Double-edged sword role of Fe(II) in the dynamics of anammox coupled Fe-P crystals granulation and phosphorus removal

• First study revealing Fe’s dual role in mediating P removal in the anammox system. • Achieved 71.9% P removal efficiency through Anammox-FePs granules formation. • Formation process and mechanism of Anammox-FePs granules were identified. • Fe(II) at 0–60 mg/L stimulated EPS secretion, promoting biomineralization process. • Fe(II) > 70 mg/L caused FePs dissolution, reducing phosphorus removal efficiency. This study investigated the double-edged sword role of Fe(II) in anammox coupled Fe-P crystals (Anammox-FePs) granulation and accompanying phosphorus removal processes. At Fe(II) concentrations of 0–60 mg/L, increased Fe(II) enhanced solution supersaturation, promoting FePs nucleation and stimulating anammox bacteria (AnAOB) to secrete more extracellular polymeric substances (EPS). The electronegative groups in EPS, such as –OH and –COOH, bounded to Fe ions, facilitating PO 4 3− aggregation. AnAOB and EPS contributed to biomineralization, inducing FePs formation in localized regions of high ionic concentration and alkaline pH. The generated FePs acted as carriers for AnAOB, forming stable Anammox-FePs granules under the adhesive influence of EPS, which enhanced operational stability, sludge settling, and AnAOB retention, achieving a phosphorus removal efficiency (PRE) of 71.9 %. However, when Fe(II) concentrations exceeded 70 mg/L, the hydrolysis of Fe ions lowered pH, thereby increasing the solubility of FePs and inhibiting crystal formation. Meanwhile, excessive Fe(II) inhibited AnAOB activity and EPS secretion. The dual inhibitory effects, stemming from the dissolution of inorganic FePs and the compromised activity of the organic biomass, culminated in Anammox-FePs granules disintegration and decreased PRE. Genomic analysis further revealed that Fe(II) facilitated phosphorus transport and regulation, enhancing the transformation of organic and inorganic phosphorus. The double-edged sword role of Fe(II) was attributed to the complex interplay of chemical precipitation, microbial activity, and biomineralization. These findings provide insights into the application of the Anammox-FePs process for simultaneous nitrogen and phosphorus removal.