At Aaby and Marselisborg Wastewater Treatment Plants Aarhus Vand extract phosphorus from the wastewater and sell it as valuable fertiliser under the name PhosphorCare.

In 2013, Aarhus Vand opened the first full scale demonstration plant in Denmark at Aaby Wastewater Treatment Plant to test the viability of renewable phosphate production in Denmark. Today the plant is able to extract 60 percent of the amount of phosphorus in the wastewater.

At Aaby WWTP we use a simple technique to extract phosphorus from the wastewater and turn it into a valuable fertiliser. We recover phosphorus and nitrogen from wastewater through the use of a separate reactor. We need the reactor because the concentration of phosphorus and nitrogen is low in the main wastewater flow. To increase it, we send a side stream of wastewater through the reactor and add magnesium salt. The precipitation process refines the phosphorus, discarding heavy metals and environmentally unfriendly substances. The outcome is a granulate that contains phosphorus, nitrogen and magnesium – well-suited for use as a fertiliser. With it comes the added bonus that the challenges of clogged-up pipes in the sewage works have been considerably minimised. This, in turn, reduces the expenses of maintaining the sewage works.

Marselisborg WWTP – the biggest phosphorus-recovery plant in the Nordic countries
In 2019 we opened the largest phosphorus-recovery plant in the nordic countries which recovers phosphorus from a concentrated side stream in the wastewater treatment plant. This plant is a further development of the experimental plant in Aaby and is situated at Marselisborg Wastewater Treatment Plant. The plant has the potential to utilize up to 30 percent of the phosphorus that enters the treatment plant.

At both plants, the struvite is precipitated as a ‘ready-to-use fertiliser’ and sold to the market. An official approval of the product as commercial fertiliser has been obtained for the struvite produced at both plants under the name PhosphorCare. Both plants are the result of a collaboration between a number of private and public partners. The partnership behind the plant at Aaby includes Aarhus Vand, Herning Vand, Grundfos, Norconsult and the Danish Agricultural Advisory Service. The partnership behind the project at Marselisborg includes Aarhus Vand, SUEZ and Stjernholm.

Recovering phosphorus has great potential
If similar systems were established at the 50 largest plants in Denmark, the country would be more or less self-sufficient in phosphorus.

Denmark’s phosphorus demand for crop production requires imports of 11,000 tonnes of phosphorus. If similar recovery plants are built at the 50 largest water treatment plants in Denmark, we can produce 3,000 tonnes of pure phosphorus fertilisers per year, representing more than 25% of the country’s imports. Thereby, the country would be more or less self-sufficient in phosphorus. Phosphate is a finite resource with few global suppliers. With the world facing twin crises of excessive energy use and diminishing resources, the project is consistent with the Aarhus Vand corporate focus on sustainability.

There is another benefit of separating phosphorus from sewage sludge. Sludge is normally distributed to farmers who spread it on their fields. But sludge is potentially harmful to certain types of crops and can only be used for fields where crops fed to animals are grown. Furthermore, phosphorus left in sewage sludge binds to iron and aluminium salts, which make it difficult for the plants to take up phosphorus.

See more in this video from Marselisborg WWTP.

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Aarhus Vand
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