Analysis of struvite precipitation in real and synthetic liquors

Struvite, magnesium ammonium phosphate, is a mineral that causes a range of problems due to fouling in wastewater and sludge treatment works. Equipment susceptible to struvite encrustation includes pumps and pipework. Struvite formation was investigated at Coleshill Sludge Destruction Plant (SDP) using jar tests on real and synthetic sludge liquors. A computer model predicting struvite precipitation potential (SPP) was compared to these results to develop a greater understanding of struvite precipitation. The pH values ranged from 6.8 to 9.5, with masses of struvite formed ranging from 0 mg l(-1) at pH values below 7.5 to values exceeding 350 mg l(-1) at pH values above 8.8. Data from the jar tests were compared with a crystalline deposit taken from Coleshill SDP. Struvite was identified as the scale that had reduced the bore of a pipe carrying centrate liquor from 150 mm to 60 mm in just 12 weeks. X-ray diffraction analysis and dissolution experiments were used to identify the purity of struvite precipitates formed in both real systems and the simulated precipitation experiments using jar tests with respect to magnesium and phosphorus concentrations.