Researchers studying sewage sludge from wastewater treatment plants across the United States estimated that the metals contained in the waste could be worth up to $13 million annually for a community of one million people. According to Environmental Science and Technology , $2.6 million of the estimated value came from gold and silver, according to a study that examined the concentration, origin and potential economic value of metals accumulating in municipal sewage sludge. The estimate reflects the combined value of metals present in the large volumes of biosolids produced by wastewater treatment.
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Sewage sludge contains more than waste
Wastewater treatment plants separate solid material from water during the treatment process. The resulting sewage sludge, also known as biosolids, can contain a wide range of substances that enter the wastewater system from households, businesses and industries.
Environmental Science and Technology also notes that researchers examined 58 regulated and non-regulated elements in sewage sludge. The elements included gold and silver, as well as copper, palladium, iron, manganese, zinc and others used in industrial and consumer products. The researchers found that some metals were concentrated in biosolids rather than in treated water. The study also found that different metals appeared to have different sources. Rare-earth elements and some minor metals showed little enrichment compared with surrounding material, suggesting that dust and soil could be important sources. Platinum-group metals, by contrast, showed stronger evidence of human-related sources. The researchers identified 13 elements with the greatest potential economic value, including silver, copper, gold and palladium.
According to research published in PubMed Central (PMC) , gold was present in extremely small concentrations, but the quantities in the large volumes of sludge produced by cities could still add up. A separate account of the research described gold concentrations of roughly one part per million in some samples, with silver present at substantially higher concentrations. The researchers then used the measured concentrations and metal prices to estimate the potential value of the metals associated with a population of one million people.
The $13 million figure is a potential value, not a treasure chest
Recovering metals from sludge is technically complicated. The valuable elements are mixed with organic material, minerals and other substances, often at very low concentrations. Separating them requires additional processing, energy and infrastructure. The costs of extraction could therefore reduce or even outweigh the value of the metals recovered.
The study described the estimate as an assessment of economic potential and put the 13 most promising elements at about $280 per tonne of sludge. That does not mean every treatment plant could recover that amount profitably. The concentration of metals can vary depending on the population served, local industries and the type of waste entering the system. Metals can also originate not just from human excretions. There are everyday items and materials used by industries that contribute materials that find their way to wastewater. Metals can come from cosmetics, detergents, electronic devices and other materials that are flushed away or enter the drainage system. The industrial factor can have a great influence on sewage sludge composition.
That variability is important when interpreting the headline figure. A city with a different mix of industries, wastewater sources or treatment processes could have a very different metal profile in its sludge. The estimate therefore points to a potential resource contained in urban wastewater, rather than suggesting that municipalities have a fixed amount of recoverable gold, silver or other metals waiting to be extracted.
Wastewater plants could become urban mines
The discovery has made scientists wonder about the possibility of using sewage treatment plants as an alternative source of some raw materials. Wastewater plants collect large amounts of sewage sludge, creating a steady stream of material that can contain metals from across the city. The idea fits into broader efforts to recover useful resources from waste materials. Researchers are also exploring recovery of phosphorus, nitrogen and organic compounds from wastewater, while metals such as copper, silver and palladium may be worth extracting when concentrations are high enough.
However, the economic aspect plays a key role. Even though some metals can have a relatively high market price, their extraction can be economically inefficient from the given waste stream. The extraction of a metal can be more profitable when the facility processes industrial waste that contains relatively high levels of certain metals.
Wastewater systems are designed to remove contaminants and carry away waste, but they also collect traces of everyday materials. Tiny amounts of metals in the organic matter and discarded material flowing through urban treatment systems may add up to millions of dollars. The challenge is separating enough metal to make recovery economically and environmentally worthwhile. Until then, the precious metals in sewage remain less a hidden fortune waiting to be mined than a reminder of how much material modern cities continuously consume, use and lose.
