The La Paz water supply is currently treated by the city's water works division but the procedure is very expensive and does not adequately remove all metals.
Younger worked with a local engineer, Marcos Arce, to test the feasibility of treating the Milluni mine water with llama droppings.
They feared that the region's cold temperatures could pose a problem. The mine sits at 4,425 meters (14,500 feet) above sea level and the water often freezes, which could kill or impair the bacteria that neutralize the water and remove the metals.
But preliminary experiments produced encouraging results. As mine water passed through four plastic containers partially filled with llama droppings, the pH of the water (a measure of acidity, with seven being neutral and zero being more acidic than battery acid) rose from 3.2 to 6.3—only slightly more acidic than corn.
The treated water was almost neutralized and the levels of many of the metals were reduced to quantities declared safe by the World Health Organization.
Busy Bacteria
The technique works because bacteria of the genus Desulfovibreo, commonly found in manure, "use sulfur the way we use oxygen," said Younger.
The bacteria use the dissolved sulfate in the water to produce a compound called sulfide. The sulfide then reacts with the iron in the water, producing iron sulfide, which becomes trapped in the compost and manure beds.
The results of microbial water treatments such as this can be extremely dramatic, said ecologist Bob Hedin of Hedin Environmental in Pittsburgh, Pennsylvania, who specializes in stream restoration and passive treatment of mine drainage. "When the treatments are properly done they produce a double whammy, turning red water clear and removing most of the toxic metals," he said.
The method is particularly effective at removing heavy metals—such as zinc, lead, and copper—from the water, as well as iron and aluminum.
Younger is currently seeking funding to implement large-scale bioreactors that will treat water from the Milluni mine.
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