Arsenic Removal from Drinking Water by Adsorptive Media U.S. EPA Demonstration Project at Richmond Elementary School in Susanville, CA Six-Month Evaluation Report
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This report documents the activities performed during and the results obtained from the first six months of the performance evaluation of the arsenic removal treatment technology at Richmond Elementary School in Susanville, California. The objectives of the project are to evaluate 1) the effectiveness of an Aquatic Treatment Systems, Inc. (ATS) arsenic removal system in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 µg/L, 2) the reliability of the treatment system, 3) the system operation and maintenance (O&M) and operator skill requirements, 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 process.
The ATS system consisted of one 25-µm sediment filter, two 10-in diameter, 54-in tall oxidation columns, and three 10-in diameter, 54-in tall adsorption columns connected in series. The columns were constructed of sealed polyglass and 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). 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.
Between September 7, 2005, and March 9, 2006, the As/1200CS system operated an average of 1.7 hr/day for a total of 207 hr, treating approximately 101,000 gal of water. This volume throughput was equivalent to 9,000 bed volumes (BV) based on 1.5 ft3 of media in the lead adsorption column or 3,000 BV based on 4.5 ft3 of media in three adsorption columns. The average system flowrate was 9.0 gpm, which yielded an average EBCT of 1.2 min in one adsorption column or 3.6 min in three adsorption columns.
The oxidizing media was effective at converting As(III), the predominant arsenic species, to As(V) throughout the six month period, typically lowering the As(III) concentrations from 16.7 ± 9.2 µg/L to <0.5 µg/L. Oxidation of As(III) was achieved, presumably, through a reaction with sodium metaperiodate, resulting in I- in the column effluent. Analyses of the column effluent indicated elevated iodine concentrations, which averaged 86.1 µg/L (as I) following the oxidation columns and 112 µg/L (as I) following the adsorption columns (compared to 11 µg/L [as I], on average, in raw water). Iodine measured in the column effluent most likely was leached from the oxidation columns as IO4- or other reaction intermediates. The oxidizing media also showed a significant adsorptive capacity for arsenic (i.e., 0.20 µg/mg of media), effectively removing it to <10 µg/L when processing the first 4,800 BV of water through the lead oxidation column. Arsenic concentrations after the lead oxidation column reached the influent levels after approximately 7,500 BV, based on the 1.5-ft3 media bed in the column. After 9,000 BV or six months of system operation, the arsenic concentration after the second oxidation column was 10.7 µg/L, which was still below the influent concentrations of about 31 µg/L.
Arsenic concentrations remained below 0.2 µg/L in the effluent of the lead adsorption column during the first six months of operation. This is because the oxidation columns had removed the majority of arsenic from source water before it reached the adsorption columns.
Aluminum concentrations (existing primarily in the soluble form) in the treated water following adsorption columns were about 14 to 35 µg/L higher than those in raw water, indicating leaching of aluminum from the oxidizing and/or adsorptive media. Even with the increase in aluminum concentrations following the treatment system, the concentrations were below the secondary drinking water standard for aluminum of 50 to 200 µg/L. Leaching of aluminum continued throughout the six-month study period.
Comparison of distribution system sampling results before and after the operation of the As/1200CS system showed a significant decrease in arsenic concentration at the three sampling locations during the first six months of system operation.
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). Annualized capital cost was $1,598/yr based upon 7% interest rate and 20 year life. 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, the unit capital cost increased to $7.91/1,000 gal.
The O<M cost included only incremental cost associated with the adsorption system, such as media replacement and disposal (for both oxidizing and adsorptive media), electricity consumption, and labor. Incremental cost for electricity consumption was negligible. Although media replacement and disposal did not take place during the first six months of operation, the estimated cost was $2,755, $3,850, and $4,945 for replacing one, two, or three columns, respectively (including replacement media, spent media disposal, shipping, labor and travel). Cost curves were constructed for replacing one, two, or three columns to estimate media replacement cost per 1,000 gal of water treated as a function of the media working capacity.
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