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 EPA/600/R-08/142

 

Arsenic Removal from Drinking Water by Adsorptive Media U.S. EPA Demonstration Project at Brown City, MI
December 2008

This report documents the activities performed and the results obtained from the arsenic removal treatment technology demonstration project in Brown City, MI. The objectives of the project were to evaluate

  1. the effectiveness of a Severn Trent Services (STS) adsorptive media system – Arsenic Package Unit (APU) – with the use of 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 skills, 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.

The STS system consisted of two APU-300 units each comprising two 63-in-diameter, 86-in-tall fiberglass reinforced plastic (FRP) vessels in parallel configuration. Each adsorption vessel contained approximately 80 ft3 of SORB 33™ media, which is an iron-based adsorptive media developed by Bayer AG and packaged under the name SORB 33™ by STS. The system was designed for a flowrate of 640 gal/min (gpm) (160 gpm to each vessel), corresponding to a design empty bed contact time (EBCT) of about 3.7 min and a hydraulic loading rate of 7.4 gpm/ft2. Actual flowrate through the system averaged 564 gpm during the performance evaluation study, corresponding to an EBCT of 4.2 min.

The STS treatment system started on May 11, 2004, and continued to operate through May 2, 2007, with a total operating time of 4,547 hr. Averaged daily operating time was approximately 4.5 hr/day or a 19% utilization rate. During the performance evaluation, approximately 154,000,000 gal or 64,370 bed volumes (BV) of water were treated. The system continued to operate through the three-year demonstration study with only a few minor repairs and adjustments. The flowrate, pressure data and other operational parameters were within the vendor specifications after a system retrofit that was completed before system startup on May 11, 2004. The system continued to operate within the vendor equipment specifications through May 2, 2007.

Arsenic in source water existed primarily as soluble As(III) (i.e., 85% at 13.1 μg/L), with a small amount also present as soluble As(V) (i.e., 0.7 μg/L ) and particulate As (i.e., 1.6 μg/L). Per the vendor’s recommendations, raw water was fed directly through the adsorption vessels without prechlorination to evaluate the capacity of the SORB 33™ media for As(III) adsorption from May 11, 2004, through May 15, 2005. Because of premature arsenic breakthrough over 10 μg/L, prechlorination was implemented on May 16, 2005, to extend the media bed life through oxidation of As(III) to As(V). Since then, the system operated with prechlorination through the end of the performance evaluation on May 2, 2007.

From May 11, 2004, to May 10, 2005, without prechlorination, concentrations of total arsenic in the treated water primarily as As(III) ranged from 0.7 to 12.8 μg/L, with >10-μg/L breakthrough occurring at 20,800 BV. From May 16, 2005, to May 2, 2007, with prechlorination, concentrations of arsenic in the treated water primarily as As[V] ranged from 0.6 to 7.5 μg/L. The amount of water treated during the entire study period was 64,370 BV, representing about 80% of the vendor-estimated working capacity. Prechlorination was effective in extending the media bed life by removing soluble As(V) and particulate As (about 19% of the arsenic removed) by the media beds. Particulate iron averaged 144 μg/L after prechlorination was removed to below the method detection limit of 25 μg/L.

Distribution system water samples were collected to determine any impact of arsenic 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 STS system showed a decrease in arsenic concentration at all three sampling locations. Total aresnic levels in the distribution system decreased from an average of 10.3 to 5.3 g/L, and generally mirrored those in the treatment plant effluent. Iron levels decreased to the non-detect level, while manganese levels increased slightly. Lead concentrations did not appear to have been affected by the operation of the system. Copper concentrations were generally lower.

Backwash wastewater contained lower-than-raw-water-level of soluble arsenic, indicating removal of soluble As(III) by the media during backwash. (Note that raw water was used for backwash.) As expected, particulate arsenic, iron, and manganese concentrations were considerably higher than respectively soluble concentrations. Particulate As might be associated with either iron particles intercepted by the media beds during the service cycle or media fines. Based on the total suspended solid (TSS) values, approximately 15.6 lb of suspended solids were produced in 20,000 gal of backwash wastewater during each backwash event.

The capital investment cost of $305,000 included $218,000 for equipment, $35,500 for site engineering, and $51,500 for installation. Using the system’s rated capacity of 640 gpm, the capital cost was $477/gpm ($0.33/gpd) and equipment-only cost was $340/gpm ($0.24/gpd). 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 640 gpm. The system operated only 4.5 hr/day on average, producing an average of 51,333,670 gal of water per year. At this reduced usage rate, the unit annualized capital cost increased to $0.56/1,000 gal.
O&M cost included only incremental cost associated with the APU-300 system, such as media replacement and disposal, chemical supply, electricity, and labor. The estimated media changeout cost is $53,600 for both APU-300 units, which would represent the majority of the O&M cost. Media changeout did not occur during the performance evaluation period.

Contact

Thomas Sorg


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