Acid Mine Drainage Treatment With a Combined Wetland/Anoxic Limestione Drain: Greenhouse and Field Systems

摘要

The most common methods for treating acid mine drainage (AMD) involve applying a strong base to neutralize the acidity and to precipitate metals. Because of the enormous expense of chemical treatment, passive treatment systems have been installed on many sites to treat AMD. Constructed wetlands have been used for over a decade on hundreds of sites. Wetlands remove acidity and metals from AMD under most conditions, but problems arise hwen acid and metal loads are too great for wetland size or as metal hydroxides fill the wetland, restricting substrate and water contact. Passive systems often incorporate limestone, which is a very cost effective acid neutralizer, to treat AMD. Complications also arise in limestone systems because ferric iron (Fe~3+) in AMD coats the limestone (armoring) reducing limestone's contact with acid water and because aluminum (Al) and Fe hydroxides plug limestone pores decreasing water flow. Limestone use in AMD treatment has been largely confined to anaerobic wetlands, anoxic limestone drains (ALDs) and open limestone systems, then the use of limestone for AMD treatment could be greatly expanded. We developed and monitored a passive AMD system to determien if AMD containing Fe~3+ and Al could be pre-treated by flowing through a wetland making it more suitable for introduciton into an underlying limestone drain, similar to a vertical flow wetland. Theoretically, pre-treatment by wetlands could reduce Fe~3+ to ferrous iron (Fe~2+) through Fe reduction and precipitate Fr~2+ as ferrous sulfides (FeS_x) through sulfate reduction of metal hydroxides with subsequent plugging of limestone pores. A field scale wetland/anoxic limestone drain (WALD) system located at Douglas, WV exported net alkaline water (mean of 127 mg/L as CaCO_3) for one year. Howeve,r dissolved oxygen and Fe data suggest that poor hydraulic conductivity caused this system to act as an Fe-oxidizing system, rather than an Fe-reducing system. As such, the system's long term effectiveness for treating AMD was compromised. After five years of operation, the system still reduces the acidity of the water from about 500 mg/L as CaCO_3 to about 150 mg/L. A small sclae Greenhouse system performed more like an Fe-reducing system, decreasing acidity for seven months and exporting Fe~2+, although the water exiting the wetland did not contain excess alkalinity. While complications arose in our sytems due to high flows in the Douglas system and high acidity in the Greenhouse system ,pre-treating AMD with organic material can improve the condition of the water for proper treatment by an ALD or underlying limestone. For low to moderate flows (<400 L/min) and low Fe concentrations (<50 mg/L), a passive sytem that pre-treats AMD with orgnaic substrates and hten directs the water into limestone may be effective for many years.