|Arsenic and Antimony Removal from Drinking Water by Adsorptive Media
U.S. EPA Demonstration Project at South Truckee Meadows General Improvement District (STMGID), NV - Final Performance Evaluation Report (EPA/600/R-09/016) February 2009
This report documents the activities performed during and the results obtained from the operation of an arsenic and antimony removal technology demonstrated at the South Truckee Meadows General Improvement District (STMGID) in Washoe County, NV. The objectives of the project were to evaluate (1) the effectiveness of a Siemens 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 GFH system was a fixed-bed adsorption system that used GFH, an iron-based media, to adsorb dissolved arsenic and antimony in drinking water supplies. When the media reached its adsorption capacity, it was removed from the vessels and replaced with new media. Spent media was disposed of at a sanitary landfill after passing the Toxicity Characteristic Leaching Procedure (TCLP) test. GFH was produced by GEH Wasserchemie Gmbh and marketed by Siemens under an exclusive agreement. Designed to treat up to 350 gal/min (gpm) of water, the GFH system at the STMGID site consisted of three 66-in diameter, 72-in tall vertical carbon steel pressure vessels configured in parallel. Based on the total media volume of 240 ft3, the empty bed contact time (EBCT) in each vessel (and the entire system) was 5.1 min and the hydraulic loading rate was 4.9 gpm/ft2.
During Run 1 extending from September 27, 2005 through 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 21% lower than the design flowrate. The lower average flowrate resulted in a higher average EBCT, i.e., 6.5 min. The system experienced little pressure buildup during operation. Major operational difficulties involved the system control and data acquisition (SCADA) and programmable logic controller (PLC) interface and a mechanical problem with the pneumatic butterfly valves for the backwash discharge line. Otherwise the system was relatively simple to operate, requiring little attention from the operator. The daily demand on the operator was typically 30 min for routine activities, including visual inspection of the system and recording of operational parameters.
Breakthrough of arsenic at 10 μg/L from the GFH system occurred at approximately 7,200 bed volumes (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 was probably caused by the presence of competing anions, such as silica and phosphorous. 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 (as P) concentrations ranged from 89 to 150 μg/L and averaged 115 μg/L 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.
Because of the short run lengths experienced, another adsorptive media, CFH-0818, a dry iron-based media supplied by Kemira Water Solutions, Inc., was selected, in conjunction with GFH, for a follow-on study in Run 2. The selection of the media was based on the results of a series of rapid small-scale column tests (RSSCT) performed under two separate projects. Prior to Run 2, one of the three vessels was replaced with GFH while the other two were replaced with CFH-0818. Run 2 took place from April 5, 2007, through July 3, 2007, during which time the system operated for a total of 1,166 hr. The system was operated at similar flowrates, which averaged 276 gpm. The system daily operating time was longer than that in Run 1, with an average of 13 hr/day.
Treated water was blended with water from four other STMGID wells about one mile downstream of the adsorption system. During Run 1, 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 adsorption 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. The O&M cost evaluated in this report included only the incremental costs associated with media replacement and disposal, electricity consumption, and labor. The actual cost to change out the media in all three adsorption tanks was $58,188, including replacement media, shipping, spent media analysis and disposal, and labor. At the time this report was prepared, the CFH-0818 was taken off the market indefinitely for making improvements. The unit O&M cost curve per 1,000 gal of water treated was developed based on the unit cost of GFH and 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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