In recent years, researchers have reported a correlation between specific conductivity (SC) and total dissolved solids (TDS) and the health of aquatic organisms in receiving streams near coal mine valley-fill structures. Due to the potential for regulatory limits on SC and TDS, coal mining operations have begun to explore treatment technologies that could be used to reduce the levels of SC and TDS in receiving streams. Nanofiltration was evaluated based on its ability to reduce SC and meet a proposed limit of 500 S/cm for mine water samples with moderate and high levels of SC from southwestern VA. Three nanofilters (NF270, DK, NFX) were tested without any pretreatment in the first phase of this project. The DK and NFX nanofilters were able to meet this SC limit for both mine waters tested with an average reduction of 84 percent for both mine waters tested. The NF270 nanofilters tested provided for an average SC reduction of 69 and 59 percent for the moderate and high SC mine waters tested, respectively. Performance in terms of SC reduction declined as ion concentrations increased in the influent tested. In the second phase of this project, microfiltration and simulated-sand filtration were introduced as a pretreatment stage in order to determine if SC reduction could be enhanced. Neither of the pretreatment options improved the SC reductions accomplished by nanofiltration.

Abstract:In recent years, the specific conductance (SC) of Appalachian coal mining runoff waters has become a parameter of concern with the EPA due to its negative effect on aquatic life and water quality. In order to comply with the EPA guidance suggesting an effluent SC of 500 µS/cm, the Appalachian Research Initiative for Environmental Science (ARIES) Center at Virginia Tech requested that testing be conducted to determine the most effective technologies for reduction of SC. Runoff water was collected from two sites in southwestern Virginia and characterized to determine the source of SC. The primary contributing ions were determined to be Na + , Mg 2+ , Ca 2+ , and SO4 2-. Once characterized, the sample water was treated using a two-step precipitation method called the Cost-Effective Sulfate Removal (CESR) process. Study results indicate that source water with an SC of 1,500-2,500 µS/cm could be successfully reduced below the proposed EPA limit of 500 µS/cm when the second step of the CESR process lasted 18 hours and the reagent dose was 1.25x the source water sulfate concentration. The success of this process was due to its ability to remove more than 85% of the Ca, Mg, and SO4 2-from the water, which together accounted for more than 90% of ions in the source water.