By Hilmar Schmundt
But that could soon change, as the Japanese delegation found out during its tour of the Leoben pilot plant. The surrounding region was once the heart of Austria's proud mining industry, which has since gone into visible decline. Many houses in Leoben stand empty today.
Now urban mining could give the region a new lease on life. The ash is delivered to the plant by truck from sewage treatment plants in Vienna, about 150 kilometers (94 miles) away. The light brown, fine dust is stored in large plastic bags, and nothing about it suggests that its particles may recently have been part of a delicious meal, before it was consumed, digested, excreted and flushed away through the city's sewer system.
When it is delivered to the pilot plant, the sewage sludge ash is unfit for use as a fertilizer, because it contains excessively high levels of heavy metals like cadmium. As gears groan and conveyor belts squeak, the ash, combined with chemical additives, is moved into a rotary kiln, where a hissing natural gas flame heats it to 1,000 degrees Celsius (1,832 degrees Fahrenheit). After half an hour in the rotary kiln, the ash, which has now passed through two purification steps, has a phosphate content of about 16 percent. It is then enriched with other plant nutrients, like potassium and nitrogen, to yield the final product: urban fertilizer.
"After recycling, the heavy metal content is even significantly lower than in most conventional fertilizers," says Ludwig Hermann, co-founder of Ash Dec. "At the beginning, everything went wrong that could possibly go wrong," he says, referring to the 2 million ($2.7 million) pilot plant. "The furnaces gummed up and the dust was baked into hard pieces." The machine that Hermann now uses for urban mining was once used to process aluminum.
'Miraculous Bearer of Light'
His experiences are typical of the nascent phosphate recycling industry, where there is much speculation and trial-and-error -- almost the way it was 340 years ago, when phosphorus was discovered. It was Hamburg alchemist Hennig Brand who, while searching for the "philosopher's stone" in 1669, discovered a promising substance that glowed mysteriously in the dark.
The alchemist's recipe was somewhat idiosyncratic: Take "golden yellow" urine, distill it and heat the residue. Using this crude method, he obtained a few grams of phosphorus from several hundred liters of urine. He then sold the glowing crumbs to other scientists for large sums of money. German mathematician Gottfried Wilhelm Leibniz was also keenly interested in the secret of the "phosphorus mirabilis" ("miraculous bearer of light").
The prosaic use of phosphate as a fertilizer was discovered by accident more than 200 years later. The material was a by-product of steel production in England and was known as Thomas meal. This industrial waste proved to be an outstanding fertilizer.
After that, phosphorus came to be used in many different applications: to promote growth in plants, in animal feed, on the strike surface of matchboxes and in weapons. Ironically, the Allies used phosphorus incendiary bombs in World War II to destroy Hamburg, the place where the "miraculous bearer of light" was first discovered.
Salvation from the Sewers
With the advent of urban mining, a new source of the coveted element is now being tapped. Many methods of deriving raw materials from sewage are being tested in the process. In the Netherlands, for example, the company Thermphos is producing high-quality white phosphorus for use in industry from vast amounts of sewage sludge ash. Germany, too, could soon assume a pioneering role with an innovative research center for phosphate recycling.
By 2012 at the latest, Hermann plans to build a circa 12 million reclamation plant in the context of a joint research project with the Berlin-based Federal Institute for Materials Research and Testing. A site in the eastern state of Brandenburg is under consideration.
"With our technology, recycling could satisfy a third of German fertilizer demand," says Hermann. His first industrial-scale plant is expected to be seven times as large as the pilot plant in Austria, and would produce 29,000 tons of fertilizer a year. He plans to obtain the raw material from sewage treatment plants within a radius of 300 kilometers. But the bulk of it will come from what is potentially Germany's most important phosphate mine: Berlin's sewers.
Translated from the German by Christopher Sultan
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