Spreading human excrement and urine on farmers’ fields used to be common practice. This manure would be collected in buckets from urban districts and villages or retrieved from cesspits. That came to an end when sewer systems were built. The sewers were a big improvement in terms of stopping the spread of infectious diseases, but this also meant that the closed loop of soil–food–humans–fertilizer had been broken.
It was not long, though, before an alternative solution was found: many places started using the residue from the sewage treatment process — slurry — on farmland. But that too stopped in the Netherlands in the 1990s. The slurry was too heavily contaminated, for instance by heavy metals and medicine residues. Also, the use of inorganic fertilizer and animal manure on livestock farms has shot up since the mid-twentieth century.
Circular fertilizers
But there are downsides to using such large quantities of inorganic fertilizer and manure. ‘Inorganic fertilizers are made from mineral resources with finite supplies, for example from mines,’ says Kimo van Dijk from Wageningen Environmental Research, who researches circular fertilizers, ‘and their production involves the use of natural gas, a fossil resource. Extracting and using these products causes environmental problems, including greenhouse gas emissions and the destruction of landscapes in mining. We need to put a halt to this.’
“Inorganic fertilizers are made from mineral resources, using fossil fuels like natural gas. We have to end that”
‘Livestock farming is responsible for high emissions of CO₂, methane and nitrogen compounds,’ Van Dijk continues. ‘To combat that, livestock numbers need to be reduced. That will mean less manure. Anyway, European rules on the use of animal manure are already getting more restrictive. If we don’t come up with alternatives, farmers will resolve this issue by applying yet more inorganic fertilizer to the soil.’ That is why Van Dijk has been commissioned by the Ministry of Agriculture to look into fertilizers that don’t have to be transported long distances and have less of a negative impact on the environment.
The new fertilizers would need to satisfy the same high requirements that apply to the products the farmers are already using. ‘Their quality should be the same as the existing fertilizers. Farmers place high demands on the effectiveness, whereas the government is mainly concerned with safety and the environment.’ Van Dijk found 20 new fertilizers that satisfy these criteria. Only one of them is already available on the market. It consists of fertilizer granules made from the ash of burnt slurry from sewage treatment. Other fertilizers came from the wastewater of a brewery and a French fries factory, or processed urine. ‘In 2024, in an initial field experiment in Lelystad, we found that the nitrogen in urine is taken up very effectively by plants, up to 25 per cent better than the nitrogen in conventional inorganic fertilizers. That could be because the nitrogen in urine is more likely to stay in the soil, with less loss to the air and groundwater.’
Measuring emissions
Each fertilizer looks different. Some are liquid and reminiscent of animal manure while others are dry granules that look more like inorganic materials. Van Dijk wants to know whether the fertilizers lead to substantial emissions of ammonia and nitrous oxide, both of which are nitrogen compounds. ‘Crops never absorb all the nutrients in the fertilizer, regardless of the form. Some are gone from the soil again in about three weeks, ending up in the air or water as ammonia or nitrates, for example. We hope that the new fertilizers leave more of these nitrogen compounds in the soil than the fertilizers that farmers use currently.’
In an experiment that has recently started, bags of the fertilizers have been placed next to a long row of transparent cylinders containing earth, on a table. Each cylinder is opened in turn and Van Dijk and his colleagues add one of the fertilizers. In half the cylinders, the fertilizer is placed on top of the earth; in the other half, the researchers cover the fertilizer. ‘In practice on farms, fertilizer has to be applied using a zero-emissions approach. That means below the surface, so as to minimize ammonia emissions. That is why we are testing the fertilizer both above ground and below the surface.’
In a laboratory close by, a research project is being carried out to investigate the effect of the fertilizers in the soil and the emissions over a four-month period. ‘We look at CO₂ and methane, the main gases contributing to climate change, and emissions of nitrogen from the soil into the air.’ Van Dijk points to various jars containing soil. He inserts two needles with sensors through the lid of one of the jars. Graphs appear on a monitor showing the researchers the amount of greenhouse gases released after the fertilizers were applied.
Growing potatoes with French fries wastewater
Van Dijk and his colleagues use these two kinds of measurements to determine which fertilizers have the lowest emissions. Agronomic field trials in 2024 and 2025 will determine whether crops also grow well with these fertilisers. ‘We decided to do this with potatoes to show how you can directly close the loop of fertilizer-soil-crop-food-fertilizer. The potato plants grow in soil with fertilizer, for example from treated household wastewater, or water from food processing factories such as a potato processor. That completes the circle.’
In the field trials, human urine has given the most promising results so far. The urine came from a residential development in which the toilets separate out the urine from the excrement. After treatment, the urine has no medicine residues and is safe to use. The treatment involves an electric field and a membrane that separates the dissolved substances from the water. This residential complex also uses the treated urine in its own vegetable garden. ‘These kinds of toilets will become increasingly common in newly built homes and offices,’ thinks Van Dijk. ‘At festivals too, the urine can be collected separately and then treated thoroughly to remove medicine residues and drugs.’
“Water from the food processing industry is often much cleaner and therefore much safer as a basis for fertilizers”
He can also imagine a number of the other circular fertilizers coming on the market in the future. ‘In addition to the human urine, I think the prime candidates are ammonium sulphate and ammonium nitrate from sewage treatment, and the slurry that remains after the processing water from the food industry has been purified.’
However, changes are needed to the law first. Using fertilizer granules from incinerated slurry ash has been permitted since the start of 2024, but Van Dijk hopes more of these products will be allowed by Dutch law as fertilizers in the near future. ‘The slurry from the processing water in the food industry is not used on farms either. The problem is that the rules for products from industrial wastewater are just as strict as the rules for slurry from sewage treatment. Yet the water from the food processing industry is often much cleaner and therefore much safer as a basis for fertilizers. The rules need to change, from a sustainability perspective. We have to find a happy medium, because there are far more kinds of contamination in sewage slurry than in slurry from the food processing industry.
Getting used to the idea
Van Dijk is continuing to collect data as further evidence for which circular fertilizers have most potential. He expects to be able to announce the results from this year at the start of 2026. The only aspect he can’t influence directly is people’s attitude to the use of their own treated urine or excrement for fertilizing food crops. ‘Many people think it’s a disgusting idea, even though human urine doesn’t smell of anything after treatment, for instance. What is more, we will need fertilizers like this badly in the future if we want to be able to cope with less animal manure and inorganic fertilizer.’
This research is being carried out in collaboration with KWR and LeAF, with funding from TKI Agri & Food and TKI Water Technology.