STRUVITE FORMATION AND CONTROL IN SLUDGE TREATMENT

Struvite, {MgNH4PO4}, is a mineral that has caused a range of problems in wastewater and sludge treatment. Struvite precipitates upon pumps, pipes and associated equipment resulting in reduced capacity, maintenance costs and downtime. Struvite was identified at two sludge treatment works operated by Severn Trent water (Coleshill and Wanlip). A series of jar tests upon real and synthetic sludge liquors were carried out to determine the fundamental characteristics of struvite formation. A computer model predicting struvite precipitation potential was compared to these results. Struvite formation was examined over a pH range of 6.8 to 9.5 with masses of precipitate 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 Severn Trent's Sludge Destruction Plant (SDP) at Coleshill. Struvite was identified as the scale that had reduced the bore from 150mm to 60mm in 12 weeks. Dissolution experiments were used to identify the purity of struvite precipitates formed with respect to magnesium and phosphorus concentrations. Scales from two other plants were also examined, one was identified as struvite and the other was identified as calcite.