Recovery of Ammonia and Phosphorus from Liquid Manure.

Conservation and recovery of nitrogen (N) and P from animal wastes is important in agriculture because of the high cost of commercial fertilizers and for environmental reasons. Production of biogas from manure using anaerobic digesters (AD) is projected to be important in the future. The removal and recovery of N and P is a desirable feature because it can be exported off the farm, which solves the problems of N and P surpluses in concentrated livestock production. The objective of this work was to develop new technology for simultaneous N and P recovery suitable for digester effluents.  It combines a gas-permeable membrane technology (N recovery) with P recovery as solid using precipitation.  The gaseous NH₃ in the manure is removed from the liquid before it escapes into the atmosphere. The N removal is done with low-rate aeration in the reactors that naturally increases the pH of the liquid and accelerates the rate of passage of NH₃ (> 96%) through the submerged gas-permeable membrane manifold and further concentration in an acid stripping solution reservoir.  In turn, the high pH and unbuffered conditions promote phosphorus precipitation reactions. Phosphorus precipitating compounds such as for example, magnesium chloride (MgCl₂), are added to the system either before or after the N removal.  A  prototype system was tested using digester effluent from swine covered lagoons containing ~ 2,400 mg/L of NH₄-N /L and 450 mg/L of P.  The recovered phosphorus material was mostly magnesium phosphate with very high phosphate content: 46.4% P₂O₅ and 17.1% Mg and with low concentration of N, 1.8%; Ca, 0.4%; and K, 1.7%.  As a comparison, phosphate rock mineral in the USA typically contains about 27.5 to 37.9% phosphorus.  The recovered phosphorus materials had >99% plant available phosphorus. Our results show that the simultaneous N and P recovery method is technically feasible producing high quality N and P products.