Urban Mining – Gold digging in the Sewers
Resource-poor Japan discovered a new source of mineral wealth: sewage sludge. In its first month of operation, a sewage plant in Japan’s Nagano prefecture has mined 5 million yen ($56,000) worth of gold from sludge.
Sewage plant operator Nagano Prefecture Suwa Construction Office announced that approximately four pounds of gold can be mined from each ton of molten fly ash generated when incinerating sludge at its facility in the town of Suwa. That is better than the 20 to 40 grams of golden metal retrieved from each ton of ore at Japan’s Hishikare mine, according to Reuters.
Joint research conducted in 2007 by Nagano prefecture and the Japan Sewage Works Agency found that the concentration of gold in the ash was comparable to that of a high-grade ore. But because the cost of extracting the gold outweighed the potential profit, the operator continued treating the ash as an industrial waste material.
However, with the rise in the price of gold, Suwa decided to start mining the molten fly ash. October 2008, they sold 1.4 tons of the ash to a smelting company. At the end of January 2009, Suwa is received its first payment of 5 million yen ($56,000) for the recovered gold.
By the end of March 2009, Suwa mined a total of 5 tons of ash for a profit of 15 million yen ($167,000). The sewage operator says it will use the revenue to help pay for plant maintenance and operating costs.
The facility treats about 100,000 tons of wastewater each day, generating about 3 tons of ash in the process. Located in central Nagano prefecture, the Suwa Basin is home to numerous precision machining companies, metal plating, electronics facilities and hot springs, which may explain the high concentration of gold in the wastewater sludge.