'Manure is a raw material, we should no longer see it as waste'

The cover of Marrit van der Wal's (26) dissertation features a self-drawn pink pig with a satisfied smile and a red handkerchief tied around its head. The handkerchief, known as a symbol of the farmers' protests, has a more personal meaning for her: "I understand that the farmers are protesting, because they are in a difficult position. But for me it doesn't so much stand for protest, but for being proud of the farmers, because they keep going despite all the challenges."

It was precisely these farmers that Van der Wal wanted to support with her PhD research into manure processing at the Eindhoven University of Technology. She grew up among the Frisian meadows and saw up close how complex farming life is. Her father is a cattle dealer and many of her friends are farmers. "If you live in Friesland, everyone knows a farmer," she says with a smile.

The biggest challenge, says the chemical technologist, is the nitrogen problem in the Netherlands, in which the manure surplus plays an important role. Animal manure is rich in nutrients, but too much of it causes pollution of groundwater and surface water. In addition, ammonia is released when spreading manure, which is bad for air quality. That is why farmers are not allowed to spread manure without limits, which creates a surplus. "And that could amount to as much as five million tons in Friesland in the coming years."

As an alternative, artificial fertilizer is often used, but this solution also has disadvantages. It is produced via a chemical process: the energy-intensive Haber-Bosch process , which removes nitrogen from the air. According to Van der Wal, this is responsible for about 1 percent of global CO ₂ emissions. "It is of course a bit harsh," she emphasizes. "That we have to throw away something that we actually already had, only to have to remove it from the air again."

Tailor to the need

That was the impetus for her to work on a practical solution. “I thought: if I do a PhD, I want to do something that hopefully helps farmers,” she recalls. She developed a technique to separate nitrogen and potassium from manure. That is important because every plant needs nitrogen and potassium, but in different proportions. By applying them separately, farmers can tailor the nutrients more precisely to the needs of the crop.

The title of her dissertation ( No time to waste manure ) succinctly summarizes her message: manure is not waste, but a raw material. This raw material can be processed with membrane technology, comparable to a coffee filter. "When you make coffee, the larger solid particles remain in the filter: the coffee grounds. The water passes through, together with the caffeine and the flavor." By playing with the acidity in her 'coffee machine', the chemical engineer can ensure that, for example, only nitrogen and water pass through the membrane, while the rest remains behind.

The technology already existed, but up until now, membranes had only succeeded in recovering nitrogen together with potassium. Van der Wal improved the process, in collaboration with a manure processing company. This way, manure processors can eventually obtain and sell three products locally from one source: nitrogen, potassium and a phosphorus-rich solid.

This technique is a step towards circular agriculture, in which farmers reuse 'waste' as much as possible. In the future, the processed manure could replace artificial fertilizer .

In the lab, Van der Wal's research attracted attention. "I was testing a lot of membranes, but I also had to go to the manure processing plant to pick up samples. I didn't make my colleagues very happy with the smell in the lab," she says, laughing.

In theory, she has achieved her goal of helping farmers. The environment benefits and the farmer also benefits: lower costs for artificial fertilizers and less manure to dispose of. In addition, they could apply the technology to their own farm to process the manure from their stables.

In the eyes of the law it remains manure

Unfortunately, the law is in the way. Because the end product is still classified as 'manure', it falls under the same restrictions on use as raw manure. "If you are not allowed to use the technology, what is the point?" Van der Wal hopes that the legislation will change. "If you do research, you have to be a little hopeful. But now the government has fallen - then everything will take longer before there is a real conclusion."

Disappointed, Van der Wal notes that her research will not be applied in practice for the time being. The improved technique has only been tested in the laboratory so far. "It would of course have been very useful if we could have tested this on a somewhat larger scale. That opportunity was not yet available because there was no company ready to take it on."

"I have seen that it is very difficult to really contribute something big. Doing research is already very difficult in itself. So you can be happy that a good result comes out after only four years of PhD research."

Although she would have liked to do even more, she hopes that her technology can actually help farmers in the future: “In the Netherlands we can be a little more proud of the farmer, and help them a little more instead of seeing them as a problem – maybe then we can make a step forward together.”