The history of passive treatment of acid rock drainage dates back over 20 years. It is only recently that engineers and scientists have been able to discern how Mother Nature has been immobilizing metals in natural wetlands and to mimic her handiwork. Since 1988 (when engineers and scientists gathered at two major technical conferences in Pittsburgh and Chattanooga), the geochemistry of metal precipitation in oxidizing and reducing environments has become better understood and the capacity of passive treatment systems for mine drainage has reached levels of 1,200 gpm. Systems operating in tropical and alpine environments indicate that this technology has broad application. While there have been advances, a "cook book" approach to design has yet to be realized. However, a staged design protocol of laboratory, bench-, and pilot-scale testing has yielded full-scale designs that have been functioning as intended. Future advancements needed include a focus on sulfate removal and the recovery of resources that might make this already economical water treatment method even more so.
Abstract:The Peerless Jenny King treatment system is a series of four sulfate reducing bioreactor cells installed to treat acid mine drainage in the Upper Tenmile Creek Superfund Site located in the Rimini Mining District, near Helena MT. The system consists of a wetland pretreatment followed by the four cells connected in a serpentine manner. The mining impacted water flows from the wetland through each cell before discharge. Sulfate reducing bioreactors mitigate acidity and metal contamination through the microbial production of sulfide. The produced biogenic sulfide precipitates metals, and the microbial process of reducing sulfate to sulfide produces alkalinity.The health of the entire microbial community present in such systems is important for remediation to be effective. Classes of microbes generally present in such systems include fermenters, methanogens and sulfate reducers. The health can be measured in terms of active microbial populations and positive interactions between populations for the support of sulfate reduction. The goal of this research is to measure the activity of each class utilizing analyses that quantify the groups by their function, as opposed to the traditional molecular techniques of identifying bacteria. Gas chromatography, HPLC-DAD, and ICP-AES are used to identify and quantify the end products of metabolism. The microbial activity can then be characterized and changes can be monitored over time. Results from 2005 sampling of Cell 3 within the system indicate that the activity of sulfate reducing bacteria is much higher than the numbers present would indicate. These results combined with those from 2006 sampling indicate that methanogenesis is a minor process within this cell. The calculation of the stoichiometry of carbon utilization by SRB is much higher than what would be predicted from known stoichiometric ratios of carbon used per sulfate reduced.
A pilot biochemical reactor (BCR) was designed and constructed to treat mine-influenced water emanating from an adit at a remote site in southern Colorado which receives an average of 400 inches (10.2 m) of snowfall each season. The objective of the study is to operate and monitor a BCR on a yearround basis in a harsh mountain environment. There are several unique attributes of the treatment and monitoring system. It has been constructed at an elevation of 11,000 ft a.m.s.l. (3353 m), and is designed to operate year-round. Since the site has limited winter accessibility due to snowfall, a remote monitoring system was designed to collect samples and field parameters throughout winter months. An automated sampling system powered by solar cells is used to sample the system influent and effluent on a weekly basis and an elaborate Teledyne ISCO™ (ISCO) satellite monitoring system tracks data on an hourly basis with data being uploaded to a web site. Winter water samples will be gathered from the autosamplers in the spring and analyzed for metals. Fall influent and effluent water quality results from the treatment system are reviewed. These include field parameters reported via satellite and metal concentrations from water quality samples. Since there are limited data on biochemical and sulfate-reducing reactors operating in elevated and harsh winter locations, the acquired data are unique for mine-influenced water remediation.
Acid rock drainage from a closed gold-mining operation in northern California was studied first in the laboratory and then on the pilot scale to determine the technical feasibility of passive u-eatment. The drainage has a pH of 3.8, and concentrations of Cu, Fe, Mn, Ni, and Zn of 140, 190, 28, 0.93, and 40 mg/L respectively. The laboratory studies concentrated on the question of whether local organic and soil materials could be used to support sulfate reduction in a passive treatment system. Samples were incubated at laboratory temperatures for a period of 4 weeks. Soil and wood processing wastes from the immediate vicinity proved to be too acidic to maintain a large population of sulfate reducers. The most reasonable material for sulfate reduction was a mixture of equal amounts by weight of cow manure, planter mix soil, and limestone chips. The final solutions had pH's of 6.5 to 6.9, and average Cu, Fe, Mn, Ni, and Zn concentrations of 0.02, I, 5, 0.05, and 0.1 mg/L, respectively. Based on the laboratory results, a pilot system was constructed that consisted of a lined steel container filled with a substrate volume that measured 2 by 3 by 12 m. The substrate mixture was the same as used in the laboratory tests. Raw manure from a dairy farm was mixed into the substrate for the sulfate-reducing bacterial (SRB) inoculum. Loading of the system was based on the estimate that 0.3 mo! sulfide per cubic meter of substrate per day would be generated, and the inflow of heavy metals should not exceed the sulfide generated. Using these principles, the flow was set at approximately 800 mL/min. Over the course of 9 months, the pilot system achieved removal of Cu and Ni below the effluent standards of 1.0 and 0. 7 mg/L. Dissolved Zn concentrations in the effluent averaged approximately 0.1 mg/L, compared with an effluent standard of 0.02 mg/L. Dissolved Fe concentrations in the effluent varied with the seasons, reaching a minimum of 1 mg/Lin the summer and rising to a maximum of 120 mg/Lin the winter. There is a significant increase in concentrations of Fe in unfiltered waters. This implies that, in a full-scale system, a settlingpolishing pond will be needed.
An active underground lead mine produces water having a pH of 8.0 with 0.4 to 0.6 mg!L of Pb and 0.18 mg!L of Zn. A full-scale 1,200 gpm capacity bioreactor system was designed and permitted based on a phased program of laboratory, bench and pilot scale bioreactor testing; it was constructed in mid-1996. The gravity flow system, covering a total surface area of about five acres (2 ha), is composed of a settling basin followed by two anaerobic bioreactors arranged in parallel which discharge into a rock filter polishing cell that is followed by a fmal aeration polishing pond. The primary lead removal mechanism is sulfate reduction/sulfide precipitation. The discharge has met stringent in-stream water quality requirements since its commissioning. The system was designed to last about 12 years, but estimates suggest a much longer life based on anticipated carbon consumption in the anaerobic cells.
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