IWA Resource Recovery 2025
To make fries from potatoes, you need large-sized potatoes that have grown well thanks to a healthy soil. Typically, farmers use synthetic fertilizers containing N-P-K: nitrogen (N), phosphate (P), and potassium (K). Plants need these nutrients to grow and produce fruit. However, producing synthetic fertilizers requires a lot of energy and results in high CO₂ emissions. Phosphate, in particular, is becoming increasingly scarce, and global supplies are finite. That’s why it’s important to retain valuable nutrients (like N, P, and K) that naturally occur in our food.
Within the KNAP project, 46 partners are working together to return valuable nutrients from residual streams back to agriculture. In doing so, we close the loop – safely, responsibly, and effectively.
From wastewater treatment to circular fertilizer
Currently, phosphate and many other nutrients end up in wastewater. After the treatment of this wastewater by water authorities, sewage sludge remains – rich in natural nutrients, but not yet clean enough for direct reuse. Sludge processors such as HVC and SNB already extract heat and energy from this sludge by drying and incineration. The incineration process also destroys and removes pollutants like pathogens, microplastics, and pharmaceutical residues. What remains is a mineral ash full of usable raw materials, from which nutrients are recovered.
ICL uses this ash in the production of Puraloop – a circular phosphate fertilizer that is completely safe, effective, and certified. In this way, a residual product from the water purification chain gets a second life on the land. And it contributes sustainably to a fertile soil for the potato – and thus also for the fries.
Field trial: circular and effective
Together with our partners, KNAP carried out a field trial with potato cultivation. Fourteen innovative recovered fertilizers were applied, including one already approved for the market: Puraloop.
The results from these fouteen test fields are promising: healthy potato plants with a strong harvest, and a proven contribution to circular agriculture.
We are therefore happy to share both the results and the fries made from potatoes grown in the trial field fertilized with Puraloop. This demonstrates that high-quality recycling is not only possible – it also pays off.
Why not all residual streams are being used yet
Although more and more circular fertilizers are becoming available, many nutrient streams remain unused. Why? Technical challenges, legislation, and the need for up-scaling.
One example of an untapped stream is the nutrients found in process water from the agri-food industry – such as the potato processor that makes your fries, as well as breweries and dairy factories. This process water, used during food production, is rich in valuable nutrients and minerals. When it can no longer be used, it ends up in water treatment. While cleaner than domestic wastewater, it still contains too many nutrients to be discharged and thus needs purification. During purification, the nutrients are consumed by bacteria, which clean the water. The nutrients remain in what is called ‘aerobic biomass’. Currently, this nutrient rich biomass is also incinerated, just like sludge. But wouldn’t direct use be more efficient?
Within the KNAP project, we are also exploring how this biomass can be used as a source for safe, circular, and natural fertilizers. This way, we bring the hidden value of the agri-food industry back to the land – and close the loop.
Together, we complete the cycle
At KNAP, we work on solutions – from innovation to policy – so that more and more of these residual streams, such as aerobic biomass, can safely return to the land.
This collaboration between HVC, SNB, Lamb Weston, ICL, and the Nutrient Platform shows that it can be done: using residual streams to grow new, healthy crops. From ash from water treatment to nutrients for potato plants – from waste to valuable, sustainable fertilizer. Together, we are building an agriculture that makes smarter use of our resources – and is ready for the future.