There's gold in them thar sewers—and silver and platinum and copper, too. A study by Arizona State University (ASU) researchers estimates there is £8.57 million worth of precious elements in the sewage produced by a million-person city every year. Never think of sewage as stinky worthless waste again.
The ASU study looked at sewage samples from across the U.S. that had been stored at the National Biosolids Repository, which I am personally delighted to find out exists. By heating up the biosolids to a plasma, they measured the concentrations of elements in the sewage samples. All in all, the 13 most lucrative elements in sewage added up to an average of £185 per tonne of sludge—or £8.57 million every year for a city with a million people.
The thirteen elements were, in order: silver, copper, gold, phosphorous, iron, palladium, manganese, zinc, iridium, aluminium, cadmium, titanium, gallium, and chromium.
So where did all this metal come from? Mostly likely industrial facilities. Labs and factories that work with precious metals like gold already do a fair bit to keep money from literally going down the drain. Dental labs and jewellery repair shops incinerate their waste to recapture gold. (The San Francisco mint once burned all its carpet to find £132,000 worth of gold dust in today's money.) But some precious metals will still end up down the drain—multiplied across tonnes of sewage, you end up with a literal gold mine in the sewer.
Still, that doesn't mean it's economically feasible to start extracting precious metals from sewer. It's too dilute to be worthwhile, except in special cases. Science notes that a Japanese wastewater treatment plant near precision equipment manufacturers already burns it sludge to find about £33,000 a year in gold.
For the enterprising sewage thinker, though, there is always metaphorical gold to be found in poop. Sewage contains large quantities of nitrogen and phosphorous, limited natural resources that are used in all fertiliser. It can also be used to make biogas for fuel. Not all that glitters is gold, but some that stinks could be. [Science, Environmental Science Technology]