Arsenic Removal from Drinking Water by Adsorptive Media EPA Demonstration Project at Rimrock, AZ - Six-Month Evaluation Report December 2005
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This report documents the activities performed and the results obtained from the first six months of the arsenic removal treatment technology demonstration project in Rimrock, AZ. The objectives of the project are to evaluate: (1) the effectiveness of the AdEdge Arsenic Package Unit-100 (APU-100) AD-33T media system in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 µg/L, (2) the reliability of the treatment system, (3) the simplicity of required system operation, (4) the maintenance (O&M) and operator's skill levels, and (5) the cost-effectiveness of the technology. The project also is characterizing water in the distribution system and process 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 APU-100 treatment system consisted of two 36-inch-diameter, 72-inch-tall fiberglass-reinforced-plastic (FRP) vessels, each containing 22 ft3 of AD-33T media. The media is Bayoxide E33 iron-based adsorption media developed by Bayer AG and branded under the name of AD-33T by AdEdge. The system was originally designed to treat 90 gpm of water supplied by two production wells. Due to the loss of one well, the treatment flowrate was reduced by more than half, which prompted a change in system configuration from parallel to series (lead/lag). Following the conversion to series configuration in the field, the APU-100 system with a design capacity of 45 gpm began regular operation on June 24, 2004. The average flowrate through each vessel was 31.5 gpm, corresponding to an average empty bed contact time (EBCT) of 5.2 minutes per vessel and 10.4 minutes for both vessels.
Through the period June 24, 2004 through December 22, 2004, the APU-100 system operated for 12 hours a day on a timer for a total of 2,172 hours. The system treated approximately 4,109,000 gallons of water, or 25,000 bed volumes (BV), which was approximately 38% of the vendor-estimated working capacity for adsorptive media. Arsenic breakthrough from the lead and lag vessels was 3 µg/L and 1.3 µg/L, respectively. Total arsenic concentration in raw water ranged from 48.3 to 81.4 µg/L with As(V) being the predominating species, averaging 57.3 µg/L. Prechlorination, although not required for oxidation, was performed for disinfection. The residual chlorine measured before and after the treatment vessels was comparable, indicating little or no chlorine consumption by the AD-33T media. Concentrations of iron, manganese, silica, orthophosphate, and other ions in raw water were not high enough to impact arsenic removal by the media.
Backwash was performed monthly since August 2004 with raw water at approximately 50 gpm, or 7 gpm/ft2. Each vessel was backwashed for 15 minutes, producing between 631 to 910 gallons of water. Two sets of backwash water samples were collected during the first six months of system operation. Arsenic concentrations in the backwash water from the lead and lag vessels were approximately 48.0 µg/L and <3.0 µg/L, respectively, indicating that the lead vessel had less capacity to remove arsenic, and that the lag vessel was still very effective at removing arsenic during backwash. A backwash recycle loop enabled the system to reclaim nearly 100% of the wastewater produced by blending it with the chlorinated water at a rate of 0.5 gpm.
Comparison of the distribution system sampling results before and after operation of the APU-100 system began showed a decrease in the average arsenic concentration (from 44.6-55.2 µg/L to 18.8-21.8 µg/L) at each of the three sampling locations. However, the concentrations measured after system operation began were higher than those at the plant effluent. This was probably caused by the blending of treated water by the APU-100 system with untreated water from other wells in the distribution system. Neither lead nor copper concentrations appeared to have been affected by the operation of the system.
The capital investment cost of $90,757 includes $66,235 for equipment, $11,372 for site engineering, and $13,150 for installation. Using the system's rated capacity of 45 gpm (or 64,800 gpd), the capital cost was $2,017/gpm (or $1.40/gpd) and the equipment-only cost was $1,472/gpm (or $1.02/gpd). These calculations did not include the cost of the building construction.
O&M costs included only incremental costs associated with the APU-100 system, such as media replacement and disposal, chemical supply, electricity, and labor. Although not incurred during the first six months of operation, the media replacement cost would represent the majority of the O&M cost, and was estimated to be $9,940 per vessel. This cost was used to estimate the media replacement cost per 1,000 gallons of water treated as a function of the projected media run length to the 10-µg/L arsenic breakthrough. O&M costs will be refined once the actual throughput and cost at the time of the media replacement become available.