IWA Resource Recovery 2025

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Potato fries grown with recycled fertilizers: Closing the phosphate loop

The KNAP project is dedicated to closing nutrient cycles. For more information about this project, please read the KNAP page on our website.

Why do we want to close the cycle?

There is growing attention worldwide on closing loops, both from the perspective of bringing balance to flows and from the idea of a circular economy. But there are also other reasons: the sources of, for example, phosphate, are located in different countries, and for the EU, phosphate and phosphate ore are considered a ‘critical raw resource’. This is due to geopolitical dependence on countries largely outside the EU. Although a large stock of phosphate has been found in Norway, the demand for phosphate continues to rise. In addition to availability, extracting phosphate (ores) from mines also has a negative environmental impact. On the other hand, in the Netherlands, there are problems with an excess of nutrients in some waters. The challenge is to recover nutrients from various sources so that they can be used functionally in areas where they are needed, such as in European agriculture. Incidentally, nutrients can also be used in other applications outside of agriculture; however, the KNAP project focuses on agriculture.

Why from wastewater?

Our food contains nutrients that we consume. These substances then end up in water treatment plants through our sewage system. A significant leak in the nutrient loop occurs in the wastewater chain when these nutrients cannot be reused. It is possible to smartly and safely recover these nutrients and use them as fertilizers. For example, the company ICL Fertilisers produces the fertilizer Puraloop from ash derived from burned sludge. This fertilizer is approved, meaning it can be used as a fertilizer. The potatoes that participants of IWA were able to taste were fertilized with this recycled fertilizer. But there is more potential. Within KNAP, the project partners are also looking at other products and whether those products can be safely and effectively used as fertilizers.

Why from process water?

Process water is water that comes from factories and is used or produced in the production process. In this case, think for example of water used to wash potatoes or rinse pre-cut potatoes. This water eventually exits the factory. This water is a different type of wastewater compared to sewage treatment plant water, and therefore it is regulated differently under the law. This water also contains many nutrients that could be recovered. That is why within KNAP, the project partners are also looking at water streams from breweries, as well as the dairy and potato processing industries, to explore whether recycled fertilizer can be made from those streams.