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Treatment for Acid Mine Drainage
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Two methods can treat water to eliminate or reduce contamination by acidity and heavy metals. The active treatment method uses alkaline chemicals to neutralize acid-polluted waters . However, the chemicals are expensive and the treatment facility is expensive to construct and operate. The passive treatment method uses a treatment system that employs naturally occurring chemical and biological reactions to minimize acid mine drainage with little maintenance.
On this page:
- Active Treatment
- Passive Treatment
Active Treatment
Limestone (calcium carbonate)
Limestone with the highest calcium content is preferred. The advantages of using limestone include low cost, ease of use, and formation of a dense, easily handled sludge. The disadvantages include slow reaction time, loss in efficiency of the system because of coating of the limestone particles with iron precipitates, difficulty in treating acid mine drainage with a high ferrous-ferric ratio, and ineffectiveness in removing manganese.
Hydrated Lime (calcium hydroxide)
The coal mining industry uses hydrated lime as the preferred neutralizing agent because it is easy and safe to use, effective, and relatively inexpensive. The major disadvantages are the voluminous sludge that is produced (when compared to limestone) and high initial costs incurred because of the size of the treatment plant.
Soda Ash (sodium carbonate)
Soda ash briquettes are effective for treating small acid mine drainage flows in remote areas. Major disadvantages are higher reagent cost (relative to limestone) and poor settling properties of the sludge.
Caustic Soda (sodium hydroxide)
Caustic soda is especially effective for treating low flows in remote locations and for treating acid mine drainage having a high manganese content. Major disadvantages are its high cost, dangers involved with handling the chemical, poor sludge properties, and freezing problems in cold weather.
Ammonia
Anhydrous ammonia effectively treats acid mine drainage having a high ferrous iron and/or manganese content. Ammonia costs less than caustic soda and has many of the same advantages. However, ammonia is difficult and dangerous to use, and can affect biological conditions downstream from the mining operation. The possible off-site impacts are toxicity to fish and other aquatic life forms, eutrophication and nitrification. Fish species generally have low tolerance to un-ionized ammonia and toxicity levels can be affected by pH, temperature, dissolved oxygen and other factors. Ammonia use is not allowed in all states and, where permitted, additional monitoring is required.
Passive Treatment
Constructed Wetlands
Wetlands are passive systems, a relatively new treatment technology. Constructed wetlands use soil-borne and water-borne microbes associated with wetland plants to remove dissolved metals from mine drainage. Wetlands are generally more effective in removing iron than manganese. Wetlands are most useful in the treatment of small flows of a few gallons per minute. Initial design and construction costs may exceed tens of thousands of dollars. Optimum sizing and configuration criteria are still under study. Seasonal variations in metal removal efficiency have been noted, with lesser amounts removed in cold weather.
Open Limestone Channels/Anoxic Limestone Drains
This simply constructed passive treatment method uses open ditches filled with limestone (anoxic drains are covered). The dissolution of limestone adds alkalinity and raises pH, but a coating of limestone by iron and aluminum precipitates affects the performance of this treatment method.
Diversion Wells
Acidic water is diverted to a well containing crushed limestone. Iron precipitate coating is prevented by the turbulence of the flow through the well. The system works well, but needs periodic replenishment of limestone.
