Fertilizer-Grade Calcium Phosphate From Wastewater Treatment Plants.

Excess phosphorus in wastewater streams is viewed as an operational nuisance and an environmental threat.  Research has been directed to recover P fertilizers from waste streams, thereby creating a high demand commodity from waste. Previously, this research has focused on producing struvite from the gravity belt thickened (GBT) filtrate of anaerobic digests through the addition of magnesium hydroxide, which provides a magnesium source and increases the pH to 8.5-8.8, the point at which struvite precipitation occurs. This study endeavored to improve the P recovery system by producing fertilizer-grade calcium phosphates from the organic acid (OA) digest through the addition of calcium hydroxide. Through computer modeling and a fluidized bed reactor (FBR) simulation, this study determined that P could be effectively recovered as brushite, a calcium phosphate mineral, at a pH of 6.5, thus reducing the amount of base required and maximizing the amount of P recovered. Using a FBR system garnered additional benefits by applying the principles of Stoke’s law to increase the particle size of the recovered calcium phosphates. This study determined that the OA digest was the optimal location for a P recovery system as it had the highest concentration of available, reactive P throughout the wastewater treatment process and would maximize the benefits that ensue, particularly decreased nuisance struvite/vivianite formation and improved biosolid quality with lower P concentrations. To further research in this field, a small-scale OA digester was developed and batch experiments with its digest were utilized to quantify the P recovery potential through the formation of calcium phosphates.