Effects of ferric-phosphate forms on phosphorus release and the performance of anaerobic fermentation of waste activated sludge

Five ferric-phosphate (Fe(III)Ps) with amorphous or crystalline structures were added to waste activated sludge (WAS) for anaerobic fermentation, aiming to investigate effects of Fe(III)Ps forms on phosphorus (P) release and the performance of WAS fermentation. The results revealed that the Fe(III) reduction rate of hexagonal-FePO 4 was faster than that of monoclinic-FePO 4 ·2H 2 O, thanks to its lower crystal field stabilization energy. FePO 4 ·nH 2 O was reduced to vivianite and part of the phosphate was released as orthophosphate (PO 4 -P). Giniite (Fe 5 (PO 4 ) 4 (OH) 3 ·2H 2 O) as an iron hydroxyphosphate was transformed to β Fe(III)Fe(II)(PO 4 )O-like compounds without PO 4 -P release. In addition, Fe(III)Ps had an adverse effect on the anaerobic fermentation of WAS. The specific hydrolysis rate constant and volatile fatty acids (VFAs) yield decreased by 38.4% and 41.9%, respectively, for the sludge sample with amorphous-FePO 4 ·3H 2 O, which dropped the most. This study provides new insights into various forms of Fe(III)Ps performance during anaerobic fermentation and is beneficial to enhancing P recovery efficiency.