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 EPA/600/R-08/140

 

Arsenic Removal from Drinking Water by Adsorptive Media U.S. EPA Demonstration Project at Desert Sands MDWCA, NM
December 2008

This report documents the activities performed and the results obtained for the arsenic removal treatment technology demonstration project at the Desert Sands Mutual Domestic Water Consumers Association (MDWCA) facility in Anthony, NM. The objectives of the project were to evaluate 1) the effectiveness of Severn Trent Services’ (STS) Arsenic Package Unit-300 (APU-300) SORB 33TM media in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 g/L, 2) the reliability of the treatment system, 3) the simplicity of required system operation and maintenance (O&M) and operator skill levels, and 4) the cost-effectiveness of the technology. The project also characterized water in the distribution system and process residuals produced by the treatment system.

SORB 33TM media is an iron-based adsorptive media developed by Bayer AG and marketed by STS. Two media runs were conducted in the performance study, with the first utilizing the granular form of the media (–S) and the second utilizing the pelletized form (–P). According to the vendor, SORB 33TM–P is an improved version of the granular form (SORB 33TM–S) and has more robust physical integrity. The two forms of media have the same composition, but a different bulk density and media particle size distribution.

The APU-300 system consisted of two 63-in-diameter, 86-in-tall pressure vessels in parallel configuration. Each pressure vessel initially contained 80 ft3 of SORB 33TM–S media or 62 ft3 of SORB 33TM–P media. The adsorptive media were supported by a gravel underbed. Based on a design flowrate of 320 gal/min (gpm), empty bed contact times (EBCTs) for the system were 3.7 and 2.9 min, for the SORB 33TM –S and –P media, respectively. Hydraulic loading to each vessel based on the design flowrate was 7.4 gpm/ft2.

The first media run operated from January 16, 2004 through July 14, 2005, treating approximately 52,645,000 gal of water based on totalizer readings from each vessel. The APU-300 system operated 6.2 hr/day with an average flowrate of 271 gpm. The second media run operated from July 29, 2005 through August 16, 2006, and treated approximately 46,553,000 gal of water based on totalizer readings from each vessel. The APU-300 system operated 7.8 hr/day with an average flowrate of 251 gpm. The EBCTs ranged from 3.1 to 7.5 min for the first media run, and from 2.5 to 6.2 min for the second.

Breakthrough of total arsenic at concentration above the 10 μg/L target MCL occurred at approximately 40,600 bed volumes (BV) during the first media run, representing approximately 62% of the vendor-estimated working capacity of 66,000 BV. During the second media run, breakthrough of arsenic occurred at approximately 49,500 BV, representing about 58% of the estimated working capacity of 85,200 BV.

During the study, total arsenic concentrations in source water ranged from 18.6 to 30.1 μg/L with As(III) comprising a significant portion of the total soluble arsenic with concentrations ranging from 17.6 to 25.2 μg/L. Prechlorination was effective in oxidizing As(III) to As(V), as evident by the low As(III) concentrations (i.e., averaged 2.0 g/L) in water sampled immediately after prechlorination. Total and free chlorine residuals measured before and after the adsorption vessels were at the equivalent levels of 0.4 to 0.8 mg/L (as Cl2) and 0.3 to 1.0 mg/L (as Cl2), 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 impact arsenic removal by the media.

Backwash wastewater contained soluble arsenic concentrations ranging from 6.4 to 22.2 μg/L, and averaging 13.3 μg/L. The average soluble arsenic concentration was lower than that in raw water, indicating removal of some soluble arsenic by the media during backwash. Soluble iron and soluble

manganese concentrations ranged from <25 to 373 and 1.8 to 27.1 μg/L, respectively. As expected, total arsenic, iron, and manganese concentrations were considerably higher than soluble concentrations, indicating the presence of particulate metals in the backwash wastewater. Particulate As might be associated with either iron particles intercepted by the media beds during the service cycle or the media fines. Based on the total suspended solid (TSS) values, approximately 9.1 lb of suspended solid was produced in 10,000 gal of backwash wastewater from both vessels during each backwash event.

The spent media passed the Toxicity Characteristic Leaching Procedure (TCLP) test for all Resource Conservation and Recovery Act (RCRA) metals, with only barium showing detectable concentrations ranging from 0.61 to 0.76 mg/L. The average arsenic loading on the spent media as analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) was 2.2 mg/g or 0.22% on SORB 33TM–S media and 1.6 mg/g or 0.16% on SORB 33TM–P media. These loadings compared well with the average adsorptive capacities, i.e., 2.1 and 1.7 mg/g, respectively, as calculated by dividing the area between the influent and effluent breakthrough curves by the amount of dry media in each vessel.

Distribution system water samples were collected to determine any impact of arenic treatment on the lead and copper levels and water chemistry in the distribution system. Comparison of the distribution system sampling results before and after the operation of the APU-300 system showed a decrease in arsenic concentrations (from 22.4 to 28.2 μg/L to 1.8 to 19.0 μg/L) at all three sampling locations. However, the concentrations measured at the distribution system were higher than those in the system effluent. This likely was 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 appear to have been affected by the operation of the system.

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 gpm, the capital cost was $478/gpm of design capacity and the equipment-only cost was $350/gpm of design capacity. These calculations did not include the cost of a building addition to house the treatment system. The unit annualized capital cost was $0.09/1,000 gal, assuming the system operated 24 hours a day, 7 days a week, at the system design flowrate of 320 gpm for 20 years at 7% interest. The system operated only 7 hr/day on average, producing 40,395,000 gal of water per year. At this reduced usage rate, the unit annualized capital cost increased to $0.37/1,000 gal.

The O&M cost of the APU-300 system was estimated at $0.74/1,000 gal, which included media replacement and disposal, chemical supply, electricity consumption, and labor. Because the incremental costs for chemical supply and electricity were negligible, only media replacement and disposal and O&M labor would impact O&M costs.
The APU-300 system experienced excessive flow restriction, imbalanced flow, and/or 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, able to operate according to the original design specifications.

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Thomas Sorg


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