There are goldmines lurking in sewage processed by cities across the globe, with millions of pounds worth of metals waiting to be harvested from human and industrial waste.

Scientists at the Arizona State University, Tempe, have analysed the sludge (the substance left behind after treating sewage) and found a number of different valuable metals – including gold and silver – within.

They then quantified the different metals to estimate what it might be worth, reports Science magazine.

Sewage is a mix of water from toilets and industrial manufacturing, storm run-off and everything else put down the drain.

Researchers took sludge samples from sites across the US and analysed it with a mass spectrometer that can identify different elements.

They found there is about $13m (£8.5m) worth of metal produced every year by a city with a population of around one million. This includes $2.6m of gold and silver.

Although some sludge is used as fertiliser, most ends up being burned or dumped in landfills.

Jordan Peccia, a Yale University engineer who was not involved in the work, said: "We need to make this push where we stop thinking about it as a liability and instead we think about it as a resource. And anything we can find in sewage sludge that's valuable, it's good."

Publishing their findings in Environmental Science & Technology, the authors say it could be a good idea for cities to boost their finances by utilising this waste gold if they can find a cost-effective way of doing so.

One city in Japan already doing this yields about 2kg of gold from every metric ton of ash left over from burning sludge.

Paul Westerhoff, lead author of the study, said: "The next thing is to look at whether it's economically or technically viable. We think it is."