01 January 2018 – 30 September 2020


Phosphorus (P) use needs to become more sustainable and should include P-recycling from secondary resources. This will not only prevent eutrophication of surface waters but will also minimize costs for disposal of phosphorus-rich wastes. Phosphorus is a key nutrient in fertilizers and therefore essential for our food production. Phosphate rock is mined and processed to produce phosphorus-containing fertilizers, like PK-fertilizers. However, exploitable phosphate rock reserves are found in just a few countries. The EU itself has hardly any phosphate rock reserves and depends nearly entirely on the import of this crucial resource. The EU has therefore added phosphate rock to the list of critical raw materials.

Previous initiatives to recover P from sewage sludge have focused on the production of struvite, a magnesium ammonium phosphate mineral. This approach is now successfully applied but only at STPs that rely solely on enhanced biological phosphorus removal, while the majority of Europe’s STPs apply chemical precipitation with an iron-based coagulant to achieve sufficient phosphorus removal (64-92% in Germany and 83% in the Netherlands). Iron-based coagulants remove phosphate very effectively because the formed iron phosphates precipitates have a low solubility, however, this low solubility does prevent the formation of less soluble minerals like struvite. More stringent legislation on phosphate discharge and a move to higher-loaded STPs (to increase the energy efficiency) are expected to further increase the reliance on chemical precipitation.

The solution (technology)

This project intends to develop a separation process to recover the insoluble iron phosphate mineral vivianite from the STPs that rely on chemical precipitation. The separation process will be applied on sewage sludge after anaerobic digestion. Anaerobic digestion of sewage sludge is widely applied in 24 countries of the EU-27. Anaerobic digestion reduces the sludge volume and recovers energy in the form of biogas.

Phosphorus-rich waste streams such as sewage have a high potential for P recovery. The P has to be eliminated from these wastewaters anyway to prevent environmental damage, like harmful algae blooms. Europe’s Sewage Treatment Plants (STPs) annually remove circa 370 kton of P by immobilization in the sewage sludge. The direct use of the sludge as fertilizer is problematic due to its bulky volume, the fixed nutrient ratios and the low bioavailability of a nutrient like phosphates. Therefore there is an increasing interest to separate the phosphorus in concentrated form from the bulk sludge for subsequent reuse in the fertilizer industry.


  • Stichting Wetsus European Centre of Excellence for Sustainable Water Technology (Wetsus), Netherlands (Lead partner)
  • ICL Fertilizers Europe C.V., Netherlands
  • Kemira Rotterdam B.V., Netherlands
  • Outotec GmbH & Co. KG, Germany
  • Delft University of Technology, Netherlands