Outotec (AshDec®) Process for P Fertilizers from Sludge Ash

  • Ludwig HermannEmail author
  • Tanja Schaaf


Outotec offers the AshDec® process by which inorganic calcined phosphates (thermophosphates) are produced from phosphate-rich ashes remaining from incineration or gasification of sewage sludge, animal by-products, poultry litter, and other nutrient-rich organic waste. In the thermochemical process, solid, potassium and/or sodium-based, alkaline compounds admixed to the ash decompose at a temperature of >900 °C and react with the ash-borne phosphates to form bioavailable (ammonium citrate-soluble) alkaline phosphate compounds. Simultaneously, the toxic arsenic, cadmium, and lead compounds become gaseous and evaporate from the reactor bed. As soon as the process gas is being cooled, the particles condensate and are captured and removed in an electrostatic precipitator as metal concentrate. The process produces a P or PK fertilizer with relevant mass fractions of silicates, sodium, and trace elements. Phosphates are released and taken up by crops when root exudates decompose the Ca-K/Na-PO4 compounds preventing losses of P in solution if water-soluble P fertilizers are used. The recently explored, partial replacement of sodium sulfates by potassium phosphates avoids high sodium concentrations and leads to a PK 16-7 + 4S fertilizer with >25% total macro nutrient content and >10% sodium/potassium silicates that may enhance crop resilience. In a recent (published 2016) report, the Expert Group for Technical Advice on Organic Production (EGTOP) came to the conclusion to recommend calcined phosphates and struvite for organic production.


P recovery and recycling Calcined phosphates Heavy metal removal Citrate-soluble phosphates Organic farming Root controlled release fertilizer 


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© Springer Nature Singapore Pte Ltd. 2019

Authors and Affiliations

  1. 1.Outotec GmbH & Co. KGOberurselGermany

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