Golden Poo: Are we flushing down a fortune?
One man’s trash is another man’s treasure. Or in this case, one man’s poop is another man’s goldmine.
It turns out our sewage contains a crap load of gold (pun intended). In some cases, it can contain more than a mine. A recent study found that the sewage of a community of 1 million people could produce US$13M worth of metals in a year. If we were to tap into this resource, its benefits would be threefold. It would both reduce the need for mining as well as help remove harmful metals from our sewage, all while making money.
Gold rush, or gold flush?
Gold is originally sourced from the earth and we’ve been mining it for thousands of years. But in many mines it is getting harder and more expensive to find gold. This is what prompted researchers to explore the deep unknown (aka our sewage) in a mission to find the goldmine of the future.
When I first heard this, I was pretty grossed out, but my curiosity was piqued. I think I speak for many of us when I say that I’ve never really put much thought into what happens to our waste once we are relieved of it. In countries such as the US, its put to good use – as fertiliser. That’s right, it’s a beautiful circle of life. We eat the food, we poop out the food, the poop helps to grow the food.
Weirded out yet? Yeah, me too. But don’t worry, before its ready to be plant food the sewage must be treated.
Doing the dirty work
In Melbourne, some of our sewage is processed through the Western Treatment Plant. If you’ve ever been to or driven past Werribee, this is the source of that pungent smell you’ve likely noticed. Here, our sewage goes through a rigorous treatment process. The end products are clean recycled water and ‘biosolids’.
Biosolids are nutrient-rich organic material. Because they contain high amounts of nitrogen, carbon and phosphorous it’s an ideal material to be used as fertiliser in the forestry and farming industries. Currently, Melbourne Water (who have the lucky job of managing our sewage) are stockpiling these biosolids, but hope to put them to use in the future.
However, sewage also contains other elements, including heavy metals. A study on the historic sewage sludge pile in Werribee found it contained 31 elements, including Au, Ag, Sb, As, Cd, Hg, Zn, Cu, and Pb.
Digging for gold – through sewage?
While modern regulations mean that modern sewage piles are less contaminated, our historic sludge piles are pretty chock full of metals. If we want to use these biosolids for fertiliser, these metals need to be removed. More ideally, some could even be recovered for a profit.
Metals of interest include gold (Au), silver (Ag) and copper (Cu), but metals such as arsenic (As) and mercury (Hg) are contaminants, and can leach out of the sewage and into water and soils. A benefit of removing the economically important metals means that harmful metals (Cd, As, Sb, Hg and Cr) are also removed. This means that our sewage can be safely used as fertiliser without the risk of contaminating soil and groundwater.
But how did the gold get there? Researchers believe that precious metal sources include the dental industry, automobile exhaust (Pt, Pd and Rh), and abrasion of jewellery (gold). And, unsurprisingly, gold concentrations are particularly high in areas with a historic gold mining industry. These elements end up in our waste water and make their way to sewage treatment plants.
Flushing down a fortune
But I wouldn’t recommend panning through your own waste. Human feces itself doesn’t contain that much gold, probably only around 0.2ppb Au. In other words, in every kilogram there is only 0.2 micrograms of gold.
“What’s a microgram?” I hear you ask. Well, its equal to one-millionth of a gram. Or one-billionth of a kilogram. Hence the unit “ppb”, which stands for “parts-per-billion”.
Sitting on a gold mine
Sewage sludge isn’t going anywhere, and this is quickly being recognised worldwide. The next step is working out a method of processing which is cheap and easy enough to extract precious metals from sewage. After this the treated sewage can be used as fertiliser without the risk of metal contamination, and precious metals such as gold and silver can be sold.
It’s a win-win.