Struvite Precipitation from Anaerobically Digested Dairy Manure
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When manure is applied to crops on a nitrogen basis, it often creates a buildup of phosphorus (P) in the soil. Phosphorus recovery as struvite is one strategy to capture excess P prior to land application. To form struvite from dairy cow manure, an acid pH is required to break the calcium phosphate bonds present in the manure. Oxalic acid is desirable because, in addition to breaking bonds, its anion binds calcium. An experiment was designed to measure struvite precipitation from dairy manure after addition of commercial grade oxalic acid to result in varying pH points for maximum struvite production. Initially decreasing the pH of the dairy manure to 6.0 and later increasing the pH to 8.7 removed the most P (90%); however, this high pH can lead to magnesium phosphate precipitation and a pH of 6.0 was not low enough to completely disassociate the calcium from phosphate. Therefore, we recommend initially decreasing the pH to 5.5 and later increasing it to pH 8.2 which achieved 80%P removal.
KeywordsStruvite Oxalic acid Dairy Manure Calcium
The project is partially funded by the Washington State Dairy Products Commission.
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