Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/26/2016
Publication Date: N/A
Technical Abstract: Conservation and recovery of nitrogen (N) and phosphorus (P) from animal wastes is important in agriculture because of the high cost of commercial fertilizers and for environmental reasons. The objective of this work was to develop new technology for simultaneous N and P recovery suitable for anaerobic digester effluents. It combines gas-permeable membrane technology (N recovery) with precipitation of phosphate solids products (P recovery). The N recovery is done with low-rate aeration that naturally increases the pH of the liquid and accelerates the rate of passage of ammonia (> 96%) through gas-permeable membrane manifolds submerged in the manure. The ammonia is separated into an N concentration reservoir. The effluent after ammonia extraction has a high pH of about greater than 9 and is low in buffering compounds (ammonia and carbonates). 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. Two prototype systems were tested using digester effluent from swine covered lagoons containing about 2,400 milligrams per litre (mg/L) of ammonia-N and 450 mg/L of P. In a first configuration, MgCl2 was added to the effluent after the N removal step. During N removal the pH of the effluent increased from 8.36 to 9.33. The recovered phosphorus material was magnesium phosphate with very high phosphate content: 46.4 percent (%) phosphorus-2-oxygen-5 (P2O5), 17.1% magnesium (Mg), and low concentration of N, 1.8%. As a comparison, phosphate rock mineral in the USA typically contains about 27.5 to 37.9% P2O5 and triple superphosphate contains 46%. The recovered phosphorus materials had >99% plant available phosphorus. In a second configuration, MgCl2 was added to the digester effluent before the N removal step and transferred to an N separation tank fitted with gas permeable membrane module. Low-rate aeration was also used in the N separation tank to increase pH (from 8.4 to 9.5) that enhanced both the capture and recovery of the ammonia and the formation of P solids. The combined process was completed at the end of the ammonia extraction treatment (5 days). At that time, all the phosphate was in recoverable solid flocs. It was recovered at the bottom of a contiguous settling tank. This second configuration produced phosphates containing 4.5% N, 26.4% P2O5, and 10% Mg. The composition of this product was similar to struvite phosphates (5.7% N, 29% P2O5, and 9.9% Mg). The recovered phosphate was 99% plant available. The process provided quantitative recovery (100%) of the P in solid forms. For ammonia, the combined process recovered approximately 92%: about 3% was recovered in the phosphorus solids, and approximately 89% was recovered in the ammonia concentrate by the N recovery module. Our results show that the simultaneous N and P recovery method is technically feasible producing high quality N and P products. The technology has many positives: 1) it uses low-rate aeration instead of alkali chemical to increase the manure pH; 2) the increased pH promotes both the ammonia recovery and the formation of phosphate solids and P recovery; 3) the P and N are recovered in purified forms; and 4) the nutrients recovered have high plant availability, which is a valued property in commercial fertilizers.