Natural phosphate sources low in heavy metals are getting scarce. Containing about 15 mass-% of P2O5, sewage sludge ash can be considered a secondary phosphorus (P-) source. The P-content in the European sewage sludge could currently replace roughly 15% of the phosphate imports into the EU.Hence already for many years, almost decades, it has been tried to recover phosphorus from sewage, sludge and ...