Predicting struvite formation in digestion

The solubility constant for struvite (MgNH4PO4•6H2O) was measured in formation and dissolution equilibrium experiments. The proposed solubility constant (pKSO=13.26) accurately predicted struvite precipitation in sludge handling facilities at the Sacramento Regional Wastewater Treatment Plant, whereas the solubility constant in common use did not. An improved method was developed for predicting struvite precipitation potential in anaerobic digestion and post-digestion processes. Method improvements include consideration of ionic strength effects on ion activities, magnesium phosphate complexation effects on ion speciation, and an experimentally derived struvite solubility constant. The improved method accurately predicted struvite to be less soluble than predicted by earlier methods.