Due to strict environmental regulations, farmers are not allowed to spread all of their manure on their land. At the same time, they use synthetic fertilizers to provide their crops with sufficient nutrients. The result: a growing manure surplus and the loss of valuable nutrients. PhD researcher Marrit van der Wal is working on a circular solution: a method to separate nitrogen and potassium from manure and make them reusable.
Source: Cursor / Martina Silbrníková
"I'm from Friesland, my father works as a livestock feed trader, and many of my friends and acquaintances are farmers. So I'm very familiar with the challenges they face," Van der Wal explains. When she read the project description, she didn't hesitate for a second. "I even moved to the south of the country for it." From the very beginning, her goal was clear: through her PhD research, she wants to make a real contribution to solving the current problems in the agricultural sector.
The manure problem
According to her, it's not an easy time for farmers. "They're dealing with several major challenges, and manure is one of them." Manure contains important nutrients such as nitrogen (N), phosphate (P), and potassium (K). These substances are essential for plant growth. But an excess of manure can harm the environment. "When more manure is applied than the crops can absorb, nitrogen and phosphate leach into the groundwater and surface water," Van der Wal explains. This causes algae to grow in ditches and lakes, leading to suffocation of fish and other aquatic life. Moreover, untreated manure emits ammonia, which also contributes to environmental pollution.
To reduce these environmental effects, farmers must comply with strict regulations that limit how much nitrogen and phosphate from animal manure they can use on their land. In the Netherlands, where a relatively high number of animals are kept on a relatively small area of farmland, this quickly leads to a manure surplus that farmers are not allowed to use. The excess manure must be exported or processed, often at additional cost.
A striking paradox
Because farmers can't use all their manure, their crops don't get enough nitrogen or potassium, so they compensate with synthetic fertilizers. These are usually produced via the energy-intensive Haber-Bosch process, which emits large amounts of CO₂ and causes additional environmental strain.
This leads to a paradox: manure full of valuable nutrients must be disposed of or treated at a cost, while at the same time, polluting fertilizers are purchased to reintroduce those same nutrients. "That's quite harsh, and such a waste," says Van der Wal.
"This whole fertilization process has become very complicated," she continues. The amount of manure farmers can apply - and how much synthetic fertilizer they're allowed to add - depends on soil type, weather, crops, surrounding water systems, and many other factors. "It's really hard to align all those variables. You almost need a PhD to understand it all and get it just right," she says with a smile.
From waste to circulation
To break this vicious cycle, researchers are exploring ways to use manure more efficiently. "What if we could process the manure in a way that allows us to extract the nitrogen?" The processed product is no longer classified as manure, but as so-called RENURE - Recovered Nitrogen from Manure. This is covered by evolving European regulations that define how much synthetic nitrogen can be replaced by RENURE. "You can't fully replace synthetic fertilizers yet, but using RENURE is an important step toward more circular and environmentally friendly agriculture."
Van der Wal's research specifically focuses on separating nitrogen and potassium from manure, making them available in concentrated form so they can be applied more precisely in the fields. She uses membrane technology - a separation method in which a membrane acts as a filter, allowing certain substances through while blocking others. There are different types of membrane technology, and Van der Wal focused on improving those already in use
Into the field
To fully understand how manure processing works in practice - and to develop a solution that's truly useful for farmers - Van der Wal didn't stay confined to the lab. She quite literally "went out into the field."
She regularly visited the manure processing company Duurzaam Landleven Bernheze. Not only to collect real manure samples - "You can recreate a lot in the lab, but nothing beats working with the real material" - but also to have discussions about the bottlenecks and how technology can help solve them.
The right dosage
Her most important contribution is the development of a method that separates nitrogen and potassium into distinct fractions. This allows farmers to apply these nutrients separately and in the correct dosages. "If you're working with a fixed nitrogen-potassium ratio, there's no flexibility," she explains. With three separate fractions - rich in nitrogen, phosphate, and potassium, respectively - farmers can work more precisely and flexibly. "They can adjust the ratios and apply exactly what's needed, depending on the time of year, soil type, or crop."
Implementing this new technology is the next major step, but Van der Wal is optimistic. "In general, I see a strong willingness among farmers to adapt and improve their processes." And in this case, the benefits are clear: processing manure into reusable products can make their operations more efficient while helping them meet environmental regulations. This enables farmers to continue their profession in a sustainable, future-proof way. "To make that possible, we need to move toward systems where nutrients are not lost, but circulate within the agricultural business," Van der Wal concludes.
