CALCIUM EFFECT ON STRUVITE CRYSTALLIZATION OF LIQUORS FROM AN ANAEROBIC DIGESTION OF PREFERMENTED AND EBPR SLUDGE

The recovery of phosphorus from wastewater has achieved in the recent years a high importance. Struvite (MgNH4PO4·6H2O) crystallization process appears as a viable technology to accomplish this aim. In this paper two crystallization experiments has been carried out using two different rejected liquors from the sludge treatment line. The influence of calcium on struvite crystallization process has been tested proving that phosphorus recovery as struvite in the crystallizer is reduced when high calcium concentrations are present. This struvite precipitation reduction is due to the fixation of part of the phosphorus as calcium phosphates. Since XRD analyses of the crystals obtained do not show any crystalline calcium phosphates among the struvite formed, the presence of amorphous calcium phosphate has been considered.