English

Abstract

The successful long-term operation and sustainability of passive systems for the treatment of mining influenced water (MIW) affected by metals and acidity has been challenged by a lack of understanding on organic substrate design. Metabolic activity of sulfate-reducing bacteria (SRB) in bioreactors produces H2S that precipitates dissolved metals, and it is the main mechanism that removes metals from solution. Fermentation products of the organic substrate provided support the SRB indirectly. The relation between changes in bioavailable carbon (C) in the substrate, SO4 2- reduction and metal removal was thus studied. The systems under investigation are two identical 1,500-gallon tanks filled with volume fractions of 0.7 walnut shells, 0.25 corn stover, and 0.05 manure. The tanks have an operating volume of approximately 1,100 gallons, receive influent water at a nominal rate of 1 L/min, and have an estimated hydraulic residence time of about 1.5 days (based on 50% porosity). The bioreactors receive mining influenced water from the National Tunnel in Black Hawk, CO that has a typical metals concentration of 35mg/L Fe, 8mg/L Zn, and 0.6mg/L Cu; pH is circumneutral at 5.5 ±1.0. To assess bioavailable C in the substrate, hot water and acid soluble C and lignin-to- cellulose ratios were periodically analyzed. Preliminary chemical analysis of the substrate suggests that only the hot water extractable fraction (organic acids and labile polysaccharides) changed during the first three quarters of operation. The lignin-to- cellulose ratio during the same period was relatively unchanged, suggesting that the rate of organic substrate depletion was low. Sulfate reduction was variable but sufficient to promote efficient removal of Cu and Zn. After the first 12 months of operation, the average percentages for metal removal in the duplicate bioreactors were >99% for Cu and Zn and 70% for Fe, the latter ranging between 40 and 95%. The low rate of substrate depletion suggests the substrate mixture may provide long-term support of the sulfate-reducing consortium. The bioreactors were able to successfully remove Cu and Zn but Fe removal was problematic. This suggests that extent of SO4 2- reduction was not sufficient to effectively remove iron in the National Tunnel MIW.