|Arsenic Removal From Drinking Water by Adsorptive Media, U.S. EPA Demonstration Project at Desert Sands MDWCA, NM, Six-Month Evaluation Report (PDF) (63 pp, 4.91 MB) (EPA/600/R-05/079) September 2005
This report describes activities and results of the first six months of the arsenic removal treatment technology demonstration project at the Desert Sands Mutual Domestic Water Consumers Association facility in Anthony, New Mexico. The objectives of the project are to evaluate the:
The project is also characterizing the water in the distribution system and residuals produced by the treatment system 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 costs.
The STS treatment system became operational on January 16, 2004. After treating approximately 14,647,000 gallons (12,200 bed volumes) of water, which was approximately 9 percent of the vendor-estimated working capacity for the adsorptive media, total arsenic concentrations were reduced from 20.7–30.1 µg/L in raw water to 2.8 µg/L in the treated water.
Arsenic (III) was the predominating species in raw water, averaging 21.1 µg/L. Prechlorination was effective in oxidizing arsenic (III) to arsenic (V), as evidenced by the low arsenic (III) concentrations (0.5 to 1.1 µg/L) in water sampled immediately after prechlorination. Total and free chlorine residuals measured before and after the adsorption vessels were nearly identical at 0.3–0.5 milligrams per liter (mg/L) (as hydrogen chloride) and 0.4–0.6 mg/L (as hydrogen chloride), respectively, indicating little or no chlorine consumption by the SORB 33TM media. Concentrations of iron, manganese, silica, orthophosphate, and other ions in raw water were not high enough to affect arsenic removal by the media.
Comparison of the distribution system sampling results before and after the operation of the STS system showed a decrease in arsenic concentration (from 22.4–28.2 µg/L to 1.8–10.4 µg/L) at all three sampling locations. However, the concentrations measured after system operation were higher than those in the plant effluent. This was likely due to the blending with untreated water produced by a separate well in the distribution system. Neither lead nor copper concentrations at the sample sites appeared to have been affected by the operation of the system.
Two sets of backwash water samples were collected during the first six months of system operation. Dissolved arsenic concentrations in the backwash water ranged from 3.5 to 12.1 µg/L, which were significantly lower than those measured in raw water, indicating removal of arsenic by the media during backwash. Dissolved iron and manganese concentrations in backwash water correlated more closely with the influent concentrations.
The STS system experienced excessive flow restriction, imbalanced flow, and elevated pressure differential across the adsorption vessels and the entire system during the first four months of system operation. After extensive on-site and off-site investigations and hydraulic testing, the system was retrofitted in May 2004 and, thus, was able to operate according to the original design specifications. After the retrofit, the only O&M issue encountered was the temporary failure of the digital flow meters on the vessels on two separate occasions for one to two days at a time.
The capital investment cost of $153,000 included $112,000 for equipment, $23,000 for site engineering, and $18,000 for installation. Using the system's rated capacity of 320 gallons per minute, the capital cost was $476 per gallon of design capacity and the equipment-only cost was $350 per gallon of design capacity. These calculations do not include the cost of building an addition to house the treatment system.
O&M costs included only incremental costs associated with the APU-300 system, such as media replacement and disposal, chemical supply, electricity, and labor. Because the incremental costs for chemical supply and electricity were negligible, only media replacement and disposal and O&M labor would affect the O&M costs. O&M costs for media replacement were estimated based on media replacement cost and projected breakthrough, and will be determined once the actual throughput and cost at the time of the media replacement become available.
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