Phosphorus Recovery from Animal Manures using Optimized Struvite Precipitation

The over-application of phosphorus, as manure by animal feeding operations, is a threat to surface water quality. Concentration of animal feeding operations and over application of manures to cropland has lead to a build-up of phosphorus on many farms (Greaves et al., 1999). Relative to crop needs, manure slurries contain higher levels of phosphorus than nitrogen. When manure is applied to meet crop nitrogen needs, phosphorus is over applied. Recent laboratory studies show that phosphorus content in swine manure can be reduced by recovering a portion of the phosphorous as a crystalline precipitate containing struvite (magnesium ammonium phosphate hexahydrate, MgNH4PO4·6H2O) (Beal et al., 1999; Burns et al., 2001; Kalyuzhnyi et al., 2001, Nelson et al., 2000; Wrigley et al., 1992). Precipitation of phosphorus prior to land application of manure offers the potential to recover excess phosphorus from manures and move it to cropping areas that require phosphorus fertilizer inputs. The cost effective relocation of excess phosphorus would allow existing animal feeding operations to successfully implement phosphorus-based nutrient management plans on their current land base. Proposed EPA regulations regarding concentrated animal feeding operations must be finalized by December 2002. These regulations will likely limit land application of manure to phosphorous-based rates. Comparisons of nitrogen and phosphorus-based nutrient management plans indicate that some poultry-broiler producers, swine producers, and diary producers may require as much as ten, eight, and four times more land, respectively, if required to shift to a phosphorus-based plan (Burns, et. al., 1998). Producers who do not have an adequate land base will be faced with transporting manure nutrients off-site. Recovery of phosphorus as precipitated struvite has the potential to substantially reduce transportation costs by isolating the excess phosphorous and converting it into a crystalline form for low-cost movement.