Waste metal worth millions of dollars has been measured in sewer systems of municipalities around the nation, dissolved in sludge.
Sludge is thick material, left over from treating waste water from sewage systems. A new study has determined that each ton of this substance could hold hundreds of dollars worth of various metals. Even cities as large as a million people - roughly the size of San Jose, California, could extract millions of dollars worth of metals each year from municipal sludge, the investigation reveals.
Sewers gather waste water from a wide variety of sources, including industry, precipitation, and toilets. Researchers have long known that metals can collect in the runoff, and water needs to be purified before being reused by homes, businesses, and farms. Approximately 60 percent of all sludge produced in the United States each year is used in agriculture, although researchers are now questioning possible toxicity in fields, caused by metal deposits. The remaining 40 percent of the material is placed into landfills or incinerated.
Arizona State University researchers collected samples of sludge from cities around the country. They then analyzed these specimens, using a mass spectrometer. These devices are capable of identifying materials in a sample as the substance is heated.
Researchers found that each ton of sludge contains around $280 of metals, much of which could be extracted from the waste. These figures mean that a city of a million people could collect around eight million dollars each year in metals, dissolved within sludge. This figure includes an average of $2.6 million in silver and gold.
One city, Suwa, Nagano Prefecture, Japan, has started to process its sludge, extracting precious metals that would have otherwise gone to waste. Ash left over from burning sludge is producing around 70 ounces of gold for every 2,200 pounds of raw material. This makes sludge in that city richer than many gold ores recovered from mines.
Roughly eight million tons of biosolids, created from sludge, are generated in the United States each year. There would be significant costs to extract metals from the material, but any waste processed and sold would reduce landfill costs.
"We're not going to get rid of this sewage sludge. We need to make this push where we stop thinking about it as a liability and instead we think about it as a resource. And anything we can find in sewage sludge that's valuable, it's good," Jordan Peccia, an engineer at Yale University, who was not involved in the study, said.
Characterization, Recovery Opportunities, and Valuation of Metals in Municipal Sludges from U.S. Wastewater Treatment Plants Nationwide, detailing the new study, was published in the journal Environmental Science & Technology.