Abstract

As part of the Superfund Innovative Technology Evaluation Program, an evaluation of the compost-free bioreactor treatment of acid rock drainage (ARD) from the Aspen Seep was conducted at the Leviathan Mine Superfund site, located in a remote, high-altitude area of Alpine County, California. The evaluation was performed by EPA’s National Risk Management Research Laboratory, in cooperation with EPA Region 9, and Atlantic Richfield Company (ARCO), the state of California, and the University of Nevada at Reno (UNR).

The primary target metals of concern in the ARD include aluminum, copper, iron, and nickel. Secondary target metals include selenium and zinc.

Drs. Glenn Miller and Tim Tsukamoto of UNR have developed a compost-free bioreactor technology in which sulfate-reducing bacteria are nurtured to generate sulfides that scavenge dissolved metals to form metal sulfide precipitates. Unlike compost bioreactors, this technology uses a continuous liquid carbon source and a rock matrix rather than a compost or wood chip matrix, which is consumed by bacteria and collapses over time. The benefits include better control of biological activity and improved hydraulic conductivity and precipitate flushing.

Evaluation of the compost-free bioreactor technology occurred between November 2003 and July 2005. The treatment system neutralized acidity and precipitated metal sulfides from ARD at flows up to 91 liters per minute (24 gallons per minute) on a year-round basis. Multiple sampling events were conducted during both gravity flow and recirculation modes of operation. During each sampling event, EPA:

• Collected chemical data from the system influent and effluent streams
• Documented metals removal and reduction in the acidity of the bioreactors, settling ponds, and aeration channel
• Recorded operational information pertinent to the evaluation of the treatment system

The treatment system was evaluated independently based on:

• Removal efficiencies for primary and secondary target metals
• Comparison of effluent concentrations to EPA interim (pre-risk assessment and record of decision) discharge standards
• Characteristics of and disposal requirements for the resulting metals-enriched solid wastes

Removal efficiencies of individual unit operations were also evaluated.

The compost-free bioreactor treatment system was shown to be extremely effective at neutralizing acidity and reducing the concentrations of four of the five target metals in ARD flows at the Leviathan Mine to below EPA interim discharge standards. During the demonstration, pilot testing to determine optimal sodium hydroxide addition resulted in exceedance of discharge standards for iron; however, after base optimization during gravity flow operations, effluent iron concentrations met discharge standards. Iron also exceeded discharge standards during recirculation operations when base addition was stopped due to equipment failure or lack of adequate base supply.

Although the influent concentrations for the primary target metals were up to 580 times above the EPA interim discharge standards, the treatment system was successful in reducing the concentrations of the primary target metals in the ARD to between 1 and 43 times below the discharge standards. Removal efficiencies for the primary target metals exceeded 85 percent; sulfate ion was reduced by 17 percent. The metal sulfide precipitates generated by this technology were not found to be hazardous or pose a threat to water quality and could be used as a soil amendment for site reclamation.

Based on the success of bioreactor treatment at the Leviathan Mine site, ARCO will continue to treat ARD at the Aspen Seep. The state of California and ARCO are also evaluating the potential effectiveness, ease of implementation, and costs for treatment of other ARD sources at the mine site.

Contact

Edward Bates

See Also

Superfund Innovative Technology Evaluation

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