We're flushing away a fortune
Sewage sludge contains traces of gold, silver and platinum at levels that would be seen as commercially viable by traditional prospectors. “The gold we found was at the level of a minimal mineral deposit,” said Kathleen Smith, of the United States Geological Survey.
Smith and her colleagues argue that extracting metals from waste could also help limit the release of harmful metals, such as lead, into the environment in fertilisers and reduce the amount of toxic sewage that has to be buried or burnt.
“If you can get rid of some of the nuisance metals that currently limit how much of these biosolids we can use on fields and forests and, at the same time, recover valuable metals and other elements, that’s a win-win,” she said.
A previous study, by Arizona State University, estimated that a city of one million inhabitants flushes about $13-million worth of precious metals down toilets and sewer drains each year.
Unexpected benefits over traditional mining
The task of sifting through sewage for microscopic quantities of gold may sound grim but it could have a variety of unexpected benefits over traditional gold mining. The use of powerful chemicals, called leachates, used by the industry to pull metals out of rock is controversial because these chemicals can be devastating to ecosystems when they leak into the environment. In the controlled setting of a sewage plant, the chemicals could be used liberally without the ecological risks.
Precious metals are increasingly used in everyday products, such as shampoos, detergents and even clothes, in which nanoparticles are sometimes used to limit body odour. Waste containing these metals ends up being funnelled through sewage treatment plants. “There are metals everywhere,” Smith said.
More than seven million tonnes of “biosolids” come out of American sewage plants each year, about half of which is burned or sent to landfill and half used as fertiliser.
In Britain, about 500 000 tonnes of dry sewage solids are used as fertiliser each year. But the amount of waste that can be converted into fertiliser is limited, in part, by the high levels of some metals.
“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 cellphones, computers and alloys,” Smith said.
To assess the viability of mining sewage, the team collected samples from small towns in the Rocky Mountains, rural settlements and big cities and used a scanning electron microscope to observe microscopic quantities of gold, silver and platinum.
In findings presented this week at the 249th national meeting and exposition of the American Chemical Society in Denver, the scientists showed that the levels of the precious metals were comparable with those found in some commercial mines.
The eight-year study, which involved monthly testing of treated sewage samples, found that 1kg of sludge contains about 0.4mg of gold, 28mg of silver, 638mg of copper and 49mg of vanadium.
A sewage treatment facility in Tokyo has already started to extract gold from sludge and has reported a yield that rivals those found in ore at some leading gold mines. A Swedish treatment plant is testing the feasibility of making bioplastics from waste water. Elsewhere, sewage plants are removing phosphorus and nitrogen, which can be sold as fertiliser. – © Guardian News and Media 2015