Have you been adding Goldschlager liqueur to your hot chocolate again? People have been flushing millions of dollars worth of gold down the toilet, a new study suggests.

Researchers at Arizona State University measured levels of metals in sewage sludge, also known as biosolids, collected across the United States using a mass spectrometer and an electron microscope. They found that amounts of the 13 most valuable elements, including silver, copper, gold and platinum, were worth about $280 US ($350 Cdn) per tonne of sludge.

That means about $13 million worth of metals a year could theoretically be mined from the sewage produced by a city of one million people, said Paul Westerhoff, a professor at the School of Sustainable Engineering and the Built Environment at Arizona State University in an interview with CBC's Quirks & Quarks.

Sewage from around the U.S. has been found to contain metals, including gold. 8:27

Westerhoff added that cities currently pay about $300 to $400 per tonne to get rid of their sewage sludge or biosolids.

"But yet you're throwing away or giving away two or three hundred dollars worth of these metals for that same tonne of material," he said in an interview that airs Saturday.

There is as much gold in sewage sludge as there is in low-grade ore that is already mined for gold and about 1,000 times more than there is in typical soil, Westerhoff added.

However, the metal found in sewage typically comes in the form of very small particles.

"You can't go out and pick out a gold nugget," Westerhoff said.

The researchers published the results of their study in the Journal of Environmental Science & Technology.

Westerhoff and his colleagues aren't sure exactly where all that metal in sewage is coming from, but he suggested some might be from drug therapies, dentist's offices, foil coatings on food, or companies that make semiconductors or different coating materials.

"If you go to wealthy houses, you can find gold-plated toilets, right?" he added.

Westerhoff said he is now working with two mining companies in Arizona to try and develop an environmentally friendly way to extract the gold from sewage sludge.