Formation Mechanism of hydroxyapatite encapsulation in Anammox-HAP Coupled Granular Sludge

The potential of the formation of anammox-hydroxyapatite (HAP) granule composites as a cost-effective approach to removing nitrogen and phosphorus in the treatment of wastewater has been recently reported. Before these annamox granules, which consist of an anammox biofilm layer and an HAP crystallizing layer, can be used in applications, the formation mechanism of hydroxyapatite (HAP) encapsulation in the granules needs to be further studied. In this work, the role of extracellular polymeric substance (EPS) secreted by microorganisms and HAP core in Ca and P removal in anammox-HAP coupled granular sludge was investigated. According to the Lamer model, it is possible that the nucleation time of the granules becomes shorter as the crystal seeds. The enhanced buffering capacity of the granules was 0.08 mmol-H⁺ SS-g^-1 with the pH kept above 6.5 for a comfortable environment for anammox. The results of this study show that ion competition and exchange, mainly between cations of Ca²⁺ and Mg²⁺ and between anions of PO₄^3- and CO₃^2-, affects the precipitation process. The results of this study indicate that the addition of granule crystal seeds can be used as a strategy to hasten the anammox process, and therefore accelerate the overall process.