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Treatment System Harnesses Nature to Clean Toxic Wastewater

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by Michael Keller

The rural highlands farmers and herdsmen called campesinos who work the land near Bolivia’s Cerro de Potosí Mountain have had a problem for the last 468 years.

It was way back then when someone stumbling around this dry and dusty section of the Andes kicked over a rock on the mountain and found silver ore. A mine was built that fed the Spanish Empire’s thirst for the precious metal. It still operates today.

Those mining the mountain have slowly chipped away what became known as Cerro Rico—Rich Hill—and, after the silver ore started drying up, expanded their production to other valuable metals found in the ground there: copper, gold, iron, zinc, tin, lead, cadmium and chromium. More than 10,000 still travel down some 600 pitheads to make their pay underground.

The resulting environmental mess caused by metal contamination running out of the mines has made headlines for years. Dozens of toxic minerals spew out of mines, boreholes and tailing piles in the region’s wastewater discharge, including 161 tons of zinc, 157 tons of iron and more than two tons of arsenic that scientists have estimated flow out of just one study area every year. Some streams sampled around Potosí are as acidic as lemon juice from the runoff; others are as basic as milk of magnesia.

Everybody pays: The miners can expect to live for only 40 years; poverty and pollution blanket the city dwellers; and the campesinos irrigate their crops with heavily contaminated runoff, often the only water in this arid region that sees average yearly rainfall of just 16 inches.

“Water is extremely important for the campesinos and right now they’re using this really contaminated runoff,” says Robert Nairn, a University of Oklahoma civil and environmental engineering professor and director of the Center for Restoration of Ecosystems and Watersheds. “There is a serious health threat posed by eating crops irrigated with this water.”

Harnessing nature to solve a major problem

Cleaning up wastewater that is so polluted is a big job, often involving active treatment systems that use industrial chemicals and electricity to drive filtration and other equipment. It’s not something that has ever been achievable in a poor place like Potosí.

But Nairn’s team is changing all that. They’ve built what’s called a passive treatment system—a method that harnesses mechanisms in the environment to clean contaminated mine discharge.

“The big thing here is that we’re promoting natural processes to do the work, not engineering organisms or coming up with new chemicals,” he says. “It’s an attempt to marry ecology with engineering and design.”

The system works by collecting wastewater in a number of separate process units, small clay-lined filtering ponds and wetlands that each makes different conditions to promote stages of treatment. Water moves through the system by the slope of land underneath the system. That means the whole operation requires no or very little electricity, which can be supplied by small-scale solar or wind power.

Each unit’s characteristics are designed to coax specific metals out of the mine effluent. Some ponds are made to contain no oxygen while others do. One, called a vertical flow bioreactor, sends water through organic compost where microbial organisms capture some contaminants. Another, called an anoxic limestone drain, uses the mineral to increase pH to help other metals precipitate out of solution. Constructed wetlands are the final step along the processing chain, filtering out solids to boost water quality.

“There are a number of mechanisms that contribute to contaminant retention in these ponds. The major pollutant uptake happens through geochemical and microbial means,” Nairn says. “When the water reaches the last pond, it has gone from looking like orange, sediment-laden sludge to clear water.”

Design refined over decades

The system the Oklahoma team is using has been developed over the last few decades. It comes out of an increasing awareness that the sites of historical mining operations needed to be reclaimed to end the damage they continue doing to surrounding surface and ground waters. “The technology has advanced considerably in that time to where we can now pinpoint specific mechanisms that we’re trying to promote and engineer the process to match it,” Nairn says.

His group has already successfully demonstrated the passive treatment system in six projects being run in the United States.

One deployment, at an Oklahoma Superfund site, is cleaning up wastewater discharge that has been called irretrievably damaged by past mining activity. Designing, engineering and construction for this project cost $1.2 million and it is capable of processing 250 gallons of substantially polluted water a minute.

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(Passive treatment system at the Oklahoma Tar Sands Superfund site in 2009. The nearest pond is the start of processing. The farthest is the final finishing wetland before treated water is discharged into receiving waters. Courtesy Robert Nairn.)

The Potosí project, which has cost $75,000 so far for design, engineering and construction, is still in its pilot stage. But the team’s initial results are showing that heavy-metal-laden water processed through the multistep system comes out the other side with significantly lower concentrations that are acceptable for irrigating crops.

The system sequesters the target metals as hydroxides and sulfides, so it will eventually need to be cleaned out in 20 or 30 years. “So we know we’ll have to come back, but what Potosí is getting from the system is several decades of wastewater treatment at a very low cost,” Nairn says.

Top Image: University of Oklahoma students sampling mine water in Potosi, Bolivia. Courtesy Robert Nairn.

Michael Keller is the Managing Editor of Txchnologist. His science, technology and international reporting work has appeared online and in newspapers, magazines and books, including the graphic novel Charles Darwin’s On the Origin of Species. Reach him at mkeller@groupsjr.com.

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