An Evaluation of Granular Activated Carbon and Ozone/Ultraviolet/Air Stripping Technologies for the Removal of Low Levels of Mercury from Water Discharges at the Y-12 National Security Complex

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Abstract:

The Y-12 National Security Complex (Y-12 Complex) is an active manufacturing and developmental engineering facility that is located on the U.S. Department of Energy (DOE) Oak Ridge Reservation. Past operations at the Y-12 Complex led to the release of mercury to the environment. Significant environmental media at the site were contaminated by accidental releases of elemental mercury from the building process facilities piping and sumps associated with Y-12 Complex mercury handling facilities. Releases to the soil surrounding the buildings have resulted in significant levels of mercury in these areas of contamination, which is ultimately transported to Upper East Fork Poplar Creek (UEFPC), its streambed, and off-site to the Clinch River.

The Record of Decision (ROD) for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area, Oak Ridge, Tennessee, requires the construction and operation of a new 300 gpm water treatment facility to remove total mercury from groundwater discharging from Outfall 51 as well as groundwater collected in the basement sumps of Bldg. 9201-2. The water treatment system will be designed using best available technology and reduce mercury to a maximum of 200-ng/L (ppt) in the effluent. Additional methods for removal of mercury, potentially bringing levels to the ambient water quality criteria (AWQC) of 51-ng/L, or less will be evaluated as part of a pre-design characterization and testing study. (DOE 2002)

Mercury at a concentration of approximately 1400 to 5000-ng/L is found in different forms in the discharge at Outfall 51. Limited data from mass balances performed during the pilot studies at Outfall 51 indicate that approximately 28% of the mercury is present as elemental mercury [Hg (0)], 33% is present as dissolved Hg (II) and 39% is of a refractory form, which speciation is presently unknown. The term refractory is being used to designate the fraction of mercury that cannot be converted to elemental mercury in the presence of stannous chloride (SnCl2), which is added to samples before analysis. (BJC 2002) Addressing the refractory fraction of mercury is important in meeting the goal of bringing discharge levels to 200-ng/L or less as well as in the evaluation of the ability to achieve the AWQC of 51-ng/L.

A pre-design study was performed to investigate the applicability of two treatment technologies for reducing mercury discharges at Outfall 51 in support of the design of the water treatment system. This document evaluates the results of the pre-design study for selection of the mercury removal technology for the treatment system. The approach to the pre-design study was to operate/evaluate a traditional baseline technology with a field pilot plant and compare the results to an innovative technology to select the treatment system to be utilized in a production facility.

The baseline technology consisted of filtration and four granular activated carbon (GAC) adsorption columns in series operation. Each column was filled to a packed depth of 4 ft with a 12 × 40-mesh activated carbon with this configuration selected to model the operation of two production carbon columns in series, each containing an 8-ft bed depth allowing for sample collection in the middle of each bed.

The innovative technology consisted of filtration, oxidation using ozone and ultraviolet light (UV) in a contactor/reactor to oxidize the mercury to a soluble form. SnCl2 was added to reduce the oxidized mercury to the elemental form that was removed in an air stripper. The ozone/UV treatment technology was selected based on bench-scale, laboratory studies performed at the Oak Ridge National Laboratory. Results from the field ozone/UV system are compared with the baseline to determine the effectiveness and implementabilty of the system.

The GAC system was operated continuously for approximately 60 days. During its operation, the system never exceeded the 200-ng/L discharge limit and did not appear to approach exhaustion/breakthrough for the 200-ng/L limit. As a result, a treatment system consisting of two carbon columns each with an 8-ft bed depth, operated in series was determined to be adequate to consistently meet the discharge limits documented in the ROD. The columns would be 8-ft diameter to accept a flow rate of 150 gpm. To meet the 300-gpm design requirement, two parallel treatment trains would be required. The data also indicates that there is a potential for achieving the 51-ng/L discharge goal through the installation of an additional column incorporated to each treatment train. (BJC 2002)

The field ozone/UV system was operated and sampled over a period of approximately 5 days. As with the activated carbon columns, the discharge from the ozone/UV system never exceeded the 200-ng/L limit. (BJC 2002) During setup and operation of the field system, modifications from the original laboratory scale unit were required. Early field analysis determined that additional reducing agent was required downstream of the UV reactor to maximize the formation of elemental mercury in the water prior to entering the air stripper. As a result, the ozone concentration in the gas was reduced and the SnCl2 feed rate was increased. Additionally, the initial semi-batch operated air stripper was replaced with a continuous flow stripper.

Based on the information evaluated in this study, mercury discharges below 200-ng/L are achievable with both technologies. In addition, it appears that levels approaching the AWQC levels may also be within reach of the GAC technology. The GAC appears to be able to remove Hg from the water independent of the species. As a result, the full-scale water treatment system has been designed and will be constructed using GAC as the mercury removal technology. To insure high removal results, an extra column will be added to the two-column configuration used in the pilot study. The treatment system is designed with two parallel 150-gpm trains of three columns in series for a maximum 300-gpm flow rate as required in the ROD. The levels of mercury removal reported using the ozone/UV studies indicate this technology merits additional development work.

Document Type: Research Article

DOI: http://dx.doi.org/10.2175/193864703784755580

Publication date: January 1, 2003

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