Bulk pH and Carbon Source Are Key Factors for Calcium Phosphate Granulation

Recovery of calcium phosphate granules (CaP granules) from high-strength wastewater is an opportunity to reduce the natural phosphorus (P) scarcity, geographic imbalances of P reserves, and eutrophication. Formation of CaP granules was previously observed in an upflow anaerobic sludge bed (UASB) reactor treating source separated black water and is enhanced by Ca²⁺ addition. However, the required operating conditions and influent composition for CaP granulation are still unknown. In this study, we have experimentally demonstrated that the carbon source and bulk pH are crucial parameters for the formation and growth of CaP granules in a UASB reactor, operating at relatively low upflow velocity (<1 cm h^-1). Degradation of glucose yielded sufficient biomass (microbial cells and extracellular biopolymers) to cover crystal and amorphous calcium phosphate [Ca _x(PO₄) _y], forming CaP granules. Influent only containing volatile fatty acids as the carbon source did not generate CaP granules. Moreover, bulk pH between 7.0 and 7.5 was crucial for the enrichment of Ca _x(PO₄) _y in the granules over bulk precipitation. Bulk pH 8 reduced the Ca _x(PO₄) _y enrichment in granules of >1.4 mm diameter from 9 to 5 wt % P. Moreover, for bulk pH 7.5, co-precipitation of CaCO₃ with Ca _x(PO₄) _y was reduced.