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TECH GUIDE

Water Treatment: Surface Water: Passive Treatment Systems: Anoxic Limestone Drains

Anoxic limestone drains (ALDs) are used in conjunction with a settling pond, wetland or ditch in order to improve metal removal capabilities. This passive system uses trenches of buried limestone which raise the pH and alkalinity of the acid mine drainage (AMD). Limestone is a cheap and effective way to generate alkalinity, but it must be utilized under the right conditions or its effectiveness is limited. ALDs create the "right" conditions where limestone can generate alkalinity for long periods of time. In an anoxic (meaning insufficient or low oxygen) drain, limestone does not get coated with iron oxides and continues to dissolve (unlike using limestone for aerobic chemical treatment as discussed above). It is important to realize that ALDs simply raise the pH and add alkalinity to acid water. Once the water exits the drain, sufficient area must be provided for metal oxidation, hydrolysis and precipitation to occur. The type and size of the area, be it a ditch, settling pond, or wetland, depend on the metals in the water. Many times, allowing flow through an ALD before entrance into a wetland can improve metals removal since the additional alkalinity added to the acid mine drainage from the ALD is favorable for the chemical and biological processes that precipitate metals in wetlands.

For more information on anoxic limestone drains (ALDs), see Skousen and Zimkiewicz (1996) and Hedin et al. (1994). Also, check out the following websites:

 

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