Several months ago we wrote about a report from scientists at Arizona State University analyzing the heavy metal content of the nation’s wastewater and sewer sludge. After collecting samples from across the US and using a mass spectrometer to identify every atom in each sample, they concluded that a metric ton of sludge contained an average of 16.7 grams of silver and 0.3 grams of gold. The report concluded with hopes of further research into the economic viability of harvesting these metals – and the idea is starting to spread.
In the UK, the School of Earth and Ocean Sciences collaborated with Thames Water to conduct a similar study, finding that the amount of gold in sewage systems throughout the UK could be worth as much as £13.6 million. According to Hazel Prichard, a geologist from Cardiff University who has been studying precious metals in urban waste alongside Thames Water, the sewage treated at wastewater plants could contain gold in concentrations between one to three parts per million. For reference, most gold mines are profitable at around the same ratio.
Of course, equipping and staffing sewage treatment plants to harvest precious metals would have significant startup costs. But as a consolation, reclaiming gold from sewage removes two of mining’s most costly and labor intensive processes: removing ore from the earth and grinding it down into a workable powder.
However, just like gold mines, the yield from a sewage treatment plant would greatly depend on its location. The report from Arizona State University claimed that a 1 million-person city could produce sewage containing about $2.6 million in traces of gold and silver annually. Meanwhile, a sewage treatment plant in Nagano, Japan reported findings of 1,890 grams of gold per ton of ash from incinerated sludge – a ratio better than the ore from Japan’s Hishikari Mine, which contains 20-40 grams of gold per ton of ore. Officials from the plant credited the high concentrations of gold to the large number of precision equipment manufacturers working with gold in Nagano.
With all of the hardworking jewelers in the Diamond District, plus the chefs of Serendipity 3 crafting recipes containing edible gold, it makes you wonder what could be in Manhattan’s sewers…