Potential phosphorus recovery by struvite formation

Formation of struvite (MgNH4PO4•6H2O) at sewage treatment works can cause operational problems and decrease efficiency. Struvite has a commercial value and the controlled formation and recovery of it would be beneficial. A mass balance was conducted at full scale across the whole sewage treatment plant in order to identify a stream to conduct bench-scale struvite crystallisation studies. The most suitable stream was identified as the centrifuge liquors. The average flow of the liquor stream was 393 m3 d−1 and the composition was as follows: 167 mg L−1 phosphorus, 44 mg L−1 magnesium, 615 mg L−1 ammonium, 56 mg L−1 calcium and 2580 mg L−1 of alkalinity. The pH averaged at 7.6 and the stream had a predicted struvite precipitation potential of 140 mg L−1. Struvite crystallisation occurred quickly during the trials, by raising the pH of the centrifuge liquors to 9.0 and dosing with magnesium. Up to 97% phosphorus removal as struvite was achieved. Struvite formation occurred when the molar ratio of magnesium: phosphorus was at least 1.05 : 1. Below this ratio phosphorus removals of 72% were observed, but not exclusively as struvite. Annual yields of struvite were calculated to be 42–100 tonnes a year, depending on the dose regime. Revenue from the sale of produced struvite could be between £8400 and £20,000 a year.