Crucial role of extracellular polymeric substances from anammox sludge in wastewater phosphorus recovery via magnesium phosphate crystallization

Abstract

The magnesium phosphate (Mg-P) crystallization based on anaerobic ammonia oxidation is of interest for simultaneous nitrogen and phosphorus removal in wastewater treatment due to cost-effectiveness. Understanding the role of extracellular polymeric substances (EPS) in crystal growth is pivotal for bio-induced Mg-P crystallization. In this work, the effects of EPS in Mg-P crystallization were thoroughly investigated. Results indicated that EPS decreased the initial crystallization rate but slightly improved the final crystallization yield. The maximum removal efficiencies of Mg and P ions increased by 2.8 % and 3.1 %, respectively, when EPS concentration was 80 mg/L. Additionally, the presence of EPS enhanced Mg3(PO4)2·10 H2O production and facilitated the formation of larger rhombic plate-like structures crystals (maximum particle size increased by 215 %). Analysis of EPS composition changes during the crystallization process and characterization of the final crystals suggested that tryptophan-like proteins preferentially involved in the Mg-P crystallization process, while β-sheet proteins potentially influencing crystal morphology. Furthermore, the formation of phosphate ester groups, hydrogen bonding, and COOH-Mg2+ complexes were considered triggering factors for the interaction between EPS and Mg-P crystals. This study elucidated the underlying mechanism of EPS on Mg-P crystallization, further enhancing the understanding of the interaction between inorganic mineralization processes and microorganisms.