Ultrasonic treatment is an effective method to disintegrate sludge and extract organic matter and nutrients, including nitrogen and phosphorus, from the sludge. This study investigated the transformation of phosphorus species during the ultrasonic treatment of sludge, to reveal the mechanism of phosphorus migration in the activated sludge structure. The experimental results indicated that power density and ultrasonic time were critical parameters affecting the energy input for sludge integration. The optimal phosphorus release performance was achieved at 2.5 W/mL 10 min. The release of phosphorus showed as a layer-by-layer pattern from the inner sludge core to the outer sphere of the multiple-layer structure of the sludge. The complex sludge structure played an important role in buffering the ultrasonication process and transfer of phosphorus. Mg-P, Ca-P, and organic phosphorus are the main phosphorus species that can be extracted from the sludge core into the supernatant. The three-stage process of phosphorus migration through the sludge layers, including dissolution, reprecipitation, and equilibrium, has been revealed.

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