Pilot Study Using Tailored Granular Activated Carbon (t-GAC) for Treatment of Perchlorate in Groundwater
Abstract:In 2001, the Air Force implemented a treatability study to extract and treat groundwater at Site 133 located at Edwards Air Force Base (AFB), CA. One objective was to evaluate the effectiveness of using conventional granular activated carbon (c-GAC) to treat low levels of perchlorate (< 20 micrograms per liter [μg/L]) as well as chlorinated volatile organic compounds (VOCs). Based on the results of the first 3 years of operation, the c-GAC was not effective at removing perchlorate. After the California notification level (NL) for perchlorate was decreased from 18 μg/L to 4 μg/L in January 2002 (subsequently increased to 6 μg/L), the California Regional Water Quality Control Board (CRWQCB) requested the Air Force to further treat the groundwater to reduce perchlorate below the NL prior to discharge of the treated groundwater to the Air Force's wastewater treatment plant. In response, a pilot study to assess the effectiveness of perchlorate removal using a tailored granular activated carbon (t-GAC) was proposed with the following objectives:
Verify the number of bed volumes (BVs) before breakthrough of perchlorate above the NL occurs at the midpoint between t-GAC vessels.
Monitor the concentration of naturally-occurring uranium in the water before and after the t-GAC vessels to evaluate whether the t-GAC concentrates uranium.
Generate a waste profile of the spent t-GAC and evaluate disposal options.
The pilot study was designed to run for 6 months during which two 1,000-pound (lb) vessels of t-GAC would be operated in series following two existing 2,000-lb vessels of c-GAC, with a third c-GAC vessel on-line after the two t-GAC vessels. Groundwater was to be extracted from a single well at a flow rate of approximately 10 gallons per minute (gpm) and an estimated average influent perchlorate concentration of approximately 16 μg/L. Based on these assumptions and results of a bench-scale study conducted prior to selection of the t-GAC (c-GAC pre-treated with an organic monomer), over 9,900 BVs would be treated in 6 months, with breakthrough of perchlorate anticipated after 10,000 BVs.
The pilot study was initiated on 27 July 2004 and evaluated through February 2006. During this time, groundwater was extracted only from Well 133-EW01 at an average flow rate of 8 gpm. The average influent concentration to the t-GAC (following passage of the water through two c-GAC vessels) ranged from 7 to 18 μg/L.
First objective: Verify the number of BVs before breakthrough above the NL occurs at the midpoint between t-GAC vessels. The first detection of perchlorate at the t-GAC midpoint was an estimated (J-qualified) concentration of 1.5 μg/L in June 2005, after the t-GAC had treated approximately 11,500 BVs. Perchorate was not detected in July or August 2005, but was detected consistently between September 2005 and February 2006 at estimated concentrations ranging from 1.6 to 5.4 μg/L. As of February 2006, the perchlorate concentration at the midpoint had not exceeded the NL of 6 μg/L while treating over 21,000 BVs, demonstrating the effectiveness of t-GAC in perchlorate removal at low concentrations.
Second objective: Evaluate whether the t-GAC concentrates uranium. Samples for analysis of total uranium were collected from the t-GAC influent and effluent on the day following start-up of the pilot study and monthly through December 2004, after which samples were collected only from the t-GAC effluent between February and April 2005 and then discontinued. Although results from the very first samples collected in July 2004 indicated that nearly all of the uranium in the influent water (96 μg/L) was captured by the t-GAC, results for samples collected in August 2004 and subsequently show that uranium concentrations in the t-GAC influent and effluent were approximately equal. These results were interpreted to mean that uranium reached saturation in the t-GAC after the first month.
Generate a waste profile of the spent t-GAC and evaluate disposal options and costs. Samples were collected from the lead t-GAC vessel in May and July 2006 for waste profile analysis. Despite some initial concerns regarding the uranium concentration (29.7 ± 5.5 micrograms per gram) in the spent t-GAC, 6 drums of the spent t-GAC (from the lead vessel only) were successfully removed in October 2006 for disposal at US Ecology in Grand View, Idaho.
Document Type: Research Article
Publication date: January 1, 2007
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