Mimicking Nature: Nature inspires passive treatment technology for high-flow acidic mine drainage discharges
Some deep mine discharges throughout the Appalachian coal mining region have naturally occurring iron-rich mounds at their outflows. These mounds, which contain a plentiful and diverse microbial community, form via a microbially driven reaction that precipitates iron out of the water as the acidic mine drainage (AMD) is exposed to air. So the reaction that creates the mound also removes iron from the water. The passage of water across some mounds works quickly and without human intervention to reduce the water’s dissolved iron concentration – in some cases from more than 50 mg/L to about 1–5 mg/L in 15 minutes. This treatment alone may meet the discharge standard for iron in water.
Environmental engineer Bill Burgos is leading a study to engineer a passive pretreatment technology
for high-flow acidic mine drainage AMD that exploits the natural biological low-pH iron oxidation
occurring in these mounds. Other members of the research team include Mary Ann Bruns, associate
professor of soil science/microbial ecology; John Senko, postdoctoral researcher in civil and
environmental engineering; and three master’s students.
Impacts of Coal Mining in Pennsylvania
Pennsylvania has a long history of coal mining. Starting in the mid-1700s and continuing today, Pennsylvania has supplied more than 25 percent of all the coal mined in the nation and currently ranks 4th among all states in coal production. But this enduring legacy hasn’t come without a price – Pennsylvania has at least 2,400 miles of streams polluted with AMD. New mining practices and stricter environmental regulations have greatly decreased AMD from currently operating mines, but AMD from abandoned mines continues to be a major environmental issue. The U.S. Environmental Protection Agency designated AMD the single greatest environmental threat to the Appalachian region. Trout Unlimited estimates that Pennsylvania alone loses about $70 million annually in foregone tourism dollars because of AMD impacts on waterways.
Acidic Mine Drainage: Introduction and Treatment
AMD is caused by metal sulfide minerals, mainly pyrite, chemically reacting with oxygen and water to form discharges that are highly acidic and laden with dissolved metals. Some streams polluted with AMD have a pH below 3 – similar to the pH of vinegar and lemon juice – on a scale where 7 is neutral.
Cost-effective technologies for treating AMD are needed, especially for high-flow discharges that contribute high pollutant loads to receiving streams. Burgos emphasizes that the removal of iron is probably the most important aspect of AMD treatment.
AMD can be treated by both conventional active (treatment plants) and the more recently developed passive (in situ) treatment systems, in which alkalinity addition raises the pH of the discharge, reduces the acidity, and causes dissolved metals to precipitate - form solid particles – and settle out of the discharge. Passive systems offer a solution for treating AMD in remote locations.
There are many types of passive treatment systems, such as aerobic wetlands, anaerobic compost reactors, limestone ponds, and limestone channels. With high metal concentrations, the lifespan of all passive treatment systems is limited because the limestone beds used to raise the pH becomes “armored” or clogged with iron precipitates. This limits the pH buffering capacity of the limestone rocks, so the rocks have to be tumbled or replaced periodically to maintain effectiveness.
Passive Pretreatment Proposed for High-Flow Discharges
Burgos explains that his current research “advocates a spatial separation of iron oxidation and precipitation from alkalinity addition” for AMD. He thinks that pretreating the water via passage through an engineered system that mimics the iron removal taking place in the naturally occurring mounds may lengthen the useful life of passive treatment systems.
This project focuses on the Hughes Borehole in Portage, Pennsylvania, which has a high discharge
of between 800 and 3,500 gallons per minute. Its discharge flows into the Little Conemaugh River,
which drains parts of Somerset and Cambria counties. The borehole, which was drilled to relieve
water pressure in mine works in the area, discharges water with a pH of about 3.1 and about 150
mg/L iron. High-flow discharges, including this one, present such an intractable problem that
there is currently no treatment of the discharged water.
Passive biological low-pH oxidation of iron is evident at the Hughes Borehole by the naturally terraced iron mounds present. Although these iron mounds have been referred to as “kill zones” because of the general lack of vegetation, these mounds support lively microbial communities capable of rapid and significant low-pH iron oxidation.
Burgos’s team previously studied low-flow discharges (10–30 gallons per minute) at Gum Boot Run in McKean County and the Fridays-2 discharge in Clearfield County, analyzing the discharge water chemistry and characterizing the microbial communities in the mounds. Burgos explains, “We have made considerable progress in better understanding biological low-pH for the passive treatment of relatively low-flow deep-mine discharges, and now propose to extend this research to high-flow discharges. We envision a treatment technology that mimics the physical features of the natural iron mounds, for example, shallow sheet flow, where low-pH iron oxidation has been measured to be most rapid. We refer to this treatment technology as an ’aeration terrace’ and believe it could be highly effective as a passive pretreatment method for high-flow discharges.”
“Our plan,” says Burgos, “is to measure changes in water chemistry, mineralogy, and microbiology of the iron mounds to provide the scientific underpinnings required to engineer this process for remediation purposes.” The project includes field sampling, laboratory studies, and a pilot-scale field demonstration, all designed to determine the specific factors that enhance iron oxidation in the mounds. Burgos’s crew will divert some of the Hughes borehole discharge into parallel treatment channels landscaped to control and evaluate site factors such as residence time, channel roughness, and sheet flow depth.
In their previous low-flow studies, the highest rates of iron oxidation occurred when dissolved oxygen levels were the highest; therefore, they may aerate the water by having it flow over little steps before passage through the treatment mound.
Bruns, the microbial ecologist, plans plans to identify the microorganisms most responsible for the iron oxidation and evaluate whether adding slowly dissolvable organic matter such as corn stover or straw stimulates biological activity within the mound, thereby enhancing microbial oxidation. DNA-based fingerprinting will be used to directly compare the microbial communities before and after the organic matter additions.
The team hope to develop a design equation or algorithm to size aeration terraces so that similar systems could be implemented elsewhere and to develop a standardized laboratory protocol to measure the potential rate of low-pH oxidation for any field site.
Asked what excites him about this work, Burgos says, “The opportunity to turn our basic science research into field-deployable systems is very likely.” The Pennsylvania Department of Environmental Protection (PaDEP) could start using some of the findings immediately to treat AMD at low-pH, high-iron sites. While the media tends to focus on microbial research in exotic environments, like Yellowstone’s hot springs and deep sea vents, these iron mound systems are another extreme environment that is closer to home. The knowledge gained from this research can have immediate and highly positive impacts on the environment and communities of rural Pennsylvania and throughout Appalachia.
This research is funded through PaDEP’s Growing Greener II initiative, and supported through the National Science Foundation-funded Center for Environmental Kinetics Analysis (CEKA) at Penn State.
by Joy Drohan
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William Burgos is an associate professor of civil and environmental engineering. He can be reached by email at bburgos@psu.edu. |
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Mary Ann Bruns is an associate professor of soil science/microbial ecology. She can be reached by email at mvb10@psu.edu. |
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John Senko is a postdoctoral researcher in civil and environmental engineering. He can be reached by email at jms95@psu.edu. |