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The Hg-contaminated processing water produced at Y-12 facility is discharged through the storm drain system, merged at Outfall 200, and then discharged to EFPC. Most of the baseflow mercury at Outfall 200 arises from a small number of short sections of storm drain. This report discusses the waterborne mercury treatment technologies to decrease mercury loading to the surface water of EFPC at Y-12 NSC. We reviewed current available waterborne Hg treatment technologies based on the specific conditions of Y-12 and identified two possible options: SnCl2 reduction coupled with air stripping (SnCl2/air stripping) and sorption. The ORNL 2008 and 2009 field studies suggested that SnCl2/air stripping has the capability to remove waterborne mercury with efficiency higher than 90% at Outfall 200. To achieve this goal, dechlorination (i.e., removing residual chlorine from water) using dechlorinating agents such as thiosulfate has to be performed before the reduction. It is unclear whether or not SnCl2/air stripping can reduce the mercury concentration from ~1000 ng/L to 51 ng/L at a full-scale operation. Therefore, a pilot test is a logical step before a full-scale design to answer questions such as Hg removal efficiency, selection of dechlorinating agents, and so on. The major advantages of the SnCl2/air stripping system are: (1) expected low cost at high flow (e.g., the flow at Outfall 200); and (2) production of minimum secondary waste. However, there are many environmental uncertainties associated with this technology by introducing tin to EFPC ecosystem, for example tin methylation causing abiotic Hg methylation, which should be addressed before a full-scale implementation. Mercury adsorption by granular activated carbon (GAC) is a proven technology for treating Hg at Y-12. The ONRL 2010 lab sorption studies suggest that thiol-based resins hold the promise to combine with GAC to form a more cost-effective treatment system. To achieve a treatment goal of 51 ng/L at Outfall 200 (flow rate: 1300 gpm), using a single GAC system will request very large reaction vessels and cost much more than a SnCl2/air stripping system (assuming it can achieve the 51 ng/L goal). However, the treatment cost depends on the treatment goal. If the treatment goal is 200 ng/L, the cost of GAC system will be significantly reduced while the cost of SnCl2/air stripping will remain the same. In addition, a GAC coupled with thiol-based resin system may further reduce the cost. Treating the Hg-contaminated water at source area with low flow rate (e.g., 40 gpm) may be another option to reduce the treatment cost. The advantages of the sorption technology are that it has proven treatment efficiency, reliability, and no environmental uncertainties. The disadvantages include that it produces large amount of secondary wastes. Based on the information evaluated in this report, we recommend that a pilot-scale test for SnCl2/air stripping process at Outfall 200 should be carried out before a full-scale implementation to address all the engineering and environmental risk questions. We also recommend continuing the sorbent lab studies at ORNL to optimize a sorption system that may be efficient and cost-effective enough for a full-scale implementation

TECHNOLOGY EVALUATION FOR WATERBORNE MERCURY REMOVAL AT THE Y12 NATIONAL SECURITY COMPLEX