Overview of the Sulfate-Reducing Bacteria Demonstration Project Under the Mine Waste Technology Program*

Abstract

Results are presented that were gathered during pilot- and field-scale testing of an innovative technology, sulfate-reducing bacteria (SRB), designed to treat and control acid mine drainage (AMD). The project was performed under the Mine Waste Technology Program (MWTP), which is funded by the U.S. Environmental Protection Agency (EPA) and jointly administered by the EPA and the U.S. Department of Energy through an Interagency Agreement. The MWTP is implemented by MSE, Technology Applications, Inc., located in Butte, Montana. Sulfate-reducing bacteria, a common group of anaerobic bacteria, produce hydrogen sulfide and bicarbonate when supplied with sources of carbon and sulfate. Hydrogen sulfide reacts with metal ions in AMD, precipitating them as metal sulfides; the bicarbonate serves to help neutralized the drainage. Pilot-scale testing was performed within eight packed-bed reactors at a temperature representative of a field application of this technology in a northern climate, nominally 8°C. The reactors, packed with an organic substrate, were operated continuously in upflow configurations receiving AMD at a flowrate corresponding to a 5-day retention time over a test duration of 60 days. During this time, numerous physical and chemical parameters were monitored. Pilot-scale metal removal efficiencies reached 99% for zinc, 99% for aluminum, 96% for manganese, 98% for cadmium, and 96% for copper. Iron and arsenic removal was not as effective as the aforementioned metals, which was largely attributed to the high levels of iron and arsenic contaminating the organic substrate. Evidence existed that both adsorption and sulfate reduction were occurring within the reactors. Field-scale testing was performed after completion of the pilot-scale testing. The main objective of conducting this field demonstration was to determine the effectiveness of the in situ use of bacterial sulfate reduction in the mitigation of metal contamination. The SRB field demonstration involved using the flooded subsurface mine workings of the Lilly/Orphan Boy Mine near Elliston, Montana, as an “in situ biological reactor”. Two platforms were suspended by cables in both sides of the two-compartment shaft 30 feet below the static water level and were secured at the surface. An organic substrate to nourish the SRB was placed in the shaft and supported by the platforms. Numerous physical and chemical parameters are monitored at the mine through the collection and analysis of multiple samples, principally metals concentrations in the mine water. The total length of monitoring will be 3 years; data gathered during the first 1-112 years of monitoring are reported within this document. High removal efficiencies were observed for aluminum, cadmium, copper, and zinc (70 to near 100%) in the mine water of the Lilly/Orphan Boy Mine during the 1 - 112 years of monitoring the field demonstration of the SRB technology. Low removal efficiencies were observed for arsenic and iron for the similar reasons as given above. Sulfate reduction was evident by measured decreases of sulfate and the detection of soluble sulfide within the mine water.