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Do You Poop Gold?

Sewage Contains Gold and Other Valuable Metals

Gold and other valuable metals appear in the wastewater stream. No one is excreting the proverbial golden brick. Some of the flow into the sewage system goes through surface runoff, and many of the metals come from dirt and dust on roads and parking lots. Other metals come from manufacturing, especially of semiconductors and other electronics.

A sewage treatment facility in Japan discovered that their treated sludge contained more gold per ton than the ore at Japan's best gold mine. Some of the gold in that sewage system comes from the large number of precision equipment manufacturers in the area.

'Digester eggs' at the Newtown Wastewater Treatment Plant in New York City.

How much gold might be extracted from the New York City wastewater treatment system? These sludge digester "eggs" are at the Newtown Wastewater Treatment Plant in New York City.

The sewage treatment plant in Suwa, in Nagano prefecture to the northwest of Tokyo, found 1,890 grams of gold per ton in the ash from incinerated sludge.

Compare that to the 20 to 40 grams per ton found in the ore at the Hishikari Mine. That mine, owned by the Sumitomo Metal Mining Co. Ltd., is one of Japan's top gold mines and one of the top world wide.

This isn't just "In theory they might ..." speculation. The prefecture is extracting the gold. The monetary yield varies with the price of gold, but when this was first reported in 2009 the local government expected to earn about ¥15,000,000 per year after covering the expenses of the extraction process.

It's not just gold. Silver, platinum, palladium, vanadium, copper, and other valuable metals appear in the wastewater stream. Engineers are investigating how best to capture commercially useful metals from municipal sludges.

Water, Sludge, Bugs, and Biosolids

Wastewater treatment plants take in sewage and, in older systems, "storm sewer" fluids, the water draining off roads, parking lots, and other surfaces. The treatment process aims to separate reasonably clean water from remaining solids.

The early treatment stages use microorganisms in sedimentation tanks or pools. Aerobic (or oxygen-using) bacteria and protozoa consume and break down the soluble short-chain carbon molecules in the waste. They bind larger and less soluble molecules into floc, which settles out of suspension into sediment.

Water is extracted and the remaining sludge moves into digestion chambers. Anaerobic microorganisms in these chambers break down larger molecules in the absence of oxygen. The digestion chambers need to be heated, but this stage produces methane (or CH4) which can be used to drive digestion, which produces more methane, and so on.

Wastewater
Treatment

In a following stage various microorganisms oxidize ammonia (or NH3) into nitrite (or NO2-) and then to nitrate (or NO3-) Other microorganisms then reduce the nitrate to nitrogen gas (or N2), which is released into our mostly nitrogen atmosphere.

Dewatering is the process of extracting water out of the remaining biomass. The result is biosolids, moist soil-like material that can be used as fertilizer or a soil replacement.

Home, restaurant, and hotel sewage also includes detergents plus oils and other byproducts of food preparation. Unfortunately, industrial waste can also enter the system either accidentally or intentionally.

Additionally, most wastewater systems are older combined sewer systems in which rainwater flows into the same input stream. That greatly increases the volume of wastewater, and it brings in many metals and other contaminants from the surfaces of roadways, parking lots, building roofs, and the ground.

So It's Not From Eating Gold

BMC
Pregnancy
and
Childbirth

Geophagy is the deliberate consumption of soil. It is relatively common throughout sub-Saharan Africa. A paper in BMC Pregnancy and Childbirth, "Geophagy practices and the content of chemical elements in the soil eaten by pregnant women in artisanal and small scale gold mining communities in Tanzania", examined the dirt-eating practices of pregnant women in a gold-mining area of Geita District in northwestern Tanzania, on the shore of Lake Victoria. Soil sticks are sold in markets in Tanzania, pregnant women eat those in addition to soil from walls of houses, from termite mounds, and from the ground.

They found arsenic, chromium, copper, iron, manganese, nickel, and zinc in both sources of soil. Cadmium and mercury were found in the ground soil. The estimated arsenic, copper, and manganese daily intakes exceeded the oral Minimum Risk Levels used by the U.S. Agency for Toxic Substance and Disease Registry.

The arsenic and mercury are waste from the gold extraction processes. However, despite the many gold mines in the area, no gold was detected in the soil the women were eating.

How Much Wastewater, and How Much Metal?

About 16,500 municipal wastewater treatment facilities in the United States produce about 7 million tons of biosolids per year. It contains nitrogen and phosphorus and is useful as fertilizer. About 60 to 70% of that biosolid output can be used as fertilizer. Of the remainder, about two-thirds is incinerated and the remainder is buried in landfills. If more metals were removed, then a higher percentage of the biosolids could be used as fertilizer.

Initial studies by the U.S. Geological Service have shown that gold alone appears in treated sludge at concentrations that, if found in rock, would be considered commercially viable mine locations. Another research group has estimated that the sewage produced by 1 million Americans could yield US$ 13 million in precious metals.

Sludge digesters at the Wards Island Wastewater Treatment Plant in New York City.

Sludge digesters at the Wards Island Wastewater Treatment Plant in New York City.

The paper "Characterization, Recovery Opportunities, and Valuation of Metals in Municipal Sludges from U.S. Wastewater Treatment Plants Nationwide" reports the results of another study. [Environmental Science and Technology Letters, 2015, 49 (16), pp 9479-9488] One of the authors maintains the National Sewage Sludge Repository at Arizona State University. The study used sludge samples from 94 wastewater treatment plants in 32 states and the District of Columbia.

Many of the metals seem to come mostly from soil and dust, with only small or even negligible contribution from industrial sources. However, the platinum group metals (platinum, ruthenium, rhodium, palladium) mostly came from human activity. For example, automobile exhaust catalytic systems emit fine particles of platinum and palladium. These particles collect on roadways and join rainwater runoff.

Toxic metals are regulated, including arsenic, cadmium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc. Water cannot be discharged, nor can biosolids be used as fertilizer, if they contain toxic metals above allowed threshold concentrations. However, until recently work on metal extraction went no further than removing enough regulated metals to discharge the water and use a large enough fraction of the biosolids as fertilizer.

The 14 most lucrative elements — silver, copper, gold, phosphorus, iron, palladium, manganese, zinc, iridium, aluminum, cadmium, titanium, gallium, and chromium — had a combined value of US$ 280 per ton of sludge in this study. They estimate US$ 13 million worth of metals in the biosolids produced by a city of 1 million people, with over 20% of that value coming from silver and gold.

Metal recovery makes sense at large scale. World-wide, 360 tons or more of gold may accumulate in biosolids annually. But at US$ 13 per person per year, there's little market for a home metal extraction kit. You'd better keep flushing and let the government handle it.

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