Looking Beyond Struvite for P-Recovery

Abstract

P hosphorus, a nutritional element for all life forms for which no substitute exists, is a natural resource in short supply.It is estimated that the remaining accessible reserves of phosphate rock will run out in 50 years, if the growth of demand for fertilizers remains at 3% per year.1,2 Reducing usage will help the reserves last longer, but the biggest gains will probably be made from the recovery of phosphates, both from wastewaters and livestock waste. 1 With regards to wastewater, such recovery would have the additional benefits of minimizing eutrophication and alleviating the scaling of process equipment at wastewater treatment plants.For these reasons, nutrient removal from wastewater has recently become integrated with phosphate recovery.2,3 Struvite (MgNH 4 PO 4 •6H 2 O) is generally considered as the optimal phosphate mineral for recovery as it contains 51.8% of P 2 O 5 (based on MgNH 4 PO 4 ) and could potentially be used as a slow-release fertilizer.If the economic and life cycle costs are taken into account, however, it becomes clear that phosphate recovery as struvite is likely not the best approach, for the following reasons: (1) production of P-mineral with a high content of struvite from real wastewater is a difficult and costly process; and (2) struvite is not superior to other phosphatebased compounds in fertilization efficiency, nor is it an exclusive form of raw materials favored by the fertilizer industry.In literature and practice, struvite precipitation is usually performed under alkaline conditions, which are created by dosing alkalinity or CO 2 stripping.3 It is often taken for granted that precipitates harvested at a pH range between 9.0 and 10.7 are struvite-like compounds when there are appropriate molar