Influence of Humic Substances on the Solubility and Crystallization Behavior of Calcium Phosphates during Crystallization from Digester Supernatant

Conditional solubility of dicalcium phosphate dihydrate (DCPD) and hydroxyapatite (HAp) was determined in distilled water and in digester supernatant experimentally and compared with the modelling results with PHREEQC. Conditional solubility of both phases in digester supernatant increases at the pH values > 5 – 6, and for HAp this increase is more significant. It was confirmed that the increase in conditional solubility is caused by the presence of natural organic matter (especially humic substances) by determination of conditional solubility of DCPD and HAp in the solution of humic acid (10 mg/L). The obtained solid-liquid equilibrium data served as the background for designing the separation process for recovery of phosphorus from digester supernatant. A two-step process with fast precipitation and recrystallization was developed, from which DCPD, HAp or their mixture were obtained. However, it was observed that the presence of humic compounds significantly inhibited the crystallization of DCPD and HAp and affected the purity of the obtained products. Thereafter, a novel hybrid separation process was designed and developed. The new process consists of an oxidation and a crystallization steps. It could successfully overcome the negative effects of humic compounds. Either DCPD or HAp with the high purity could be produced with a P-recovery rate of about 90 %.