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Integrated Passive Biological Treatment Process Demonstration
Figure 5. Block flow diagram.
Primary Issue Addressed: Acid Drainage/Water Treatment
Secondary Issues Addressed: Biological treatment for dissolved metals removal, including manganese
Project Site: Surething Mine in the Elliston Mining District of Montana
Collaborating Entities: MSE Technology Applications, Inc. (MSE)
Cost Share: None
Project Description
The Integrated Passive Biological Treatment Process was designed by MSE as a multistage with sequential passage of acid rock drainage (ARD) from the mine adit through three adjacent anaerobic reactors and one aerobic reactor (see Figure 5).
Anaerobic treatment relied on sulfate-reducing bacteria that reduced dissolved
sulfate to hydrogen sulfide, which reacted with dissolved metals to form
insoluble metal sulfides. This bacterial metabolism also produced bicarbonates
that increased pH of the ARD and limited dissolution of metal.
The first anaerobic reactor through which ARD passively flowed
was constructed of a mixture of cow manure and walnut shells.
Cow manure provided a source of easily degradable organic carbon
and large populations of sulfate-reducing bacteria. The walnut
shells provided a longer-term source of organic carbon and the
structural strength needed to maintain permeability of the mixture.
Bench-scale tests indicated that this initial reactor would successfully
establish the sulfate-reducing conditions needed for the overall
system, but also that it would be the first to fail due to bacterial
incompatibility with the low pH of feed water. Sulfate-reducing
capabilities also were challenged by the presence of iron ion
in the ARD, 95% of which existed in the ferric state.
Drainage water then flowed passively through the second anaerobic reactor, which was constructed of limestone cobbles that added alkalinity to the water. Earlier laboratory tests indicated that the previous cell’s reduction of ferric iron to ferrous iron reduced the extent of limestone armoring from ferric iron precipitates during ARD residence in this reactor.
The third adjacent reactor, containing the same cow manure/walnut shell mixture as the first, served as the primary driver of sulfide-precipitating reactions that removed metals from solution. With the exception of manganese, concentrations of all target metals in water exiting this reactor were below the most stringent of Montana’s Water Standards.
Figure 6. Aerobic bioreactor with manganese deposits.
Drainage water leaving the final anaerobic reactor was aerated by routing it through 300 feet of corrugated pipe riprap. The water was allowed to additionally aerate in an above-ground tank for 2 to 3 hours before passively flowing into the fourth reactor for aerobic treatment. This final reactor was constructed of a shallow, baffled limestone bed that provided an environment for indigenous manganese-oxidizing bacteria to thrive and for subsequent removal of manganese as a precipitate (see Figure 6.)
Status
After 4 full years of testing and continued process development of the system, the demonstration concluded in October 2005 when maximum contaminant levels were attained for all target metals and pH of the water returned to a neutral range (see Table 1).
The system was winterized, and the site will be closed out in the spring. Samples were collected from the bioreactors for final characterization testing including identification of active bacteria and solids stability. The final project report will be completed by the end of fiscal 2006.
| ARD Parameter | Feed Concentration (mg/L) |
Discharge Concentration (mg/L) | Montana Water Quality Standard (mg/L) |
|---|---|---|---|
| Aluminum | 29.5 | <0.04 | 0.087 |
| Arsenic | 0.127 | <0.01 | 0.010 |
| Cadmium | 0.208 | <0.00009 | 0.00076 |
| Copper | 2.35 | <0.003 | 0.037 |
| Iron | 15.0 | <0.014 | 0.31 |
| Lead | 0.151 | 0.004 | 0.015 |
| Manganese | 26.7 | 0.037 | 0.0501 |
| Zinc | 22.7 | <0.007 | 0.338 |
| Sulfate | 591 | 239 | 2501 |
| pH | 2.58 | 7.31 | 6.5-8.5 |
| Ammonia/Nitrogen | 0.11 | 0.37 | 4.612 |
| 1 EPA secondary maximum contaminant level 2 16 °C, pH 7.3 | |||