Identification of phosphorus forms in sewage sludge ash during acid pre-treatment for phosphorus recovery by chemical fractionation and spectroscopy

Identification of phosphorus (P) forms is critical for the recovery of P from sewage sludge ash (SSA). During the HCl treatment of SSA Ca was selectively eluted while P mostly remained in the SSA. Chemical P fractionation of the SSA before and after HCl treatment showed that Al-P increased from 46.6% to 79.2% of total P while Ca-P decreased from 28.9% to 4.4% at pH 2.5. HCl treatment also changed chemical structure of Al-P since no crystalline Al-P was found in the SSA after the treatment, only amorphous Al-P was found from the analyses of XRD and ATR-FTIR.