Knowing that carbon-based materials are irresistible food to bacteria, Alcoa's Tennessee Operations (United States) is using an innovative wetlands technique essentially devised by nature to clean up contaminants.
The wetlands, through their resident bacteria, are successfully reducing levels of complex cyanide and diesel fuel that had seeped into the site's groundwater. The technique also leaves behind no treatment byproducts that need to be cleaned up afterwards.
"I was initially uncertain as to whether wetlands were an appropriate technology for treating the contaminants," said Andy Shivas, program manager for the Voluntary Cleanup Oversight and Assistance Program for the Tennessee Department of Environment and Conservation. "Results from Alcoa's pilot projects were encouraging and supported the technique as being a practical, cost-effective way to address the contaminants. To my knowledge, this is the first site in Tennessee to use wetlands to treat diesel and complex cyanide."
Unlike free cyanide, complex cyanide is harmless to humans and is even used in the production of wine. The complex cyanide on the Alcoa property is a by-product of eight decades of aluminum production, some of which occurred before more stringent waste disposal and management restrictions were put into place. The diesel fuel is the result of a leaky underground storage tank that has since been removed. Trace amounts of both materials reach the surface through two springs on the Alcoa property, offering a unique opportunity for elimination.
"We researched our options, and we found that using wetlands to treat carbon-based products worked successfully for other materials, such as wastewater effluents," said Robert Prezbindowski, Alcoa senior staff engineer for remediation management. "We decided to create two pilot wetlands to see if the technique would also treat diesel fuel. Not only were we successful with the diesel, the pilot wetlands also destroyed the complex cyanide in the spring water. We had a removal rate of 90% for the diesel fuel and nearly 100% for the cyanide using this natural degradation process."
The two pilot wetlands covered about one-tenth hectare (quarter acre) at nearby Blue Pond. Each used a different substrate, water depth and aquatic vegetation. The vegetation is particularly important since it is the bacteria on the surface of the plants that consume the carbon-based complex cyanide and diesel as fuel.
From the two pilot wetlands, Alcoa determined the optimum wetland substrate is a native clay material with 30% to 40% organic content, such as compost. This 0.3-meter (12-inch) base provides sufficient surface area and organic content for the vegetation, which includes duckweed, bullrush and other water plants that harbor bacteria. The bacteria also thrive in the wetland's substrate.
Alcoa will begin operating a pilot-scale 0.4-hectare (one-acre) wetland at Duck Spring in 2003 to study the effects of this technology on degrading just complex cyanide. The cyanide level at Duck Spring is ten times higher than that at Blue Pond, and the lessons learned through this pilot study will determine if a full-scale wetland will be built at this spring and whether the technology can be adapted to other Alcoa locations.
Alcoa also plans to build a full-scale 1.6-hectare (four-acre) wetland at Blue Pond in 2004. Water from the underground spring will flow through a serpentine channel in the man-made wetland at an average depth of 15.2 centimeters (six inches) to 22.9 centimeters (nine inches), with some pools of up to 61 centimeters (24 inches) to deal with the varying water flow rates throughout the year. Prezbindowski expects the full-scale wetland to exceed the 90% diesel removal rate achieved during the pilot study since the 24-month trial period did not permit the pilot wetland to reach peak efficiency, which usually occurs in three to five years.
While Shivas has no major concerns about the wetland technology, he does note that one issue is how long the substrate will last before it has to be replaced. He also feels it's important to understand how and why the wetlands are working by monitoring their effectiveness. According to Prezbindowski, monitoring will occur on an ongoing basis.
"While the success of the project will be measured by how much of the contaminants we remove, we also have to acknowledge the benefit of a remediation technique that doesn't create any additional pollutants," said Prezbindowski. "In addition, we're creating a new habitat for plants and animals, including waterfowl."
Related Sites
Alcoa Tennessee Operations
Learn more about this major Alcoa location.
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Tennessee Department of Environment and Conservation
ENVIRONMENT - PRG - ADD Sidney RCC The agency is involved in numerous projects and initiatives throughout the state.
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U.S. Environmental Protection Agency
This site provides in-depth information about wetlands.
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