Different Calcium Sources Used in HAP Reactions for Phosphorus Removal from Wastewater

The hydroxyapatite crystallization method (HAP) was employed as a phosphorus recovery technology to investigate the effects of different calcium sources on phosphorus removal and recovery from phosphorus-containing wastewater. The results indicate that when CaCl2 was used as the sole calcium source, NaOH was required to be added during the reaction to maintain an alkaline environment, resulting in a PO43--P removal efficiency exceeding 80%. When CaO was used as the sole calcium source, PO43--P removal exceeded 70%; however, the supernatant pH increased to above 11 after the reaction. When CaCl2 and CaO were combined as the calcium source, a CaCl2/CaO mass ratio of 2.6 resulted in a PO43--P removal efficiency of approximately 80%, while the supernatant pH remained at 7.8 after the reaction. The proposed mixed CaCl2/CaO calcium source provides new insights into the HAP crystallization process and supports the integration of HAP crystallization with biological phosphorus removal technologies, thereby enabling the simultaneous recovery and reuse of phosphorus resources.