Image copyright Heather Lowers Image caption Previous research suggested the waste from one million Americans could contain as much as $13m worth of metals per year

US researchers are investigating ways to extract the gold and precious metals from human faeces.

The group identified gold in waste from American sewage treatment plants at levels which if found in rock could be worth mining.

Details were outlined at the 249th national meeting of the American Chemical Society (ACS) in Denver.

Extracting metals from the waste could also help curb the release of toxic substances into the environment.

"The gold we found was at the level of a minimal mineral deposit," said co-author Dr Kathleen Smith, from the US Geological Survey (USGS).

In addition to gold and silver, human waste also contains amounts of rare earth metals such as palladium and vanadium.

"We're interested in collecting valuable metals that could be sold, including some of the more technologically important metals, such as vanadium and copper, that are in cell phones, computers and alloys," said Dr Smith.

The team estimates that seven million tonnes of solid waste come out of US wastewater facilities each year. About half of that is used as fertiliser on fields and in forests, while the other half is incinerated or sent to landfills.

The scientists are experimenting with some of the same chemicals, called leachates, which industrial mining operations use to pull metals out of rock.

While some of these leachates have a bad reputation for damaging ecosystems when they leak or spill into the environment, Smith says that in a controlled setting - they could safely be used to recover metals in treated solid waste.

In a previous study, another team of scientists calculated that the waste from one million Americans could contain as much as $13m (£8.6m) worth of metals.