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Arsenic Removal From Drinking Water by Adsorptive Media, U.S. EPA Demonstration Project at Oak Manor Municipal Utility District at Alvin, TX, Final Performance Evaluation Report (EPA/600/R-10/045) May 2010

This report documents the activities performed for and the results obtained from the EPA arsenic removal technology demonstration project at the Oak Manor Municipal Utility District (MUD) facility in Alvin, Texas. The objectives of the project were to evaluate the:

  • Effectiveness of a Severn Trent Services (STS) Adsorptive Media System – Arsenic Package Unit (APU)-30S – with the use of SORB 33 media in removing arsenic in order to meet the new arsenic Maximum Contaminant Level of 10 micrograms per liter(μg/L)
  • Reliability of the treatment system
  • Simplicity of required system operation and maintenance (O&M) and operator skills
  • Cost effectiveness of the technology

The project also characterized water in the distribution system and process residuals produced by the treatment system.

The STS APU-30S system consisted of two 63-inch by 86-inch adsorption vessels configured in series with 53.6 cubic feet (ft3) of SORB 33 media loaded in the lead vessel and 70.3 ft3 in the lag vessel. The SORB 33 media 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 150 gallons per minute (gpm), corresponding to a design empty bed contact time (EBCT) of about 6.2 minutes (or 3.1 minutes per vessel) and a hydraulic loading rate of 6.9 gpm per square foot (gpm/ft2). Actual flowrate through the system averaged 129 gpm during the performance evaluation study, yielding an EBCT of 7.2 minutes.

During the two-year performance evaluation study that began on April 25, 2006, and ended on April 8, 2008, the treatment system operated for a total of 4,628 hours (or 6.7 hours per day), treating approximately 35,358,250 gallons or 38,140 bed volumes (BV) of water. (Bed volumes were calculated based on 124 ft3 of media in both vessels.) The system continued to operate throughout the two-year study with only a few minor repairs and adjustments. The flowrate, pressure data, and other operational parameters were within the vendor specifications.

Source water from Wells 1 and 2 contained 40.2 μg/L (on average) of total arsenic, which existed primarily as soluble arsenic (III) (i.e., 31.5 μg/L). Prechlorination was effective at oxidizing arsenic (III) to arsenic (V), converting 98 percent of soluble arsenic to arsenic (V). Arsenic breakthrough at 10 μg/L occurred after treating 9,527,220 gallons (or 10,277 BV) of water following the lead vessel and 26,638,090 gallons (or 28,736 BV) following the lag vessel. At the conclusion of the performance evaluation study, the system treated approximately 35,358,250 gallons (or 38,140 BV) of water with 23.2 and 10.5 μg/L of total arsenic present in the effluent of the lead and lag vessels, respectively. Bed volumes were calculated based on 124 ft3 of media in both lead and lag vessels.

Prechlorination also was effective in oxidizing soluble iron and manganese in source water, reducing their concentrations to below the Method Detection Limit of 25 μg/L for iron and 1.9 μg/L for manganese.

Backwash was manually initiated by the operator when differential pressure across the adsorption vessels was approaching or exceeded 10 pounds per square inch (psi). During the first year of system operation, backwash was effective in restoring differential pressure across the lead vessel, reducing it from above 10 psi to the initial level of less than 4.0 psi. Since then, backwash became less effective. Gradual accumulation of precipitated solids or well sediments was thought to have contributed to the progressively less effective backwash observed. Differential pressure across the lag vessel remained low and constant around 3.1 psi throughout the performance evaluation study, indicating that precipitated solids and well sediments were removed mostly by the lead vessel. During each backwash event, approximately 7.2 kilograms of solids were discharged along with 10,800 gallons of backwash wastewater. The discharged solids comprised 2.8 grams of arsenic, 804 grams of iron, and 71.8 grams of manganese.

Comparison of the distribution system sampling results before and after system startup showed noticeable decreases in arsenic (from 38.2 to 2.6 μg/L [on average]) and manganese concentrations (from 41.8 to 1.5 μg/L [on average]) at all three distribution system sampling locations. Initially, arsenic concentrations in the distribution system water were higher than those in the plant effluent, probably due to redissolution and/or resuspension of arsenic previously accumulated in the distribution system. The concentrations then decreased and essentially mirrored those in the plant effluent. Lead and copper concentrations did not appear to have been affected by the operation of the treatment system.

The capital investment cost for the treatment system was $179,750, including $124,103 for equipment, $14,000 for site engineering, and $41,647 for installation. Using the system’s rated capacity of 150 gpm, the capital cost was $1,198 per gallon per day (or $0.83 per gpm). This calculation did not include the cost for a building addition to house the treatment system. The unit annualized capital cost was $0.22 pre 1,000 gallons, assuming the system operated 24 hours a day, 7 days a week, at the system design flowrate of 150 gpm. The system operated only 6.7 hours per day on average, producing 18,928,170 gallons of water per year. At this reduced usage rate, the unit annualized capital cost increased to $0.90 per 1,000 gallons. O&M costs included only incremental costs associated with media replacement and disposal, and labor. There was no incremental electricity or chemical consumption cost. The unit O&M costs are presented in graphical form as a function of projected media run length in this report.

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