A University of Arizona study shows why we may soon be mining sewage sludge for precious metals
A trip into New York City’s sewers involves a number of well-known hazards: ninja turtles, giant albino alligators, mole people, and of course, rivers of putrid, slimy grease. But now we have to add gold-rush-style grizzled prospectors, industrious methheads and other precious-metal fanatics to the list of unsavory characters one might encounter in the city’s miles of tunnels.
A recent University of Arizona study found that each year, the sewer sludge from a city of a million people contains about $13 million in metals like gold, silver and platinum. If the research team’s findings are scalable, that would mean over $100 million worth of annual squandered bling in a city the size of NYC. Storm run off, industrial byproducts, and whatever else people flush down the toilet all ultimately contribute to the fortune hidden in our underground poop reservoirs. And while extracting these metals might seem like an unworkable chore, doing so could easily offset some of the high waste-processing costs incurred by bigger cities. Science magazine reports:
“One city in Japan has already tried extracting gold from its sludge. In Suwa in Nagano Prefecture, a treatment plant near a large number of precision equipment manufacturers reportedly collected nearly 2 kilograms of gold in every metric ton of ash left from burning sludge, making it more gold-rich than the ore in many mines.”
And it’s possible that precious metals are just the tip of the excrement iceberg.
“A small number of sewage plants are removing phosphorous and nitrogen, which can be sold as fertilizer. A Swedish treatment plant is testing the feasibility of making bioplastics from wastewater. A model sewage incinerator that generates electricity and drinking water was just promoted by the Bill & Melinda Gates Foundation, which helped fund its construction.”
Currently, a little more than half of our sewage sludge is processed into fertilizer, with the rest headed for landfills and incinerators. But lately, the lingering presence of toxic residues, chemicals and germs has caused municipalities and agronomists to rethink the way we deploy our sludge. It will still probably be some time before mining muck becomes a regular practice; these valuable substances are usually present in microscopic amounts, and extracting them can be less than cost-efficient. But as more cities experiment with leveraging their sewage for its secret treasures, the process should become increasingly more cost efficient, with scientists working toward a future where sludge is an important commodity rather than just a disgusting byproduct.