|Arsenic Removal From Drinking Water by Adsorptive Media, U.S. EPA Demonstration Project at Nambe Pueblo, New Mexico, Final Performance Evaluation Report (EPA/600/R-10/164) December 2010
This report documents the activities performed and the results obtained from the arsenic removal treatment technology demonstration project at the Nambe Pueblo, New Mexico. 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 micrograms per liter (μg/L). In addition, this project evaluated the:
The project also characterized the water in the distribution system and residuals produced by the treatment process. The types of data collected included system operation, water quality (both across the treatment train and in the distribution system), process residuals, and capital and O&M costs.
The treatment system consisted of two 48-inch by 72-inch fiber-reinforced plastic vessels in parallel configuration, each containing 35.6 cubic feet (ft3) of AD-33 media. Delivered in granules, AD-33 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 160 gallons per minute (gpm) (80 gpm per vessel) and an empty bed contact time (EBCT) of approximately 3.3 minutes. Over the performance evaluation period, the actual average flowrate was at 114 gpm, corresponding to an EBCT of 4.7 minutes.
The treatment system began regular operation on May 15, 2007. From May 15, 2007, through the end of the performance evaluation study on September 28, 2009, the treatment system operated for a total of 10,134 hours, treating approximately 64,580,000 gallons (or 121,390 bed volumes [BV]) of water. The average daily operation time was 12.3 hours per day and the average daily demand was 78,360 gallons per day (gpd).
As part of the water treatment system, a pH control system was used to adjust pH values of source water from as high as 9.1 to a target value of 7.0. The pH adjustment 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. During the performance evaluation study, the treatment system experienced periodic losses of pH control due to lack of a constant CO2 supply. Real-time pH values monitored and recorded after pH adjustments by an in-line pH meter/datalogger cycled between 7 and 8 and over 9.
Total arsenic concentrations in source water ranged from 10.7 to 59.0 μg/L and averaged 32.2 μg/L with soluble arsenic (V) as the predominating species, ranging from 34.2 to 36.5 μg/L, based on the results of two speciation sampling events. Total uranium concentrations in source water ranged from 19.9 to 55.8 μg/L and averaged 39.3 μg/L. Except for some occasions, total arsenic and uranium concentrations were removed to below 3 and 20 μg/L, respectively, in system effluent throughout the 28-month study period. Significantly, elevated arsenic and uranium concentrations (often higher than the respective source water concentrations) were measured during a number of sampling events, which coincided with the time periods when the system was operating without pH control.
Periodic losses of pH control apparently had caused the media beds to operate under constant adsorption/desorption cycles, with the captured arsenic and uranium intermittently “flushed” out of the media beds. Therefore, the AD-33 media was not exhausted as expected, even after treating 121,390 BV of water (twice the projected working capacity estimated by the vendor). Analyses of media samples collected at 78,200 BV revealed that the adsorptive media were loaded only minimally with arsenic and uranium (i.e., 0.38 percent and 3.2 percent of the respective mass in 78,200 BV of source water), which supported the speculation that adsorbed arsenic and uranium were intermittently “flushed” out of the media beds.
Comparison of the distribution system sampling results before and after system startup showed a significant decrease in arsenic concentration (from an average of 33.7 to less than 10 μg/L), except for three occasions when the treatment system had lost pH control. Uranium concentrations in distribution water also were reduced to below the MCL of 30 μg/L, except for four occasions. Lead and copper concentrations did not appear to have been affected by the operation of the treatment system.
The capital investment cost of $143,113 included $116,645 for equipment, $11,638 for site engineering, and $14,830 for installation. Using the system’s rated capacity of 160 gpm (or 230,400 gpd), the capital cost was $894 per gpm (or $0.62 per gpd) of design capacity. The unit capital cost would be $0.16 per 1,000 gallons if the 160 gpm system were operating around the clock. Based on the average daily operating times (12.3 hours per day) and average system flowrate (114 gpm), the unit capital cost increased to $0.44 per 1,000 gallons 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 use, electricity consumption, and labor. Although media replacement did not take place during the performance evaluation study, the media replacement cost would have represented the majority of the O&M cost and was estimated to be $29,532 to change out both vessels (71.2 ft3 AD-33 media and associated labor for media change-out and disposal).
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