Struvite precipitation as a technology to be integrated in a manure anaerobic digestion treatment plant – removal efficiency, crystal characterization and agricultural assessment

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Abstract
BACKGROUND: The formation of struvite (MgNH4PO4 · 6H2O) from digested slurry is an opportunity to recover nutrients as a slow‐release fertilizer. A series of batch and continuous experiments were conducted to assay the influence of operational parameters, total solids and organic matter content on the quality and size of struvite crystals formed. Finally, the agronomic bioavailability of struvite was assessed in a greenhouse experiment.RESULTS: In terms of process conditions an optimum pH of 9 was found, while changes in temperature (from 25 °C to 36 °C) showed to exert no influence whatsoever. On the other hand, though the presence of Na+ didn't affect the efficiency, high amounts of Ca2+ decreased the percentage of Mg2+ and NH4+ removed. CO2 stripping presented a reduction in the reagents required (NaOH) to raise the pH, with a similar removal efficiency and good quality of the struvite obtained. Nevertheless, the presence of organic matter resulted in a reduction on the size of struvite crystals. Agronomic assays showed that struvite P availability is similar to that of synthesis ammonium phosphate fertilizers.CONCLUSIONS: The results point to struvite precipitation as a good strategy to recover nutrients from digested manure and to improve nutrient management. © 2014 Society of Chemical Industry

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