Clarification of phosphorus fractions and phosphorus release enhancement mechanism related to pH during waste activated sludge treatment

[Display omitted] •WAS could be selected as potential substitution of phosphate rock to recover P.•MRP accumulation at different pH was controlled by a pseudo-chemical reaction.•The releasable and recoverable P depended on both P species and applied pH.•An overall mechanism had been proposed to understand pH effects on P release.•A potential and practical concept of WW and WAS treatments was proposed. This study aimed to clarify phosphorus (P) fractions in waste activated sludge (WAS) and explore release performance and enhancement mechanism of different P species related to pH. Results showed that inorganic P (IP) was the major P fraction in raw sludge (87.86% of total solid P), and non-apatite inorganic P (NAIP), the most labile P forms, occupied 81.30% of IP, suggesting that WAS could be selected as potential substitution of phosphate rock. The optimized acid and alkaline conditions were pH=4 and pH=12 for molybdate reactive P accumulation, increased by 311.20mg/L and 479.18mg/L compared to raw sludge, which were 3.80 and 5.84 times higher than that of control, respectively. The mechanism study demonstrated that high pH promoted NAIP release, and apatite P was sensitive to low pH. Moreover, the releasable and recoverable P depended on both fractions of different P species in sludge and pH adjustment for sludge treatment.