2021-01-04_GENERAL DOCUMENTS - C1981019

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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B TOXICITY REDUCTION EVALUATION LINES OF INVESTIGATION EXECUTIVE SUMMARY The Toxicity Reduction Evaluation (TRE) performed for the Elk Ridge's Colowyo mine site in January 2020 identified three primary technologies to address the whole effluent toxicity (WET) in the discharge waters from East Taylor Pond. These technologies are membrane separation with nanofiltration (NF), bacterial sulfate reduction (BSR), and enhanced evaporation. Further development of these technology options as they relate to the site water quality and conditions has allowed the approach for each option to be refined. Given the known toxicants of sulfate and salinity in the mine water, a nanofiltration (NF) membrane is expected to provide the same results with a lower energy requirement than the previously tested reverse osmosis membrane. The poor evaporative site conditions limit the effectiveness of natural evaporation, thus management of the reject brine from the NF membrane filtration requires enhanced evaporation to optimize the size of the pond. Evaluation of site conditions, with the design intent for a passive approach, suggests that a subgrade system be developed for a BSR bioreactor. This approach includes segregation of surface stormwater from subgrade seeps in addition to installation of a seep water collection system. Subsurface seep collection allows for more consistent flow rates and water temperature in the BSR bioreactor influent. Testing will be required to demonstrate and optimize this technology in a bench -scale test and then in the field. A preliminary conceptual design of the BSR bioreactor is outlined within this document. Cost evaluation of the technology options shows that the BSR bioreactor process has the potential for lower costs over the lifecycle of the project than NF membrane filtration. Implementation of the BSR bioreactor is likely to have a lower capital cost, including treatability testing. There is also significant opportunity to further decrease the investment cost through site investigation and possible optimizations (outlined within this report). Operational cost of the BSR bioreactor is expected to be significantly less than NF membrane filtration stemming largely from the lower labor, reagent, and power requirements. There is some uncertainty as to the ethanol requirement for the targeted sulfate removal, but even at the highest published literature value the operational cost of the BSR bioreactor is expected to be less than half of the operating cost of the NF membrane separation system. Bench and pilot testing of the BSR bioreactor is recommended for future design incorporation.