Gold, silver and platinum end up in sludge (the goo left behind from treating sewage) when we wash our hands (rubbing the rings on our fingers) and brush our (gold) teeth. Cars deposit traces of platinum in drains from catalytic converters and precious metals are increasingly used in shampoos, detergents and even in nanoparticles in clothes. The result, according to Thames Water analysis, is a level of gold in our sewage systems comparable to that found in working mines. Earlier this year, researchers from Arizona State University found that a city of a million people can produce around $13m (£8.7m) worth of precious metals annually, including $2.6m in silver and gold.
The question is how to get at it. At a sewage treatment facility in the industrial town of Suwa, north-west of Tokyo, they’re attempting it already. Not by mining and crushing rock – the two biggest costs of traditional gold mining – but by incinerating sludge and processing the molten ash. In 2009, the plant, near a high concentration of precision equipment manufacturers that use precious metals, reported a yield of gold to rival production levels at the world’s leading mines. It collected nearly two kilograms of gold in every ton of ash, worth nearly $200,00 a year. To compare, at Japan’s Hishikari mine, one of the world’s most productive, just 40 grammes of gold are found per ton of ore.
As yet, no US or UK facilities have followed suit but, as gold prices rise, scientists continue to research methods of sifting sewage for tiny amounts of gold. These tend to involve the use of powerful chemicals known as leachates that are used by industrial operations to extract metals from rock. While they can be toxic to ecosystems, in the controlled setting of a sewage plant they can be safely used without environmental risk. The benefits over gold mining as we know it could be huge. Perhaps just as importantly, knowing that there’s gold in them thar sewers completely changes the way we think about waste. Could it be that the home of sludge, Slimer, and giant fatbergs crafted from wet wipes, lard, and consumer guilt is in fact precious; something to be treasured and valued, worth its weight in, erm, gold? Maybe it’s not waste at all.