Graduate Thesis Or Dissertation
 

Development of a mobile process to extract phosphorus from livestock wastes as a valuable fertilizer

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3x816q41j

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  • Environmental and regulatory concerns are causing confined animal feeding operations (CAFO's) to account for phosphorous content when applying wastewater to agricultural fields for disposal. In most cases this requires more land to spread the waste onto so that the phosphorous needs of the crop are not exceeded. A mobile process of removing phosphorous from these wastewaters could serve as a service to these operations. Phosphorous may be removed by precipitating struvite (MgNH₄PO₄⋅6H₂O) or hydroxylapatite (Ca₅(PO₄)₃OH) from the wastewater. Both compounds have the potential to be marketed as fertilizer products, which could make a mobile nutrient removal service economically viable. Several designs of on-site phosphorus removal reactors were investigated for adaptation to a mobile process. A "spontaneous nucleation reactor" design was chosen, using magnesium chloride and sodium hydroxide to separately control magnesium ion concentration and pH in the reactor. After a process of identifying reliable analytical methods, jar tests suggested that high removal of soluble orthophosphate (O-PO₄) could be achieved with a reactor pH of 8.5 and no magnesium dosing. Jar tests also suggested that reaction times greater than 30 minutes were not necessary for maximum O-PO₄ removal. Pilot plant tests at the Rickreall Dairy achieved up to 74% O-PO₄ removal. Hydroxylapatite was observed to form without supplemental magnesium, while struvite formed with supplemental magnesium. The bulk precipitate harvested from the pilot plant was found to be only 0.7% total phosphorus by weight. X-ray diffraction analysis showed that calcium carbonate was co-precipitating in much larger amounts than hydroxylapatite or struvite, reducing the marketability of the product as a fertilizer.
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