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 Abstract

  Arsenic Removal from Drinking Water by Adsorptive Media
U.S. EPA Demonstration Project at Taos, NM
Final Performance Evaluation Report
(EPA/600/R-09/015) February 2009

This report documents the activities performed and the results obtained from the arsenic removal treatment technology demonstration project at the Orchard Highlands Subdivision site in Goffstown, NH. The main objective of the project was to evaluate the effectiveness of AdEdge Technologies’ AD-33 media in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 g/L. Additionally, this project evaluates: 1) the reliability of the treatment system (Arsenic Package Unit [APU]-GOFF-LL), 2) the required system operation and maintenance (O&M) and operator’s skills, and 3) the capital and O&M cost of the technology. The project also characterized the water in the distribution system and process residuals produced by the treatment process.

The treatment system consisted of two 18-in-diameter by 65-in-tall fiberglass reinforced plastic (FRP) vessels in series configuration, each containing approximately 5 ft3 of AD-33 media. The media was an iron-based adsorptive media developed by Bayer AG under the name of Bayoxide 33, which was labeled as AD-33 by AdEdge. The system was designed for a peak flowrate of 10 gal/min (gpm), based on the pump curve provided by the facility, and an empty bed contact time (EBCT) of about 3.7 min per vessel. The actual average flowrate of 13 gpm was 30% higher than the peak flowrate. The higher flowrate decreased the EBCT from 3.7 to 2.9 min, which might have contributed, in part, to earlier than expected breakthrough of arsenic.

The AdEdge treatment system began regular operation on April 15, 2005. Between April 15, 2005, and August 6, 2007, the system operated at an average of 5.3 hr/day for a total of 4,559 hr, treating approximately 3,459,000 gal of water. Two test runs were conducted with Run 1 (from April 15, 2005, through September 6, 2006) treating approximately 2,085,000 gal and Run 2 (from September 6, 2006 through August 6, 2007) treating approximately 1,374,000 gal. Flowrates to the system, calculated based on daily totalizer and hour meter readings on the lead vessel ranged from 9 to 16 gpm and averaged 13 gpm.

Raw water contained 24.0 to 37.3 g/L of total arsenic, existing almost entirely as soluble As(V). During Run 1, total arsenic levels in the treated water reached 10 g/L at approximately 19,500 bed volumes (BV) following the lead vessel and at approximately 25,710 BV following the lag vessel. (BV following the lead vessel was calculated based on the amount of media in the lead vessel only; BV following the lag vessel, or the entire system, was calculated based on the combined media volume in both the lead and lag vessels). These results suggested that doubling the EBCT from 2.9 (1 vessel) to 5.8 min (2 vessels) increased the run length, and, therefore, removal capacity, by approximately 32%.

Concentrations of phosphorous and silica, which could interfere with arsenic adsorption by competing with arsenate for adsorption sites, ranged from 16.3 to 99.2 μg/L (as P) and from 23.1 to 31.7 mg/L (as SiO2), respectively, in raw water. Low concentrations of iron, manganese, and other ions in raw water did not impact the arsenic removal capacity of the media.

On September 6, 2006, the media in Vessel A was changed out and piping was modified to make the vessels switchable. Run 2 was carried out with the partially exhausted Vessel B in the lead position and the newly rebedded Vessel A in the lag position. After approximately 1,374,000 gal of water had been treated by the system, the effluent of the system reached 10 μg/L on August 6, 2007, when sampling was discontinued and the performance evaluation was completed.
The system was backwashed only twice during the demonstration because there had been minimal solids buildup in the vessels and because pressure differential (Δp) across the vessels had remained essentially unchanged at 3 to 6 pounds per square inch (psi). Backwash was initiated manually with each vessel backwashed with the treated water from the 2,000-gal hydropneumatic tank for 20 min at 16 gpm (or 9 gpm/ft2), producing approximately 320 gal of wastewater. Arsenic concentrations in the backwash wastewater were 30.2 g/L from the lead vessel and 3.6 g/L from the lag vessel for the first event, compared to the treated water arsenic level of 0.3 g/L, suggesting desorption and/or release of media fines. The arsenic desorption might be due to slightly higher pH of the treated water in the hydropneumatic tank following aeration for radon removal. Approximately 0.33 lb of solids were discharged from Vessel A, including 3.6 × 10-4 lb of arsenic, 0.01 lb of iron, and 3.4 × 10-3 lb of manganese. Approximately 0.04 lb of solids were discharged from Vessel B including 3.5 × 10-5 lb of arsenic, 6.5 × 10-4 lb of iron, and 6.9 × 10-5 lb of manganese.

Comparison of the distribution system sampling results before and after operation of the system showed a significant decrease in arsenic concentration (from an average of 30 μg/L to an average of 1.1 μg/L). The arsenic concentrations in the distribution system were similar to those in the system effluent. Neither lead nor copper concentrations appeared to have been affected by the operation of the system.

The capital investment cost of $34,210 included $22,431 for equipment, $4,860 for site engineering, and $6,910 for installation. Using the system’s rated capacity of 10 gpm (14,400 gal/day [gpd]), the capital cost was $3,421/gpm of design capacity ($2.38/gpd) and equipment-only cost was $2,243/gpm of design capacity ($1.56/gpd).

The O&M cost included only incremental cost associated with the adsorption system, such as media replacement and disposal, electricity consumption, and labor. The media was replaced only once during the demonstration in Vessel A which cost $4,199. The O&M cost was calculated to $2.34/1,000 gal based on the media replacement cost and the cost of labor and electricity incurred during the demonstration.

Contact

Thomas J. Sorg


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