When Europeans hear about the EU's over-reliance on imports for critical raw materials, they immediately think of exotic minerals like lithium, cobalt, or other rare earths, usually tied to Europe’s green and digital transition. Yet, more often than not, some of the most strategic and vulnerable resources under this definition are, in fact, the minerals that feed our crops, particularly phosphorus and potash. Generally grouped under the NPK acronym – nitrogen, phosphorus, and potassium – these nutrients are the backbone of modern fertilisers; and Europe imports most of its phosphates and potash, as well as a significant share of its nitrogen. That is why most of the framing about them in the news verges on security matters. However, a less-discussed problem is the environmental impact of extracting these resources in the first place.

Open-pit phosphate mining, for instance, typically involves clearing large areas of vegetation, fundamentally altering landscapes. nitrogen fertiliser production, and to a lesser extent phosphate fertiliser production, is highly energy-intensive and depends directly on hydrocarbons. The Haber–Bosch process, in particular, which is the most widely used industrial method in this sector, synthesises ammonia from nitrogen and hydrogen – the latter alone accounting for as much as 5% of global natural gas use.

But many of these nutrient-rich compounds are also found in large quantities in our wastewater, from municipal sewage to food-industry effluents. If untreated, these end up being pollutants. But if removed, they can serve as a domestic, environmentally-friendly source of nitrogen and phosphorus, partially closing the loop and reducing dependence on imported fertilisers.

Fundación CARTIF, a Spanish-based R&D institution, has developed through the project WalNUT exactly this kind of solution. But far from just addressing the extractive aspects of the matter, the people involved are also crafting the policy language that this new tech will require in the future, as explained by project coordinator and chemical engineer Dr Francisco Corona Encinas.

How did CARTIF become involved in the WalNUT project?
CARTIF is actually the project's creator and coordinator. But the idea of the project itself was born out of previous similar projects we had already worked on with other partner institutions. So, our involvement came naturally.

Unlike the other countries partnering with WALNUT, Spain hosts two pilots. What role do they play in the broader project?
Yes, one pilot is in Zamora, in central Spain, while the other is in Galicia, in the northwest of the country. But the two pilots actually use different technologies and treat different types of wastewater. Zamora’s site processes industrial wastewater, while Galicia’s treats urban sewage.

Do they have much in common?
Well, they both help us validate how nutrient-recovery technologies behave under very different conditions and wastewater characteristics. The idea is to understand how these solutions perform when the composition of the water and the operating environment change. By comparing results from both pilots, we can better match each technology to the type of wastewater and the kind of biofertiliser we want to obtain.

Let’s focus on the Zamora demonstrator. Why was this site selected, and what makes it suitable for your technology?
The pilot in Zamora is based on technology developed by CARTIF and is now being tested by Veolia, a French company specialising in water treatment, at one of their wastewater treatment plants. The site was ideal because it treats industrial wastewater from the food sector, which has the nutrient profile needed for microalgal recovery of key nutrients such as nitrogen and phosphorus. Another advantage is that the pilot is energy self-sufficient, since it uses solar power. That’s something Spain has plenty of – daylight.

So, Zamora recovers nitrogen and phosphorus from industrial wastewater. Why are these nutrients so important today?
The issue with typical inorganic fertilisers available in the market is that the raw materials we use to make them, especially phosphorus, come from non-renewable and critical sources, while nitrogen fertilisers depend heavily on fossil energy. It should also be noted that raw materials, in and out of agriculture, are becoming scarcer, meaning we need to search for new sources. Recovering nutrients from wastewater helps reduce dependence on non-renewable fertilisers and provides an alternative or renewable product. Also, this is an opportunity to make this category of fertilisers, the bio-based ones, more cost-effective.

You mean bio-based fertilisers can’t compete with synthetic ones at the moment?
Right now, no. Synthetic fertilisers benefit from economies of scale. Bio-based fertilisers are still in an early stage, and the technologies need time to mature. At the same time, synthetic fertilisers are quite energy-intensive, and energy costs are rising. The objective in the coming years will be to develop robust technologies that can offer a real alternative to conventional fertilisers. This may just be one of the most effective ones.

Could you walk us through the process being tested in Zamora?
It is a microbiological technology that uses microalgae to recover nutrients. As I said, the industrial wastewater in question comes from the food industry. We use microalgae first to consume the nutrients contained in the water. That’s the first part. The process then allows us to remove and recover nitrogen and phosphorus, transforming components that are usually difficult and costly to remove into something that can be put to use.

How does this technology complement existing wastewater treatment processes?
Mainly through reducing the complexity of the treatment itself. It makes it easier to remove nitrogen, phosphorus, and other components. Instead of just removing them, we can recover them.

What are the next steps for WalNUT, especially from your perspective as technical coordinator?
Well, right now we are working towards validating technologies at pilot scale while also conducting field testing of the obtained biofertilisers. Further steps involve conducting sustainability assessments at the environmental, economic, and social scales. We are also developing business models for each technology. But part of the work is done at the policy level too.

So, the aim here is also to lay the groundwork for new administrative-level guidelines?
Yes. The project also aims to help the European Commission by creating a common lexicon for bio-fertilisers. Current legislation is not homogeneous, and some terms need updating. Hopefully, these will be the first steps toward a shared legislation we’ll use in the future at the EU level.