Technology can make a major contribution to reducing nitrogen emissions in Dutch agriculture according to scientists from Wageningen University & Research. They have mapped out which technologies are the best investment in order to have a maximum effect on lowering nitrogen emissions in the sector.
Circular agriculture with nitrogen in balance is the title of the knowledge base programme in which six Wageningen research institutes have worked together to explore the potential of technology to tackle the nitrogen problem. There is a huge surplus in the Netherlands, mainly due to the way far more nitrogen enters the system annually via food, animal feed and the production of artificial fertiliser than is removed.
Peter Geerdink was project leader on behalf of Wageningen Food & Biobased Research. "An effective approach to the Dutch nitrogen issue requires a broad perspective that goes beyond policy measures such as reducing livestock and buying out farmers. There are many things we can do from a technological viewpoint to help resolve the problem." This conclusion is in line with the nitrogen pact drawn up by LTO, Natuurmonumenten, Natuur & Milieu, VNO-NCW and Bouwend Nederland at the end of May 2021 which proposes six billion euros of investments in 'wide-ranging innovations' for agricultural companies.
Residual streams for animal feed
The scientists started the project by looking into which buttons could be pressed to achieve a more balanced nitrogen emission level in agriculture. Reducing the import of animal feed is one such option. According to Geerdink, the import of (mainly) soybean meal adds over 400 million kilos of nitrogen to the Dutch nitrogen cycle each year.
"In addition to cutting back on livestock, import reduction can be achieved by replacing important resources with residual streams from locally grown crops. These streams are now largely processed in a low-quality way such as in compost or for the production of biogas. Farmers who process these residual streams into animal feed could potentially be more profitable. Refining and processing into the right specifications can convert them into healthy, nutritious and tasty animal feed."
Preventing ammonia losses from manure
Another button that can be pushed that will reduce the ammonia losses on and around the farm can be found in the cow shed. The cow shed and its ammonia emissions make a large contribution to the nitrogen deposits in nature, which strengthen the growth of some plant varieties (grass, nettles, blackberries) at the cost of others. Cow sheds are kept open to create natural ventilation, which is a major condition for cattle health. As a result, the air is not filtered (as is the case in closed pig sheds) and large amounts of nitrogen leak into the outside air as ammonia from manure. Technically speaking there is much to be gained from developing and installing air washers into the open sheds.
Another technological challenge involves preventing ammonia from escaping stored manure to begin with. Geerdink: "We have to work towards closed systems for manure storage to avoid the ammonia disappearing into the air. Every year, some 71 million kilos of ammonia is released from animal manure that could instead be applied as a raw material for artificial fertiliser. Artificial fertiliser is traditionally produced by converting nitrogen from the air into ammonia via an energy-intensive process. This adds 245 million kilos of nitrogen to the Dutch cycle on an annual basis. Research is currently being performed into electrochemical processes to make artificial fertiliser from captured ammonia at the farm itself. This would be extremely effective as it would mean manure would no longer have to be transported."
Affordable filtration
The remaining manure that is transported to regional manure processing plants is converted into artificial fertiliser replacement, biogas and organic slurry. Efficiently capturing nitrogen from liquid manure and fermented biomass requires further technological steps. With this in mind Wageningen University & Research is working on affordable membrane filtration that can be applied to 'dirty' streams. Geerdink: "The current processes are very costly because they demand lots of pre-processing to purify the streams and prevent blockages. The first filtration tests with dirty material from the manure processing plant are promising, but we have yet to reach the stage where the new, efficient technology can be scaled up."
Separating ammonia and potassium
Another challenge during membrane filtration is separating the ammonia and the potassium. "Both chemicals are positively charged and the molecules are approximately the same size too," Geerdink explains. "While this makes them difficult to separate, it is important to do so in order to use them in a circular way because potassium and nitrogen are needed at different times and for different crops." The scientist adds that investments in fundamental research are needed to complete this complex puzzle.
A great deal of research is already being carried out into options for 'stripping' ammonia from organic slurry in an energy-efficient way. Geerdink: "This is currently being achieved by first letting the ammonia evaporate and then capturing it in another liquid, a process that consumes a lot of energy. Transmembrane chemisorption allows ammonia to be removed from liquid manure such as RO concentrate directly via a membrane. This seems to have particular potential for liquid manure after separation (in which the potassium has been largely removed), as transmembrane chemisorption results in a low nitrogen and low potassium liquid. The technology requires very little energy compared to stripping, for example. Moreover, having been in development for quite some time, there is already lots of insight into the mechanisms involved. We expect that a breakthrough in terms of practical solutions to be achieved soon."
Another development on which the programme is working is the application of electrodialysis technology. This combines an electrochemical cell with electrodes and membrane pairs with the latest insights into the process in order to selectively remove ammonia and potassium.
The exploratory research shows that investing in technology - in addition to other measures - helps reduce nitrogen emissions in agriculture, says Geerdink. "The buttons that can be pressed function as communicating vessels: push one button a little harder and you won't have to push the other quite so hard."
