Advanced treatment of wastewater is essential to ensure pollutants such as phosphorous are not discharged into fragile water bodies. Failure to prevent this can result in environmental degradation, and a breakdown in ecosystems, where ecosystems become oxygen free, thus creating dead zones. It is estimated that sewage plants are responsible for up to 10 per cent of phosphorous found in aquatic systems. Therefore, it is essential that phosphorous is effectively removed and recovered from wastewater before it is discharged into bodies of water.

Conversely, while phosphorous can be harmful for ecosystems, it is essential to plant growth and is a key ingredient in agricultural fertilisers, thereby helping to sustain the production of food for a rising global population. However, it is a finite resource that is extracted from mines and its supplies are rapidly declining.

Unlike fossil fuels, there are no alternatives to the use of phosphorous. Neither India nor Europe possess their own phosphorous reserves, with the United Kingdom for example having already exhausted its domestic supplies back in the 19^th century.  Reserves do exist in America, Northern Africa, China, Russia, the Middle East, and Northern Africa, but some experts predict that these supplies could run out within the next 30-70 years. Countries such as China have already introduced export controls. The situation is exacerbated by the fact that global demand for the scarce resource is projected to grow by 70 % by 2050. Therefore, there is an urgent need to devise ways of recycling phosphorous

Currently, traditional wastewater plants focus on removing the phosphorous from the wastewater rather than recovering it. However, back in 2013, Aarhus Vand, the local water utility in Denmark’s second largest city, Aarhus, began experimenting with extracting phosphorous from wastewater by constructing a pilot-scale demonstration plant at Åby wastewater treatment facility. The phosphorous recovery plant converts the waste product struvite, which is found in the sludge from anaerobic digestion, into a phosphorous product named PhosphorCare, which is sold as fertiliser that can be used on crops instead of manure.

The success of the Åby pilot demonstration project led Aarhus Vand and the French company Suez to initiate a partnership to build a phosphorous recovery plant at Aarhus Vand’s largest wastewater treatment facilities, Marselisborg WWTP.  Innovative wastewater treatment experts, entrepreneurs, producers, suppliers, and research institutions were invited to develop solutions to some of the technical challenges of the project.

The resulting solution and recovery plant employs a fluidised bed technology to extract phosphorous from the wastewater, which is then is processed further into a struvite fertiliser product that can be sold commercially to be used on crops. Utilising this method allows for up to 20-30 % of the phosphorous present in the incoming wastewater to be extracted.

The commercially approved product is known as PhosphorCare™. The larger recovey plant located at Maselisborg WWTP was officially opened in 2019, and another smaller plant is operating at the Herning Vand utility.

Recovery of phosphorous means that the participating treatment plants have transformed from being wastewater treatment facilities to resource recovery plants.

Marselisborg WWTP is now the largest phosphorous recovery plant in the Nordics, producing approximately 80-100 kg of struvite daily.

The limited supplies of phosphorous means that selling the recycled phosphorous product generates income for water utilities. This is particularly the case for wastewater treatment plants with high volumes of wastewater and biological phosphorous removal.  Extracting the phosphorous from the wastewater has also generated additional, unforeseen benefits, where doing so has minimised struvite scaling and blockages in the pipes and pumps. This reduces operational and maintenance costs at the treatment plant.

Recovering and preserving scarce resources such as phosphorous allows the Danish water sector to participate in the transition to a circular economy.