Sewage Sludge Could Be a Gold Mine


By Samba Lampich

The sewage lines in your city could be lined with gold and other precious metals. Where do the metals come from?

Metals in sewage water come from many different places: natural weathering of soils and rocks as well as human activities that involve mining, processing and industrial plants. All of these sources flush down some amount of metal that makes its way into drains and water treatment plants. These metals can be corrosive and toxic, contaminating streams and other sources of water used for farming or fishing.

A new study conducted by scientists at Arizona State University, Tempe looked at the amount and value of these metals in sludge, a byproduct of sewage treatment.

In the study, the researchers used a mass spectrometer to identify 13 different metals in samples of sludge as they ionized through super-heating. They collected sludge from 94 different wastewater treatment plants in 32 different states. These elements included gold, silver, platinum and copper.

In a report published by Environmental Science & Technology, the researchers calculated that each metric ton of sludge could yield about $280 worth of these 13 metals. By extrapolating this figure, they calculated that a city of a million people could generate up to $13 million a year in these precious metals, of which $2.6 million would come from gold and silver.

“While we expected that the metals were present at low concentration, the fact that the small amounts represent such a significant economic value was definitely surprising,” says Pierre Herckes, associate professor at ASU and co-author of the study.

Extracting these metals might sound like a good idea, but it is not practically feasible. There is no current technology that could profitably extract the microscopic amounts of metals in sludge on a large scale.

However, a sewage plant in Japan has been able to harvest gold from burned sludge. They were able to get about 4.4 pounds of gold from every metric ton of ash from burned sludge.

While it may not yet be practical to extract metals from sludge, future capabilities to do so would help remove heavy metals that are detrimental to aquatic sources and not only help the environment but also earn a pretty penny.

Extension Questions

• What other resources could be extracted from sewage?

• How can heavy metals and minerals harm the environment?