Anaerobic fermentation of the enhanced biological phosphorus removal (EBPR) sludge was investigated in terms of phosphate release and volatile fatty acids (VFAs) production regarding polyphosphate accumulating organisms (PADS) activity. PAOs decay rate during fermentation was determined as 0.35 +/- 0.03 d(-1). Sludge lysis was enhanced with an increase in polyhydroxyalkanoate (PHA) content. Moreover, the phosphate release profiles and the VFAs production as well as the individual VFA fractions varied with different acetate concentrations added initially. Based on these observations, anaerobic digestion model No. 1 was extended and modified by introducing: (1) processes that PAOs store 4 VFA species as PHA, which can be degraded into varied fractions of individual VFA in dependence on PHA composition, (2) the effect of PHA content on disintegration rate, and (3) phosphorus precipitation. The proposed model adequately fitted a multi-experiment, multi-variable data set, indicating that it plays an important role in predicting phosphate and VFAs variations during EBPR sludge fermentation. (C) 2015 Elsevier B.V. All rights reserved.