Transformation and release of phosphorus from waste activated sludge upon combined acid/alkaline treatment†
This study was conducted to investigate migration and transformation of phosphorus (P) in waste activated sludge (WAS) during sequential processes of acid and alkaline treatments. The P mass balance analysis and the transformation of various P species from solid to liquid phase were also studied. Compared with single acid or alkaline experiments, the migration of total solid-P to liquid-P was promoted by the acid-alkaline sequential treatment. Acid-alkaline sequential treatment increased dissolution of non-apatite IP (NAIP) and decreased apatite P sediments in the final alkaline from the released species, resulting in enhancing the inorganic P (IP) transformation from solid to liquid phase. The maximal accumulation of molybdate reactive P (MRP) in liquid was 520 mg L−1, accounting for 54.7% of total P. However, the P release was reduced when alkaline-acid sequential treatment was used because of a low liquid P release from NAIP. It was found that solid-P transforming to liquid-MRP was mainly regulated by pseudo-chemical reactions regardless of a constant or changed pH. The release efficiencies of NAIP under both alkaline and acid-alkaline conditions were more than 88.0%, whereas it was only 6.2% with alkaline-acid treatment. The findings in this study can help to understand the phosphate release and migration rules under different WAS treatment conditions for future P recovery.