Phosphorus release during alkaline treatment of waste activated sludge from wastewater treatment plants with Al salt enhanced phosphorus removal: Speciation and mechanism clarification.

Chemical phosphorus removal (CPR) is being increasingly adopted in wastewater treatment plants (WWTPs) to enhance P elimination to comply with stringent discharge limits. However, strategies to recover P enriched in the produced waste activated sludge (WAS) are not well developed. In this study, we investigated the release of P in WAS from three WWTPs employing Al salt enhanced CPR by alkaline treatment. We also monitored P mobilization by tracking the dynamics of P fractions and species, the dissolution of major metals, and sludge cell integrities as pH was altered. The level of aqueous total phosphorus (TPaq) in the sludge increased significantly to >200 mg/L (from <11 mg/L in the raw sludge) as the pH was increased to 12, with the majority being PO4-Paq especially at high pHs. The dominance of non-apatite inorganic phosphorus (NAIP) in the sludge-P, a good correlation observed between aqueous PO4-P and aqueous Al, and the reversibility of P mobilization all suggest that the dissolution of Al-bound P was largely responsible for the sludge-P release. Sludge cell integrity, on the other hand, was not closely correlated with TPaq concentrations. Although the level of TP released in this study is among the highest, a more efficient strategy still needs to be developed to further enhance sludge-P release when TP content in the sludge mixture (TPmx) is considered (TPmx was >800 mg/L in this work).