Researchers are developing bio-based fertilisers that reduce pollution, save energy and could curb Europe’s reliance on Russian imports. One promising solution: algae grown on wastewater.
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In western France, farmers are experimenting with an unconventional fertiliser: a powder made from algae grown on wastewater.
The results are encouraging: when mixed with mineral fertilisers, this bio-based product can reduce their use by up to 25%, without sacrificing yields.
"We grew unicellular algae on dairy effluents from a food processing plant," explains Orhan Grignon, agriculture and environment advisor at the Chamber of Agriculture in Charente-Maritime.
"The algae feed on the organic matter in the wastewater, turning it into plant biomass. We then dehydrate that biomass and spread it on fields as a fertiliser, since it’s naturally rich in nitrogen."
The tests, carried out on wheat plots, compared algae powder with mineral fertilisers and other organic products. The verdict: algae alone doesn't match mineral fertilisers in terms of yield, but when combined with them, it delivers the same results, while cutting mineral fertiliser use by a quarter.
However, there are challenges. Unlike mineral fertilisers, which release nitrogen instantly and are easy to dose, algae powder works more slowly.
"Managing it requires anticipation and more expertise from farmers," says Grignon. Still, its potential is clear. And because it's dehydrated, it can be transported further and used in areas where spreading sewage sludge, another organic fertiliser, is restricted.
The tests were carried out within WALNUT, a European project aimed at giving wastewater a second life.
"Our main objective is treating different kinds of wastewaters, such as industrial effluents, urban wastewater, or brines," explains its coordinator, Francisco Corona Encinas. "By applying a circular approach, we not only reduce the pollutant load of these processes but also add value to the nutrients contained in them—using these nutrients as bio-fertilisers in agriculture."
One promising example comes from Ourense, northern Spain, home to one of Europe's most advanced water treatment plants.
Here, technicians and researchers are putting nutrient recovery into practice on a large scale.
"In this facility of nearly 30,000 square meters, more than 600 litres of urban wastewater arrive every second," explains Alicia González Míguez, project manager at CETAQUA.
"Here, water from taps, sinks, and toilets goes through advanced purification before returning to the river. But we don’t just remove harmful compounds—we also recover valuable nutrients like nitrogen and phosphorus."
Traditionally, nitrogen fertilisers are made using processes that consume vast amounts of energy and emit greenhouse gases.
At Ourense, that nitrogen comes from the residual streams left after sludge treatment. "This residual stream is very rich in nitrogen, which is an essential nutrient for plants," explains Cecilia Lores Fernández, a researcher at CETAQUA. "We recover this nitrogen using a bed of zeolites, and then extract it with sodium hydroxide to create a basic stream, which we finally transform into ammonium sulphate for application in agricultural fields."
With the growing global demand for nitrogen, she adds, "this technology can offer an alternative to conventional production, which relies on polluting and energy-intensive processes."
By recovering nutrients and developing bio-based fertilisers, Europe can cut its reliance on imports, reduce environmental impacts, and build resilience into its food systems.
While more research is needed to optimise these products, early results show real potential. From algae grown on factory effluents to nitrogen extracted from municipal wastewater, these innovations point to a future where what we flush could help feed the continent, closing the loop between waste and food.
