complex engineering meets an evolving regulatory environment. The projects that will succeed are the ones that treat water not as a utility input to be minimised, but as a core part of the system design.

Justin Borrison and Sebastian Ravn-Andersen will present this work at IFAT in Munich in May 2026. If you would like to discuss the water treatment challenges associated with your PtX project, please get in touch.