In WWTPs using EBPR, phosphorus, magnesium and potassium are accumulated by the PAO's in the waterline and later released in the digester . In digested sludges, phosphorus precipitates preferentially with iron to form vivianite (4, 6), then with magnesium to form struvite, and finally with calcium to form calcium phosphate . In digested sludge from CPR installations, a higher quantity of iron is generally available to bind the phosphate, explaining the higher concentration of soluble calcium and magnesium, and the lower concentration of phosphate. Concentration down to 50 mg/L were observed when Mg was dosed in a digester processing EBPR sludge to form struvite .