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Arsenic Removal from Drinking Water by Adsorptive Media U.S. EPA Demonstration Project at Valley Vista, AZ Six-Month Evaluation Report (61 pp, 1.6 MB) September 2006
This report documents the activities performed and the results obtained from the first six months of the EPA arsenic removal technology demonstration project at the Arizona Water Company (AWC) facility in Sedona, AZ, commonly referred to as Valley Vista. The main objective of the project is to evaluate the effectiveness of Kinetico's FA-236-AS treatment system using AAFS50 media in order to remove arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 µg/L. Additionally, this project evaluates the reliability of the treatment system for use at small water facilities, the required system operation and maintenance (O&M) and operator skill levels, and the cost-effectiveness of the technology. The project also characterizes water in the distribution system and residuals generated 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 costs.
The FA-236-AS system consists of two 36-in-diameter, 72-in-tall fiberglass tanks in series (lead/lag), each containing 22 ft3 of AAFS50 media. The media is an iron-modified activated alumina (AA) medium manufactured by Alcan. The system was designed to treat 37 gal/min (gpm) of flow with an empty bed contact time (EBCT) of 4.5 min/tank and 9.0 min for both tanks. For the first of two media runs performed during the first six months of system operation, due in part to the use of an incorrect media density, the vendor inadvertently loaded 16.7 ft3 (i.e., 1,100 lb) of AAFS50 media into each tank, resulting in a shorter EBCT of 3.4 min/tank.
After extensive engineering plan review and approval by the state and county drinking water officials, the treatment system was installed in May 2004 and became operational on June 24, 2004. During the first six months, the treatment system operated for 24 hr/day with less than 1% downtime for repairs and media replacement. The source water contained 34.8 to 47.6 µg/L of total arsenic, with As(V) being the predominating species, averaging 41.8 µg/L. Prechlorination, although not required for oxidation, was performed after a month into the study to inhibit biological growth in the adsorption tanks and to provide residual chlorine in the distribution system.
The raw water pH values, ranging from 7.5 to 8.4 and averaging 7.8, were not adjusted during the first media run. After treating approximately 8,200 and 16,900 bed volumes (BV) of water, the effluent from the lead and lag tanks exceeded the 10-µg/L arsenic breakthrough limit on July 14 and August 4, 2004, respectively. (Note that BV was calculated based on 16.7 ft3 [125 gal] of media in the lead tank.) Based on the breakthrough curves, the arsenic adsorptive capacity of the media without pH adjustment was estimated to be 0.31 mg/g of media at 10-µg/L arsenic breakthrough and 0.6 mg/g of media near exhaustion. An effort to extend the media life by lowering the pH value to 6.8 using H2SO4, beginning on September 17, 2004, reduced the arsenic concentrations after both tanks (i.e., 33 to 24 µg/L and 26 to 16 µg/L in the lead and lag tanks, respectively), but not to the desired level of 10 µg/L. Therefore, the spent media in both tanks was replaced on October 25, 2004, and disposed of as non-hazardous waste after passing the Toxicity Characteristic Leaching Procedure (TCLP) tests.
For the second media run, the raw water pH was adjusted to 6.7 to 6.9. As of December 15, 2004, the new AAFS50 media had treated approximately 2,635,000 gal, or 16,000 BV of water, leaving 4.3 µg/L and 0.1 µg/L of total arsenic in the effluent from the lead and lag tanks, respectively. Therefore, pH adjustment significantly increased the media's arsenic adsorptive capacity. Concentrations of iron, manganese, silica, orthophosphate, 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 the commencement of the system operation showed a decrease in arsenic concentration, most prominently at one sampling location close to the treatment plant (from 34.9 to 51.8 µg/L to 0.3 to 23.9 µg/L). Arsenic concentrations at the other two locations were much higher than those of the treatment effluent presumably due to the blending of untreated water from other wells supplying the distribution system. The lead and copper concentrations at the three sampling locations did not show a clear pattern of the arsenic treatment system's impact.
Backwash of the filter media was performed monthly based on a set throughput of 1,400,000 gal using treated water at 27 to 36 gpm, or 3.8 to 5.1 gpm/ft2. No significant pressure buildup was observed during service runs. Each backwash lasted for 40 min (20 min/tank), producing between 1,060 and 1,400 gal of water. Average soluble arsenic concentrations in the backwash water from the lead and lag treatment tanks were 31.9 and 15.2 µg/L, respectively. A backwash recycle loop enabled the system to reclaim nearly 100% of the wastewater produced by blending it with the feed water at a maximum rate of 3.6 gpm. The capital investment cost of the system was $228,309 consisting of $122,544 for equipment, $50,659 for site engineering, and $55,106 for installation. Using the system's rated capacity of 37 gpm (or 53,280 gal/day [gpd]), the capital cost was $6,171/gpm (or $4.29/gpd). This calculation does not include the cost of the sun shed enclosure which houses the treatment system.
The O&M cost for the treatment system included only incremental cost associated with the FA-236-AS system, such as media replacement and disposal, chemical supply, electricity consumption, and labor. Representing the majority of the O&M cost, the media replacement and disposal cost depended on the number of tanks to be changed out when the arsenic breakthrough following the lag tank reached 10 µg/L. Without pH adjustment, it might be more convenient and cost-effective to replace the media in both tanks together to reduce the changeout frequency and minimize the associated scheduling and coordinating effort. The cost for replacing media in both tanks was estimated at $6,623 (for 33.4 ft3, the total amount of media used during the first media run) or $3.15/1,000 gal of water treated. With pH adjustment, the media run length was significantly increased so that only the media in the lead tank might be replaced at an estimated cost of $4,363 for 22 ft3 of media. Adjustment of pH lowered the media replacement cost, but added a chemical cost of $0.66/1,000 gal of water treated. The total O&M cost and media replacement cost per 1,000 gal of water treated were plotted as a function of the media run length.