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EPA/600/R-07/081
Arsenic
and Antimony Removal from Drinking Water by Adsorptive Media
U.S. EPA Demonstration Project at South Truckee Meadows General Improvement
District (STMGID), NV Interim Evaluation Report (PDF)
(63 pp, 1926 Kb)
August 2007
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Abstract:
This report documents the activities performed during and the results obtained from the first 32 weeks of operation of an arsenic and antimony removal technology currently being demonstrated at the South Truckee Meadows General Improvement District (STMGID) in Washoe County, NV. The objectives of the project are to evaluate (1) the effectiveness of a granular ferric hydroxide (GFH) adsorptive media system in removing arsenic and antimony to meet the respective maximum contaminant levels (MCLs) of 10 and 6 µg/L, (2) the reliability of the treatment system, (3) the required system operation and maintenance (O&M) and operator's skills, and (4) the capital and O&M cost of the technology. The project also characterizes the water in the distribution system and process residuals produced by the treatment system.
The Siemens GFH system is a fixed-bed adsorption system that uses GFH, an iron-based media, to adsorb dissolved arsenic and antimony in drinking water supplies. When the media reaches its adsorption capacity, it will be removed from the vessels and replaced with new media. Spent media will be hauled away to a landfill after passing the Toxicity Characteristic Leaching Procedure (TCLP) test. GFH is produced by GEH Wasserchemie Gmbh and marketed by Siemens under an exclusive agreement. The GFH system for the STMGID site was designed to treat up to 350 gal/min (gpm) of water and consisted of three 66-in diameter, 72-in tall vertical carbon steel pressure tanks configured in parallel. Based on the design flow rate of 350 gpm and total media volume of 240 ft3, the empty bed contact time (EBCT) in each tank (and the entire system) was 5.1 min and the hydraulic loading rate to each tank was 4.9 gpm/ft2.
Between September 27, 2005 and May 3, 2006, the GFH system operated for a total of 943 hr. After it began normal daily operation on November 18, 2005, the system operated an average of 3.8 hr/day. The average flowrate during the 32-week study period was 275 gpm, which was 79% of the design flowrate. As a result, a longer average EBCT of 6.5 min was experienced by the media. During the 32-week study period, the volume of water processed was 15,567,000 gal or 8,677 bed volumes (BV) (one BV is equal to 240 ft3 or 1,795 gal). There were no backwash events based on headloss buildup during this study period.
Breakthrough of arsenic at 10 µg/L from the GFH system occurred at approximately 7,200 BV. Breakthrough of antimony at 6 µg/L occurred at approximately 3,000 BV. The media run length for arsenic was much shorter than the vendor-projected working capacity of 38,000 BV. The unexpectedly short run length for arsenic was probably caused by the presence of competing anions, such as silica and phosphorous, at high levels in raw water. Silica concentrations in raw water ranged from 51.5 to 95.1 mg/L (as SiO2) and averaged 72.6 mg/L (as SiO2). Total phosphorous concentration in raw water ranged from 0.27 to 0.46 mg/L and averaged 0.35 mg/L (as PO4) with some phosphorous existing as orthophosphate. Both silica and phosphorous were removed effectively by GFH, with silica reaching complete breakthrough about halfway through the 32-week study period and phosphorous never reaching complete breakthrough.
Treated water was blended with water from four other STMGID wells about one mile downstream of the GFH system. Water samples were collected at three locations in the distribution system, including one non-residential location prior to the blending point and two residences after the blending point, to evaluate the impact of the GFH system on water chemistry in the distribution system. As a combined result of treatment by the GFH system and blending with other source water, arsenic and antimony concentrations in the distribution system were significantly reduced to below the respective MCLs (except for one exceedance). There were no noticeable changes in lead or copper concentrations measured in the first draw samples from two residences. The lead concentrations remained low (i.e., 1.5 µg/L or less) in all samples; copper concentrations fluctuated from <1 to 176 µg/L, far below the action level of 1.3 mg/L.
The capital investment cost of $232,147 included $157,647 for equipment, $16,000 for site engineering, and $58,500 for installation. Using the system's rated capacity of 350 gpm (or 504,000 gpd), the capital cost was $663/gpm (or $0.46/gpd) of design capacity. O&M cost evaluated in this report included only the incremental costs associated with the GFH system, such as media replacement and disposal, electricity consumption, and labor. The media replacement and disposal did not take place during the first 32 weeks of operation; however, the cost to change out the media in all three adsorption tanks was estimated to be $70,550 by the vendor. The unit media replacement cost per 1,000 gal of water treated was developed as a function of the media run length to 10-µg/L arsenic or 6-µg/L antimony breakthrough in the combined effluent.
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Contact:
Thomas Sorg
sorg.thomas@epa.gov