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EPA/600/R-07/049


Arsenic Removal from Drinking Water by Adsorptive Media
U.S. EPA Demonstration Project at Webb Consolidated Independent School District in Bruni, TX
Six-Month Evaluation Report

(PDF, 2024 Kb, 63 pp)
June 2007

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Abstract:

This report documents the activities performed and the results obtained from the first six months of the arsenic removal treatment technology demonstration project at the Webb Consolidated Independent School District (Webb CISD) site at Bruni, TX. The main objective of the project is 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]-50LL-CS-S-2-AVH), 2) the required system operation and maintenance (O&M) and operator skills, and 3) the capital and O&M cost of the technology. The project also characterizes the water in the distribution system and residuals produced by the treatment process. The types of data collected include system operation, water quality (both across the treatment train and in the distribution system), process residuals, and capital and O&M cost.

The APU-50LL-CS-S-2-AVH treatment system consisted of two 42-in-diameter, 72-in-tall carbon steel vessels in series configuration, each containing approximately 22 ft3 of AD-33 pelletized media, which is an iron-based adsorptive media developed by Bayer AG and marketed under the name of AD-33 by AdEdge. The treatment system was designed for a peak flowrate of 40 gal/min (gpm) and an empty bed contact time (EBCT) of approximately 4.1 min per vessel. The actual average flowrate for the six-month operational period was 44 gpm, based on readings of the hour meter interlocked to the well pump and the electromagnetic flow meter/totalizer installed on each adsorption vessel.

As part of the water treatment system, a pH adjustment/control system was used to adjust the pH value of raw water from as high as 8.2 to a target value of 7.0. A prechlorination system also was used to oxidize As(III) to As(V) and maintain a target chlorine residual level of 1.2 mg/L (as Cl2) in the distribution system. The pH adjustment/control system consisted of a carbon dioxide (CO2) supply assembly, an automatic pH control panel, a CO2 membrane module (that injected CO2 into a CO2 loop), and an in-line pH probe. The prechlorination system, which was upgraded from the preexisting system, included a chemical feed pump, a sodium hypochlorite (NaOCl) feed tank, and an inject port located downstream of the CO2 loop and in-line pH probe.

The AdEdge treatment system began regular operation on December 8, 2005. The data collected included system operation, water quality (both across the treatment train and in the distribution system), process residuals, and capital and O&M cost. Between December 8, 2005, and June 9, 2006, the system operated an average of 4.3 hr/day, treating approximately 2,070,000 gal of water. This volume throughput was equivalent to 12,600 bed volumes (BV) based on the 22 ft3 of media in one adsorption vessel or 6,300 BV based on the 44 ft3 of media in the two adsorption vessels in series.

Since system start-up, the APU system has experienced component failures associated with the pH control system and flow meters/totalizers. Leaks were detected in the CO2 supply line; the proportional flow control valve malfunctioned; and the in-line pH probe failed. There were periods when the pH control system was switched from automatic to manual mode until replacement of certain system components were performed to address the problems encountered. In addition to the pH control system problems, errors were encountered with the system flow meters/totalizers. On two occasions, the system totalizers reset and began totalizing from zero, likely caused by a programming error. As of the end of the first six months of the evaluation period, the issues with the pH control system appeared to have been resolved and programming updates are being prepared to prevent future totalizer errors.

Total arsenic concentrations in raw water ranged from 46.2 to 62.9 g/L. As(III) was the predominating species, ranging from 35.8 to 40.8 g/L. Chlorination effectively oxidized As(III) to As(V), reducing As(III) concentrations to an average value of 1.7 g/L. As of June 6, 2006, the total arsenic level in the treated water following the lead adsorption vessel was 1.1 g/L at approximately 12,100 BV. The arsenic level from the lag vessel at the time was 0.8 g/L. Concentrations of phosphorus and silica, which could interfere with arsenic adsorption by competing with arsenate for adsorption sites, ranged from <0.01 to 0.03 mg/L (as PO4) and from 40.6 to 43.9 mg/L (as SiO2), respectively, in raw water. Concentrations of iron, manganese, and other ions in raw water were not high enough to impact arsenic removal by the media.

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 68.7 g/L to an average of 2.4 g/L). The arsenic concentrations in the distribution system were similar to those in the system effluent. Lead and copper concentrations did not appear to have been affected by the operation of the treatment system.

The capital investment cost of $138,642 included $94,662 for equipment, $24,300 for site engineering, and $19,680 for installation. Using the system's rated capacity of 40 gpm (or 57,600 gal/day [gpd]), the capital cost was $3,466/gpm (or $2.41/gpd) of design capacity. The capital cost also was converted to an annualized cost of $13,086/yr using a capital recovery factor (CRF) of 0.09439 based on a 7% interest rate and a 20-year return period. Assumed that the system operated 24 hours a day, 7 days a week at the system design flowrate of 40 gpm to produce 21,024,000 gal of water per year, the unit capital cost would be $0.62/1,000 gal. Because the system operated an average of 4.3 hr/day at 44 gpm, producing 2,070,000 gal of water during the six-month period, the unit capital cost increased to $3.16/1,000 gal at this reduced rate of use.

The O&M cost included only the cost associated with the adsorption system, such as media replacement and disposal, CO2 and chlorine usage, electricity consumption, and labor. Although media replacement did not occur during the first six months of system operation, the media replacement cost would represent the majority of the O&M cost and was estimated to be $11,190 to change out one vessel (including 22 ft3 AD-33 media and associated labor for media change out and disposal). This cost was used to estimate the media replacement cost per 1,000 gal of water treated as a function of the projected media run length to the 10 g/L arsenic breakthrough.

For more information on this {and similar} research, please visit our research web site.

Contact:

Thomas Sorg
sorg.thomas@epa.gov

Office of Research & Development | National Risk Management Research Laboratory


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