Types of passive treatment systems
There are many types of water treatment systems available for removing metals from acid mine drainage. Passive treatment systems are a relatively recent technology that involves using sulfate-reducing bacteria or limestone or both to neutralize acidity and precipitate metals. These systems are sometimes called “wetlands” or “bioreactors.” Passive treatment systems differ from active systems (Aerobic wetlands
Aerobic wetlands are shallow (1–3 foot deep) ponds; they may be lined or unlined and some are nearly filled with soil or limestone gravel. Such wetlands facilitate natural oxidation of the metals and precipitate iron, manganese, and other metals. Anaerobic wetlands are used to neutralize acidity and reduce metals to the sulfide form. This reaction consumes H+ and therefore acidity.Anaerobic wetlands
Anaerobic wetlands may be lined or unlined shallow ponds filled with organic matter, such as compost, and underlain by limestone gravel. Water percolates through the compost, becomes anaerobic and metals precipitate as sulfides. Microorganisms facilitate this reaction by first consuming oxygen. Alkalinity and H2S are produced. If the system is improperly sized, if flow dries up, or if extended low temperatures are encountered, the microorganisms will die and the performance will be decreased. Some anaerobic wetlands discharge a sulfide “sewage” effluent, particularly during the first few years.Anoxic limestone drains
Anoxic limestone drains consist of a buried limestone gravel system that requires the exclusion of oxygen and aluminum in the water. If oxygen or aluminum are present, iron and aluminum hydroxides clog the system, causing failure. Alkalinity producing systems are a combination of an anaerobic wetland and an anoxic limestone drain.Other types
Other types of passive treatment systems include various limestone treatment configurations, ranging from limestone ponds to open limestone channels in which water flows down a steep slope with limestone riprap. These systems oxidize and precipitate metals and add alkalinity to the water. Another passive treatment system uses lime dispensing technology to neutralize acidity and precipitate metals in a settling pond. These units do not require power or hazardous chemicals and are inexpensive. BLM is currently conducting pilot tests on the Aquafix technology.Advantages
Passive treatment systems are a valuable option for treating acid mine drainage at remote locations. The advantages of passive treatment systems are that they do not require electrical power; do not require any mechanical equipment, hazardous chemicals, or buildings; do not require daily operation and maintenance; are more natural and aesthetic in their appearance and may support plants and wildlife; and, are less expensive than active alternatives.Disadvantages
There are disadvantages with any water treatment system. The disadvantages of passive treatment systems are that they may require complex discharge permits unless taking a Comprehensive Environmental Response, Compensation, and Liability Act (Maintenance
All of the passive treatment systems described will accumulate metal precipitates and will eventually have to be replaced. Research indicates that these systems can be expected to perform for 20 years. The precipitate is not normally a hazardous waste. Nonetheless, regular monitoring, inspection, and maintenance are required, although to a much lesser extent than with active water treatment systems.See also
*References
{{USGovernment Mining and the environment