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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B <br />TOXICITY REDUCTION EVALUATION — LINES OF INVESTIGATION <br />Design Basis <br />passive treatment approach is considered preferable due to reduced operating costs and labor <br />requirements. <br />As further detail has been developed on each of these options, it is clear that enhanced evaporation will <br />not be an effective standalone treatment technology given site constraints, but can be considered as <br />supplemental treatment for residual handling for waste brine. The primary reason for enhanced <br />evaporation being infeasible as a standalone technology is the poor evaporative conditions in this area of <br />Colorado. <br />Efficient evaporation for this area in Colorado occurs from late April through September, which means <br />that flows from October through early April will need to be stored and evaporated the next summer. <br />Retention of all flow over this duration is prohibitively expensive because of the potential pond size. <br />Furthermore, the location of the site limits the enhanced evaporation equipment options. Residual liquid <br />from evaporation must be collected and controlled which eliminates the highest efficiency options <br />(atomization) which can result in droplet drift of hundreds to thousands of feet in windy conditions, likely <br />impacting native and reclaimed areas with elevated soil salinity. <br />Design calculations, notes, and assumptions are provided in Appendix A. Conceptual design drawings <br />are provided in Appendix B. <br />2.2.1 Reverse Osmosis/Nanofiltrarior <br />Membrane separation technologies that apply to Colowyo are reverse osmosis and nanofiltration. These <br />are effective technologies for removing dissolved ions from a process stream and concentrate them into a <br />reject brine stream. TDS reduction testing on the Elk Ridge water was completed on October 31, 2019. <br />The Outfall 010 water has a conductivity range of between 3,640 and 4,340 µmhos/cm which was <br />decrease by reverse osmosis to 540 µmhos/cm, a reduction of 85-88%. Although this rejection is readily <br />achievable by a conventional reverse osmosis membrane, it also concentrates several ionic constituents <br />that are not considered detrimental. As part of the reverse osmosis tests, fish were placed in varying <br />concentrations of the RO permeate and showed no lethal toxicity effects in 100% permeate and <br />reproductive toxicity effects at some point between 60% and 80% permeate concentration. The primary <br />consequence of these positive test results is that further piloting is not required to implement a strategy <br />involving membrane separation, allowing it to move into a design and construction phase if selected by <br />Colowyo. <br />Interpretation of the TDS reduction testing information leads to a few key observations. Sulfate, the <br />primary TDS component, can be removed by both reverse osmosis and nanofiltration membranes. <br />Nanofiltration offers the benefit of lower feed pressure requirements than the reverse osmosis <br />membranes because it tends to allow monovalent ions to pass through the membranes. This means that <br />constituents which do not have an effect on the WET criteria will not be reduced thus reducing the <br />effective treatment volumes. Additionally, the entirety of the flow at Outfall 010 does not need to be <br />treated to meet the requirements as some of the water can be split and blended back into the permeate <br />stream. Given this information, a split -stream treatment solution using nanofiltration is proposed for this <br />investigation. <br />dpg \\us0321-ppfss011shared_projectsV33001407\reports\4_tre lines of investigationitre lines of investigation_20200320_ifra.docx 2.4 <br />