|Arsenic Removal from Drinking Water by Adsorptive Media U.S. EPA Demonstration Project at Richmond Elementary School in Susanville, CA
This report documents the activities performed and the results obtained for the arsenic removal treatment technology demonstration project at Richmond Elementary School in Susanville, CA. The objectives of the project were to evaluate:
The project also characterizes water in the distribution system and residuals produced by the treatment process.
The ATS system consisted of three Well-X-TROL pressure tanks; one 25-μm sediment filter; two 10-in diameter, 54-in tall oxidation columns; three 10-in diameter, 54-in tall adsorption columns; and one pressure tank/booster pump assembly before entering the distribution system. Constructed of sealed polyglass, the columns were loaded with 1.5 ft3 each of either A/P Complex 2002 oxidizing media (consisting of activated alumina and sodium metaperiodate) or A/I Complex 2000 adsorptive media (consisting of activated alumina and a proprietary iron complex) for series operations. Based on the design flowrate of 12 gal/min (gpm), the empty bed contact time (EBCT) in each column was 0.9 min (or 2.8 min for three adsorption columns in series) and the hydraulic loading rate to each column was 22 gpm/ft2. Because the actual flowrate through the system was slightly lower at 9.3 gpm (on average), the actual EBCT was slightly longer at 1.2 min and the actual hydraulic loading rate was slightly lower at 17.2 gpm/ft2.
Between September 7, 2005, and June 13, 2007, the treatment system operated for an average of 1.1 hr/day for a total of 442 hr, treating approximately 303,000 gal of water containing 25.1 to 35.4 μg/L of arsenic. Arsenic in raw water existed as both soluble As(V) and soluble As(III), with As(III) concentrations remaining below 47% of the soluble arsenic throughout most of the study period (except for the first two months). Oxidation of As(III) was achieved through reactions with sodium metaperiodate (IO4-) within the oxidation columns, producing As(V) and I- as end products. The oxidation columns remained effective for As(III) oxidation throughout the study period, reducing As(III) concentrations to less than 2.7, 1.2, and 1.0 μg/L following the first and second oxidation columns and the third adsorption column, respectively. As much as 264 μg/L of IO4- (as I) had leached from the oxidation and adsorption columns, but the leaching followed an apparent decreasing trend.
The oxidizing media showed a significant adsorptive capacity for arsenic (i.e., 0.18 to 0.20 μg of As/mg of dry media), effectively reducing arsenic concentrations to <10 μg/L after processing 51,600 gal of water through the lead oxidation column (or 4,600 bed volumes [BV; 1 BV = 1.5 ft3 = 11.22 gal]). Complete arsenic breakthrough from the lead and lag oxidation columns occurred after processing 79,700 and 193,000 gal of water, respectively, which correspond to 7,100 BV (1 BV = 11.22 gal) through the lead column and 8,600 BV (1 BV = 22.44 gal) through the lead and lag columns.
Arsenic breakthrough of 10 μg/L following the lead and first lag adsorption columns occurred after processing approximately 184,000 and 221,000 gal of water. Complete arsenic breakthrough for the lead adsorption column took place after processing approximately 227,800 gal of water. The arsenic loading on the lead adsorption column was 0.23 μg of As/mg of dry media, which was very close to that on the oxidation columns as mentioned above. These adsorptive capacities were very close to those observed at another EPA arsenic demonstration site in Wales, ME, where a similar ATS system was used for arsenic removal.
The lead and the first lag adsorption columns with spent adsorptive media were replaced after approximately 18 months of operation. Before changeout, the total arsenic concentration in the system effluent was 8.4 μg/L, less than the 10 μg/L MCL. The spent media in both vessels passed the Toxicity Characteristic Leaching Procedure (TCLP) test and could be disposed off at a sanitary landfill. However, the vendor recycled the spent media into another product, thus saving the disposal cost.
Comparison of distribution system water sampling results before and after system startup showed a significant decrease in arsenic concentration at the three sampling locations during the 12 monthly sampling events. Arsenic concentrations were reduced from an average baseline level of 30.6 to 1.5 μg/L, which, although low, were still higher than the concentrations (≤0.2 μg/L) measured at the distribution entry point. Therefore, some dissolution and/or resuspension of arsenic might have occurred in the distribution system. Lead and copper values also were low and did not appear to have been affected by the treatment system.
The capital investment cost of $16,930 included $8,640 for equipment, $3,400 for site engineering, and $4,890 for installation. Using the system’s rated capacity of 12 gpm (or 17,280 gal per day [gpd]), the capital cost was $1,410/gpm (or $0.98/gpd). The annualized capital cost was $1,598/yr based upon a 7% interest rate and a 20-year return. The unit capital cost was $0.25/1,000 gal assuming the system operated continuously at 24 hr/day, 7 day/wk at 12 gpm. At the current usage rate of 180,520 gal per year, the unit capital cost increased to $8.90/1,000 gal.
The O&M cost included only incremental cost associated with the adsorption system, such as media replacement and disposal, electricity consumption, and labor. The incremental cost for electricity consumption was negligible. The cost to replace the lead and first lag adsorption columns was $2,310. Labor and travel would add approximately $1,660 to the total cost. This cost was used to estimate the O&M cost per 1,000 gal of water treated as a function of the media run length to the 10-μg/L arsenic in the system effluent.
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