Low-pH Iron Removal as Part of Multi-Stage Passive Treatment

Abstract

Through a contract with the US Army Corps of Engineers, Baltimore District, an innovative passive system to treat drainage from an abandoned underground coal mine was completed in 2008. The discharges can be characterized as highly acidic (pH 2.7, 1400 mg/L acidity) with moderate to high metal concentrations, having average total iron, manganese, and aluminum values of 300 mg/L, 20 mg/L, and 140 mg/L, respectively. The passive system utilizes a variety of treatment components including mixed-media vertical flow ponds, an anoxic limestone drain, a horizontal flow limestone bed, settling ponds, wetlands, etc. Of particular interest is the use of oxidation and precipitation channels to form iron solids at low pH. Preliminary results indicate that on average about 230 mg/L of dissolved iron is being removed at 2.6-3.2 pH. Removal of iron within the channels is expected to decrease operation & maintenance requirements while increasing the overall life of the system. Other design considerations included decreasing the potential for saturation with respect to gypsum, increasing alkalinity through anaerobic decomposition and sulfate reduction in alkaline mine drainage sources, and creating elk habitat. The final system effluent is net alkaline (7.5 pH, 214 mg/L alkalinity, -180 mg/L acidity) with total iron and aluminum less than 1 mg/L and total manganese of about 5 mg/L.