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Pilot scale application of microwave technology for dairy manure treatment and nutrient recovery through struvite crystallization Zhang, Hui


Excessive use of nutrients in agriculture has been significant surface and groundwater pollution sources. A pilot scale treatment process was developed for the purposes of phosphorus and ammonia recovery from dairy manure slurry. A novel advanced oxidation process combining hydrogen peroxide, microwave, and sulphuric acid was studied for its abilities to solubilise nutrients and metals in manure. The treated manure was then used for struvite crystallization to recover phosphates and ammonia in a commercially viable fertilizer form. Using dairy manure, as a phosphate source for struvite crystallization, had some foreseeable challenges. The majority of phosphorus in dairy manure slurry was insoluble, thus unsuitable for struvite crystallization. A synergistic nutrient solubilisation effect was found when manure was treated. As high as 101.7 ± 4.3% of total phosphorus was solubilised as phosphate, and 94.9 ± 4.7% of total nitrogen was solubilised as ammonia. High suspended solids concentration in manure slurry, thought to impede struvite crystallization, was also overcome through microwave treatment to destabilise manure solids from suspension. As high as 2.00 Log reduction in total suspended solids was achieved by gravity clarification. Manure contains high levels of calcium that would compete against struvite for phosphate precipitation. Oxalic acid was tested as a potential reagent to use for calcium removal from dairy manure. Up to 97.2% soluble Ca removal, by calcium oxalate precipitation was observed. The treated dairy manure was then used as influent material for struvite crystallization. Under various reaction conditions, it was found that phosphorus reduction between 69% and 99% was achievable. Roughly 25% nitrogen removal was observed under all reactor conditions. An interesting observation from the crystallization experiments was that pellet morphologies and compositions varied with reaction conditions. The potassium containing mineral hazenite was found to coexist in the struvite pellets. The ability to extract potassium from dairy manure was not only beneficial to the dairy industry, but may also create a fertilizer of higher economical and nutritional value, than struvite alone.

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