<p>This study investigated phosphorus release, iron reduction, and the generation of volatile fatty acids (VFAs) during the anaerobic co-fermentation of waste activated sludge (WAS) and starchy food waste (SFW). Meanwhile, the formation of vivianite crystals was compared between two different iron sources: ferric oxyhydroxide (FeOOH) and iron citrate (FeC₆H₅O₇). The results showed that when the amount of SFW added was 30%, phosphorus release increased by 3.5-5 times, and the VFAs yield reached 2857.85&#xa0;mg/L. When using FeC₆H₅O₇ as the iron source, the production of vivianite and VFAs increased by 2 times and 5 times, respectively. Additionally, citric acid enhanced the metabolic activity of acid-producing bacteria, promoting the conversion of more substrates into VFAs. The addition of organic substrates during co-fermentation promoted the growth of hydrolytic and acidogenic bacteria, enhancing hydrolysis and acidogenesis. These changes were beneficial to the biological processes of Fe²⁺ and PO₄³⁻ release and VFAs production. The experiment further revealed that in the organic iron source FeC₆H₅O₇ experimental group, the acid-producing bacterial consortium preferentially utilized tyrosine-type substrates, ultimately achieving coupling between VFAs accumulation and Fe³⁺ reduction. Overall, this study provides new insights into vivianite formation and VFAs production during the co-fermentation of WAS and SFW with different iron sources, offering a potential pathway for simultaneous nutrient recovery and waste valorization.</p> Graphical Abstract <p></p>