|Arsenic Removal From Drinking Water by Adsorptive Media, U.S. EPA Demonstration Project at Dummerston, VT, Six-Month Evaluation Report (EPA/600/R-07/003) January 2007
This report documents the activities performed during and the results obtained from the first six months (from June 22, 2005 through December 22, 2005) of the arsenic removal treatment technology demonstration project at Charette Mobile Home Park in Dummerston, Vermont. The objectives of the project are to evaluate 1) the effectiveness of an Aquatic Treatment Systems' (ATS) arsenic removal system in removing arsenic to meet the new arsenic maximum contaminant level of 10 microgram per liter (µg/L), 2) the reliability of the treatment system, 3) the required system operation and maintenance (O&M) and operator's skill, and 4) the capital and O&M costs of the technology. The project also characterizes water in the distribution system and residuals produced by the treatment process.
The ATS system consisted of two parallel treatment trains, each having three 10-inch-diameter, 54-inch-tall, sealed polyglass columns connected in series to treat up to 11 gallons per minute (gpm) of water. Water supplied from three source water wells was chlorinated to provide chlorine residuals and then passed through a 25-micrometer sediment filter and the three adsorption columns in each train. Each adsorption column was loaded with 1.5 cubic feet (ft3) of A/I Complex 2000 adsorptive media, which consisted of an activated alumina substrate and a proprietary iron complex. Based on the design flow rate of 11 gpm, the empty bed contact time (EBCT) in each column was 1 minute and the hydraulic loading rate to each column was 20.4 gallons per minute per square foot. The actual flow rate was much lower, averaging only 4.4 and 3.2 gpm for Trains A and B, respectively, for the first 23 weeks from June 27 through November 26, 2005, and 1.4 and 1.9 gpm for the remainder of the reporting period. As a result, each adsorption column had a much longer EBCT, ranging from 1.8 to 34 minutes throughout the entire study period. The highly variable and slow flow rates from the wells might be attributed, in part, to slow recovery rates of the aquifer resulting from a dry summer.
Between June 22, 2005, and December 22, 2005, the system operated an average of 5.9 hours per day for a total of 1,100 hours, treating approximately 302,000 gallons of raw water containing 30.4 to 72.2 µg/L of arsenic existing predominately as arsenic (V). Arsenic concentrations after the lead columns reached 10 µg/L at approximately 5,400 bed volumes (BV) from Train A and 5,000 BV from Train B. (Note that BV was calculated based on 1.5 ft3 [or 11.2 gallons] of media in each column.) Arsenic existing mostly as arsenic (V) approached complete breakthrough (concentration equal to those in the influent) following the lead columns at approximately 12,000 BV. Arsenic breakthrough from the lead columns occurred sooner than projected (at 40,000 BV) by the vendor. It is presumed that relatively high pH values of source water (averaging 7.8), competing anions, such as silica, and higher influent arsenic concentrations (i.e., 45.1 µg/L, on average, compared to 30 µg/L observed during the initial site visit) might have contributed to early arsenic breakthrough from the adsorption columns.
Aluminum concentrations (existing primarily in the soluble form) in the treated water following adsorption columns were approximately 10 to 30 µg/L higher than those in raw water, indicating leaching of aluminum from the adsorptive media. Leaching of aluminum continued throughout the study period; however, there was a decreasing trend in aluminum in the treated water during the six months of evaluation.
Comparison of distribution system sampling results before and after operation of the system showed a significant decrease in arsenic concentrations at two of the three residences during the first six months of system operation. One residence had arsenic concentrations ranging from 16.3 to 26.0 µg/L through the first three months of system operation. Starting from the fourth month, all three residences had arsenic concentrations below 3.1 µg/L. Lead and copper levels did not appear to have been impacted by the treatment system.
The capital investment cost of $14,000 included $8,990 for equipment, $2,400 for site engineering, and $2,610 for installation. Using the system's rated capacity of 22 gpm (or 31,680 gallons per day [gpd]), the capital cost was $636 per gpm (or $0.44 per gpd).
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