Manure Has Promise as Filter
A little-known and unpleasant fact from the Department of Agriculture: Food animals in the United States produce 350 billion tons of manure each year. A lot of it returns to the fields as fertilizer, but not all -- not by a long shot.
Last week, USDA's Agricultural Research Service reported that research chemist Isabel Lima has devised a way to turn chicken manure into porous activated carbon with the ability to leach heavy metals from wastewater.
"Animals leave behind incredible amounts of manure," Lima said in a telephone interview from her office at USDA's Southern Regional Research Center in New Orleans. "We wanted to see if there was any way to transform it and add value."
Lima's team first "charred" the manure -- heated it in an oxygen-free environment to boil off volatiles and pollutants, leaving a carbon-rich, charcoallike residue. Then the team bombarded the residue with steam, imparting porosity to give it a large amount of surface with which to catch impurities.
Lima said that most activated carbons are made from coal, wood or plant "residuals" such as peanut shells, soybean hulls or coconut husks, and are best used in removing odor and organic materials from drinking water.
Unlike these products, chicken manure turns out to have an unusual ability to extract from industrial wastewater positively charged metal particles such as zinc, copper and cadmium, many of which are carcinogens.
Lima's team is conducting further tests to find out how this happens.
The team is also trying to reduce the cost of making manure carbon, but in the targeted industrial market, where coal is currently the predominant raw material, manure has an advantage: "Coal is expensive and nonrenewable," Lima said. "With manure, the raw material is free."
-- Guy Gugliotta
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