Arsenic Removal from Drinking Water by Adsorptive Media U.S. EPA Demonstration Project at Webb Consolidated Independent School District in Bruni, Texas: Final Performance Evaluation Report (EPA/600/R-10/040) April 2010
This report documents the activities performed and the results obtained from the arsenic removal treatment technology demonstration project at the Webb Consolidated Independent School District (Webb CISD) in Bruni, TX. The main objective of the project was to evaluate the effectiveness of AdEdge Technologies’ AD-33 media in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 µg/L. Additionally, this project evaluated 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 characterized 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 treatment system consisted of two 42-in × 72-in 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 Technologies. 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. Over the performance evaluation period, the actual average flowrate was estimated at 40 gpm (although with quite a bit of fluctuation), based on readings of an 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 pH values of raw water from as high as 8.3 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 inline pH probe. The prechlorination system, which was upgraded from the pre-existing system, included a chemical feed pump, a sodium hypochlorite (NaOCl) feed tank, and an inject port located downstream of the CO2 loop and inline pH probe.
The 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 29, 2007, the treatment system treated 5,658,728 gal of water. Since then, a system operator was not available and therefore system measurements were sporadic from June 30, 2007 through the end of the system performance evaluation on May 15, 2008. Based on an average daily operating time of 4.2 hr/day and total number of operational days (i.e., 889 days), the total amount of water treated was estimated at 8,841,000 gal. This estimated volume throughput was equivalent to 27,000 bed volumes (BV) based on the 44 ft3 of media in both lead and lag vessels.
Since system startup, the treatment 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 inline 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, 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. In the first few months of the performance evaluation study, the issues with the pH control system were resolved and programming updates were prepared to prevent future totalizer errors. On June 29, 2007, the licensed system operator working for Webb CISD resigned, which impeded the data collection efforts for the remainder of the system performance evaluation. Operational and water quality data provided by Webb CISD after June 29, 2007, were collected by a temporary operator who was not formally trained on operating the system.
Total arsenic concentrations in raw water ranged from 46.0 to 68.7 µg/L. Soluble As(III) was the predominating species, ranging from 31.3 to 42.0 µg/L. Chlorine effectively oxidized soluble As(III) to soluble As(V), reducing soluble As(III) concentrations to an average value of 1.2 µg/L. At the end of the performance evaluation study on May 15, 2008, total arsenic levels in the treated water were 45.4 and 5.2 µg/L following the lead and lag adsorption vessels, respectively. Concentrations of phosphorus and silica, which could interfere with arsenic adsorption by competing with arsenate for adsorption sites, ranged from ‹10.0 to 13.7 mg/L (as P) and from 39.1 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 µg/L to an average of 2.4 µg/L). The arsenic concentrations in the distribution system were similar to those in the system effluent. Lead and copper concentrations appeared to have been affected to some extent by the operation of the treatment system. However, the effects were not trendy, with the lead concentrations becoming mostly lower and the copper concentrations becoming mostly higher after system startup.
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. Assuming 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 4.2 hr/day at 40 gpm, producing an estimated 3,679,200 gal of water annually, the unit capital cost increased to $3.56/1,000 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 system performance evaluation, the media replacement cost would have represented 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 changeout and disposal).
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