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Arsenic Removal from Drinking Water by Adsorptive Media
EPA Demonstration Project at Goffstown, NH
Six-Month Evaluation Report (EPA/600/R-06/125) November 2006
This report documents the activities performed and the results obtained from the first six months of the arsenic removal treatment technology demonstration project at the Orchard Highlands Subdivision site at Goffstown, NH. The objectives of the project are 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 evaluates the reliability of the treatment system (Arsenic Package Unit [APU]-GOFF-LL), the required system operation and maintenance (O&M) and operator's skills, and the capital and O&M cost of the technology. The project also characterizes the water in the distribution system and process residuals produced by the treatment process.
The APU-GOFF-LL treatment system consists of two 18-in.-diameter, 65-in.-tall fiberglass reinforced plastic (FRP) vessels in series configuration, each containing approximately 5 ft3 of AD-33 media. The media is an iron-based adsorptive media developed by Bayer AG and marketed under the name of AD-33 by AdEdge. The system was designed for a peak flowrate of 10 gal/min (gpm) based on the pump curve provided by the site. The system design had an empty bed contact time (EBCT) of about 3.7 min per vessel based on the 10 gpm flowrate. The actual average flowrate of 13 gpm was 30% higher than the design flowrate. The higher flowrate decreased the EBCT from 3.7 to 2.9 min, which might have contributed, in part, to earlier than expected breakthrough of arsenic.
The AdEdge treatment system began regular operation on April 15, 2005. The 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. Between April 15 and October 22, 2005, the system operated an average of 5 hr/day for a total of 1,032 hr, treating approximately 807,300 gal of water (that contained total arsenic ranging from 24.1 to 34.0 µg/L, and existing almost entirely as As[V]). This volume throughput was equivalent to about 21,600 bed volumes [BV] based on the 5 ft3 bed volume in the lead adsorption vessel. Total arsenic levels in the treated water following the lead vessel reached 10 µg/L at approximately 19,500 BV. The arsenic level from the lag vessel at the time was <1 µg/L. Concentrations of orthophosphate and silica, which could interfere with arsenic adsorption by competing with arsenate for adsorption sites, ranged from <0.05 to 0.3 mg/L (as PO4) and from 24.2 to 31.7 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.
The system was backwashed only once during the first six months of system operation because there had been minimal solids buildup in the vessels and because pressure differential (Δp) across the vessels had remained essentially unchanged at 3 to 6 pounds per square inch (psi). The backwash was initiated manually with each vessel backwashed with the treated water from the 2,000-gal hydropneumatic tank for 20 min at 16 gpm (or 9 gpm/ft2), producing approximately 320 gal of wastewater. Arsenic concentrations in the backwash water were 30.2 µg/L from the lead vessel and 3.6 µg/L from the lag vessel, compared to the treated water arsenic level of 0.3 µg/L, suggesting desorption from the media. The arsenic desorption might be due to slightly higher pH of the treated water in the hydropneumatic tank following aeration for radon removal.
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 30 µg/L to an average of 1.1 µg/L). The arsenic concentrations in the distribution system were similar to those in the system effluent. Neither lead nor copper concentrations appeared to have been affected by the operation of the system.
The capital investment cost of $34,210 included $22,431 for equipment, $4,860 for site engineering, and $6,910 for installation. Using the system's rated capacity of 10 gpm (14,400 gal/day [gpd]), the capital cost was $3,421/gpm of design capacity ($2.38/gpd) and equipment-only cost was $2,243/gpm of design capacity ($1.56/gpd).
The O&M cost included only incremental cost associated with the adsorption system, such as media replacement and disposal, electricity consumption, and labor. Although not incurred during the first six months of system operation, the media replacement cost would represent the majority of the O&M cost and was estimated to be $4,199 to change out one vessel. This cost was used to estimate the media replacement cost per 1,000 gal of water treated as a function of the projected media run length to the 10 µg/L arsenic breakthrough.