When we think of sources of gold, we usually think of elaborate gold mines with precision equipment digging deep down into the Earth. But in Nagano Prefecture in central Japan (and location of the 1998 winter Olypmics) gold is found in sewage.
The Suwa sewage treatment facility in Nagano reports that it finds 1890 grammes of gold per tonne of ash taken from incinerated sludge. Compare that with the 20-40 grammes of gold per tonne of ore mined at the Hishikari Mine, one of Japan's and one of the world's top gold mines. That is roughly 50-100 times more gold per ton!
But where does that gold content come from? Officials in Nagano believe that it comes from the large number of equipment manufacturers in the area who use gold in their operations. Traces of gold that are lost as waste products are retrieved from the sewage.
The lucky prefecture gets to bank the gold, and expects to get about $150,000 dollars in revenue from the retrieved gold during this fiscal year. The exact amount of course depends on the price of gold at the time.