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European universities, small and medium-sized enterprises and multi-national industry have joined forces in the RecoPhos consortium to develop a sustainable and highly efficient process for recovering phosphorus from sewage sludge ash.

Phosphorus recovery

RecoPhos is a thermo-chemical process involving the fractioned extraction of phosphate and heavy metals from sewage sludge ash at high temperatures under reducing conditions. The chemical principle of the core reaction is modelled on the “Wöhler process”, in which phosphates react with carbon and silicon dioxide in a furnace and is reduced to phosphorus. The RecoPhos process follows a novel approach by using the innovative InduCarb retort, where a coke bed is heated inductively, and the reduction of the phosphorus contained in the sewage sludge ash takes place in a thin melt film on the surface of the coke particles. The reduced phosphorus can evaporate from the film without significantly reacting with other elements and can subsequently be retrieved either as white phosphorus or oxidised into phosphoric acid.

Valuable secondary materials for industry

A special advantage of the RecoPhos process is that it not only transforms a problematic waste with high heavy metal content into valuable phosphorus but also produces several streams of secondary raw materials that can be used in other industrial applications. Such materials include an iron alloy, a silicate slag for use as a binder in cement industry and a high calorific syngas for thermal energy generation.

Use of waste materials in the process

The process will enable optional use of waste materials as heat sources, reducing agents or additives, including dried sewage sludge, foundry ash, waste salts or meat-and-bone meal. Concepts to recover a maximum of the heat of the high-temperature process will be developed, turning RecoPhos into a near-zero waste process.


The RecoPhos process will be developed using lab-scale experiments and state-of-the-art modelling and simulation methods to investigate the thermodynamic and chemical processes taking place within the reactor. The simulations and experiments will provide the basis for the implementation of a fully operational bench scale reactor and the design of a pilot scale plant. The economic, environmental and social impact of the RecoPhos process will be investigated in a market study and a Life Cycle Assessment.

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