|Arsenic Removal from Drinking Water by Adsorptive Media, U.S. EPA Demonstration Project at Bow, NH, Final Performance Evaluation Report (EPA/600/R-08/006) March 2008
This report describes the activities and results of the arsenic removal treatment technology demonstration at the White Rock Water Company public water system, a small residential drinking water facility in Bow, NH. The objectives of the project were to evaluate the:
The project also characterized the water in the distribution system and process residuals generated by the treatment system. The types of data collected included system operation, water quality, process residuals, and capital and O&M costs.
After engineering plan review and approval by the state, the ADI arsenic removal system was installed and became operational on October 13, 2004. The system consisted of two vertical 72-inch diameter and 72-inch-sidewall-height stainless steel vessels configured in series. The treatment system operated in three configurations under three separate test runs.
Due to its high pH value, the G2 media required onsite conditioning with sulfuric acid before service. Project personnel used a 93 percent sulfuric acid solution to lower raw water pH progressively from an average of 7.3 to 6.8, 6.4, and 6.0 in an unsuccessful attempt to improve media performance. Treated water pH was readjusted to 7.5 using 25 percent caustic sodium hydroxide solution before entering the storage tanks and distribution system.
The first test run, with both vessels A and B configured in series, ran for 3,714 hours from October 13, 2004, through November 29, 2005. The system removed arsenic from an average of 46.4 µg/L, present almost entirely as arsenic (V), to less than 10 µg/L for about 3,890,000 gallons, or 3,050 bed volumes (BV), of throughput. Treated water arsenic levels spiked to 37.5 µg/L after system startup, but leveled off to about 15 µg/L following the lead vessel and about 5 µg/L following the lag vessel at approximately 800 BV. Arsenic concentrations greater than 10 µg/L were observed in all samples taken following the lead vessel until reaching the influent level by the end of the test run. Personnel measured elevated manganese as high as 35.8 µg/L and silica concentrations as high as 61.8 milligrams per liter in the effluent of both vessels after system startup.
The second and third runs occurred after rebedding of both vessels in late December 2005. The second run, with only vessel A in service, ran from January 13 through April 14, 2006, for 119 hours. The third run, with only vessel B in service, ran from April 15 through September 26, 2006, for 2,705 hours. Both runs produced similar observations as those made in the first run. During the second test run, arsenic concentrations initially spiked, but progressively decreased to less than 10 µg/L for only two weekly sampling events (1,100 BV) before steadily increasing to 15.2 µg/L. The third test run also produced an initial spike in arsenic concentrations. However, these decreased to 1.2 µg/L, rising above the 10 µg/L level after treating roughly 1,900,000 gallons, or 3,000 BV, of water.
The system was backwashed only three times because of low headloss across each filter. Analysis of the backwash water indicated soluble arsenic concentrations 11 to 40 µg/L higher than levels in the finished water. Because the finished water was used for backwash, some arsenic appeared to have been desorbed from the media.
Comparison of the distribution system sampling results before and after system startup showed a decrease in arsenic concentration at all three Lead and Copper Rule sampling locations until the media reached capacity. Manganese concentrations in the distribution system generally followed those measured at the entry point.
The most significant operational issue was the need for additional acid and caustic to maintain the desired pH ranges of the feed water to the treatment system and the finish water to the storage tanks and distribution system. Discrepancies in pH readings made proper chemical dosing difficult.
Capital investment was $166,050, including $105,350 for equipment, $17,200 for site engineering, and $43,500 for installation. Using the system’s actual capacity of 40 gallons per minute (gpm) (57,600 gallons per day [gpd]), the capital cost was $4,150 per gpm ($2.88 per gpd). These calculations did not include the cost of the building construction.
O&M costs, estimated at $5.11 per 1,000 gallons, included $4.30 for media replacement and disposal, $0.47 for chemical supply, and $0.34 for labor. Incremental costs for electricity were negligible.
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