Web Date: August 26, 2014
Recovering Phosphorus From Animal Manure
Phosphorus is an essential nutrient for food crops. But some scientists worry that the agricultural demands of the world’s growing population will overwhelm the supply of the natural source of this mineral, phosphate rock. So researchers have looked for other sources of phosphate.
One team now reports that they can recover 80 to 90% of the phosphorus in animal manure by heating the biomass suspended in water (Environ. Sci. Technol. 2014, DOI: 10.1021/es501872k). The process is called hydrothermal carbonization, and it uses less energy than previous methods to extract phosphorus from waste, the researchers say.
Each year, large-scale farming operations in the U.S. produce amounts of manure that contain the equivalent of 2.7 billion lb of P2O5 used in fertilizer. One commercial method of recovering phosphorus from sewage involves incinerating sewage sludge and processing the resulting ash. Steven M. Heilmann of the University of Minnesota, Twin Cities, and his colleagues picked hydrothermal carbonization over this method for their work on animal waste because the process requires less energy.
“This process intrigued me because it was simple,” says Heilmann, who is also a retired chemist from 3M. “Simple things tend to work industrially and commercially.” Hydrothermal carbonization involves mixing biomass with water and heating it in a closed container at moderate temperatures relative to those used in incineration. The result is a solid hydrochar and a solution of dissolved salts.
To test the method on animal waste, the researchers suspended manure from cows, pigs, or chickens in water and heated the slurry to between 200 and 250 °C, depending on the
type of manure. They analyzed the elements in the resulting mixture and found that most of the phosphorus was trapped in the solid hydrochar. The researchers thought the phosphorus might be bound to metal ions, such as aluminum, calcium, magnesium, and iron. So they treated the solid with aqueous acid to dissolve these metal phosphate salts and then added base to precipitate predominantly calcium phosphate. The researchers filtered the solution to collect the phosphate, recovering more than 80% of the phosphorus present in the manure.
Rolf Krebs of Zurich University of Applied Sciences wonders if it would be possible to perform the hydrothermal carbonization so that the phosphates end up in the liquid instead of the solid hydrochar. If so, the process might be more energy efficient than producing the dry hydrochar and then rewetting it with acid.
Heilmann points out that a commercial application of hydrothermal carbonization to recover phosphorus from sewage sludge did something similar by performing the reaction in acidic water. The resulting phosphate salts dissolved in the water, though the final product was ferric phosphate. That form of phosphate is a less effective fertilizer than the calcium phosphate produced in this process, which is similar to that mined from rock, Heilmann says.
For the new process to be economically viable, Heilmann envisions also using the leftover hydrochars to produce useful products, such as sorbents for water treatment.
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