Kinetics of struvite precipitation in synthetic biologically treated swine wastewaters.

An experimental design was set up to understand the influence of five process parameters on the kinetics of struvite precipitation in synthetic swine wastewaters. The responses studied were the kinetics of phosphorus (P) removal, the struvite precipitation rate and the dissolution rate of amorphous calcium phosphates (ACP). The kinetic study showed that the P-removal was complete in less than 1 h and was influenced positively by the added MgO. The precipitation of struvite with MgO was confirmed to follow a first-order kinetic. This study showed that ACP co-precipitated with struvite during the first 30 min. Afterwards, ACP dissolved to maintain the phosphates balance limiting the struvite growth. An initial Mg:Ca > 1.5 induced a complete dissolution of ACP in 1 h. Another experiment was conducted and it validated the results of the statistical model. This experiment also determined that 7-10 h was the best time to recover large crystals. After 10 h, the crystals were broken by stirring.