Potential phosphorus recovery by struvite formation

Formation of struvite (MgNH(4)PO(4)(.)6H(2)O) 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 m(3) 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 pound8400 and pound20,000 a year. (C) 2002 Elsevier Science Ltd. All rights reserved.