SULFATE REMOVAL FROM COAL MINE WATER IN WESTERN PENNSYLVANIA: REGULATORY REQUIREMENTS, DESIGN, AND PERFORMANCE

The listing of the Monongahela River as an impaired waterway prompted the Pennsylvania Department of Environmental Protection (PADEP) to adjust aqueous discharge limits to the river to no more than 250 mg/L of sulfate. In response to this, an analysis of water treatment options for a coal mining company was conducted at several non-operating mines in western Pennsylvania that discharge directly or indirectly to the Monongahela River. Given the extremely high capital and operations costs for typical sulfate reduction methods such as reverse osmosis and ion exchange, novel passive and semi-passive treatment options were explored. An ethanol-fed bioreactor system was selected, designed, and constructed in 2014 to test whether sulfate reducing bacteria could be utilized to remove sulfate in alkaline mine water to meet discharge limits. The unique design elements consist of metals removal circuit, ethanol feed circuit, and twin bioreactors bedded with large cobbles and seeded with sulfate reducing bacteria, but containing no additional carbon source. Biochemical performance has shown that sulfate reduction approaches 1500 mmol SO4 m -3/ day during warmer weather, one of the highest rates recorded in the literature. Effluent sulfate ranged from 58 to 400 mg/L at 16oC and about 1400 mg/L at 2oC compared to influent sulfate concentrations that averaged 2800 mg/L. In addition, the bioreactor produced 500-1500 mg/L of total alkalinity due to microbial metabolism supported by the ethanol, typically corresponding with sulfate decreases. Effluent metal concentrations were decreased to 1 mg/L Fe and 0.2 mg/L Mn. The recirculation loop was found to remove 90% of iron in the original settling pond prior to entering the reactors to minimize sludge accumulation. Additional