Algae grown on dairy effluent cuts mineral fertiliser use by 25% – scientists report

FRANCE – According to recent reports, scientists have found that algae cultivated on dairy effluent—the nutrient-rich wastewater from dairy farms—can reduce the need for mineral fertilisers by up to 25%.

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.

“We grew unicellular algae on dairy effluents from a food processing plant. 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,” said Orhan Grignon, agriculture and environment advisor at the Chamber of Agriculture in Charente-Maritime.

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 of innovation in sustainable water management comes from Ourense, a city in northern Spain, which hosts one of Europe’s most advanced water treatment facilities. Technicians and researchers are implementing nutrient recovery on a large scale.

“In this facility of nearly 30,000 square meters, more than 600 litres of urban wastewater arrive every second. 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,” explained Alicia González Míguez, Project Manager at CETAQUA.

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. Cecilia Lores Fernández, a researcher at CETAQUA, explained that the residual stream contains a high concentration of nitrogen, an essential nutrient for plants.

She described how they recover this nitrogen by using a bed of zeolites, then extract it with sodium hydroxide to create a basic stream, which is ultimately transformed into ammonium sulphate for use in agricultural fields.

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