Reclaiming phosphorus as struvite from hog manure
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The over application of manure phosphorus (P) to farmlands can lead to P build up in the soil and eventual runoff to surface waters causing eutrophication. Reducing P in manure by precipitation of struvite enables P capture and reuse as a fertilizer in the agricultural supply stream. Struvite precipitation is dictated by levels of soluble P which can be highly variable according to manure treatment and management. This research studied P forms in liquid pig manure, ways of increasing P-PO4 levels in manure from strategic storage conditions, novel struvite reactors, and the effectiveness of struvite as a fertilizer. Studies that monitored soluble nutrients during manure storage revealed that P-PO4 concentration was sensitive to pH and time. Anaerobic fermentation of manure increased P-PO4 by 2.5 fold with 12 days of storage, provided the buffering by alkalinity was low. Two different struvite reactors were operated, a batch-type system for processing lagoon supernatant and an upflow air sparged reactor that used supernatant from a rotary press solids separator. They achieved 75% and 31% total P removal, respectively. The upflow reactor operated without chemical addition at pH 6.8 to produce high purity struvite free of calcium phosphates. Costs of both reactors were comparable ($0.0139 and $0.0167/kg live pig wt) and similar to other pilot struvite reactors. Manure derived struvite was compared with pure struvite and commercial fertilizer for agronomic value in canola production. Results of a greenhouse pot experiment showed no significant difference between the two struvites despite impurities in the manure precipitate.