Environmental concern with phosphorus (P) transport in runoff or drainage is equally important as the concern with nitrogen transport regarding eutrophication or degradation of receiving waters. Numerous fields that have been receiving animal wastes have a high P index in soil-test samples, meaning the soil already has enough P to provide crop nutrients for several years; in some cases for the next 20 or more years. Applying typical animal manure or wastewater based on N requirements means excess application of P compared to crop uptake. If farms continue to apply P in excess of crop needs, it is only a matter of time until more fields are limited to applying P at crop uptake rates or no manure P at all. This project conducted demonstration and evaluation of a struvite crystallizer at a commercial swine farm for recovering P in a product that is relatively easy to dry and transport and could have substantial fertilizer value for slow-release P and magnesium (Mg). Struvite is a common name for magnesium ammonium phosphate hexahydrate (MgNH4PO4• 6(H2O)). The fertilizer analysis of pure struvite is 5.7-29-0 for N-P2O5-K20 and 16.4 for MgO. The struvite crystallizer works well with liquid wastewater such as from an anaerobic lagoon or anaerobic digester. A bed of struvite particles is initially put into the crystallizer, and these particles grow as more struvite is formed. Increasing Mg concentration and raising pH in the flowing liquid promotes struvite formation in wastewaters that already have high concentrations of ammonium and phosphate. The objectives of this project were to: (1) Construct a struvite crystallizer system for a commercial swine farm, (2) Conduct preliminary lab tests with the farm’s lagoon or digester liquid to determine the appropriate amounts of Mg addition and pH adjustment to obtain 80 % removal of P in a struvite product, (3) Determine crystallizer performance for farm conditions and full scale crystallizer, and (4) Perform cost analysis of the struvite crystallizer system. Lab tests for phosphorus reduction by increasing magnesium and pH to form struvite were conducted using effluent from a covered earthen digester for swine manure. A cone-shaped crystallizer system was constructed in the field and operated with direct pumping of covered digester liquid at a flow rate of 1.43 gal./min. (5.4 L/min). Using the field system, twenty-four combinations of pH increase (0 to 1.5 pH units) and magnesium (Mg) addition (increase by 0, 20, 40 and 60 ppm) were tested in short-term (30 min.) tests. Up to 80 % of the total phosphorus (TP) could be removed with the highest increases in pH and Mg. About 65 % of TP was removed with the combination of 0.5 pH unit increase and addition of 40 ppm of Mg. To test performance over longer test-periods and different seasons, this combination was utilized in 40 tests each of 2-hr duration during the period of September 2007 through October 2008. Reductions averaged 55 % removal of TP and 65 % removal of orthophosphate phosphorus (OP). Analyses of samples of the solids removed from the crystallizer on six different dates indicated that N, P and Mg were lower on average than theoretical values for pure struvite (5.71 % N, 12.62 % P and 9.90 % Mg) by 9.9 %, 4.4 % and 6.2 %. The solids included 1.8 % calcium, indicating calcium compounds were being included in the formed material. Costs and returns were estimated for a commercial scale system and chemical costs and TP removal were estimated at selected levels of Mg addition and increase in pH. The net annual cost of the system for 60 % removal of TP from digester effluent for a 1,000-sow farrow-to-finish operation was estimated to be $0.01/lb of live hog marketed. The largest portions of net cost of the system are labor (53%), depreciation (24%), chemicals (12 %) and electricity (9%). Economies of scale are expected for this system. Scaling up of the struvite system could be accomplished by increasing the size of the crystallizer or by using multiple units. For farms with anaerobic digesters or lagoons, the struvite crystallizer offers a method to reduce phosphorus in the liquid and recover it relatively easily in a potentially valuable slow-release fertilizer.

Contact information: Dr. Philip Westerman, NC State University, Dept. of Biological and Agricultural Engineering, Raleigh, NC 27695-7625. Email address: phil_westerman@ncsu.edu