Arsenic Treatment Technology Saves the Day in a Land of Oil Strikes & Rattlesnakes
Arsenic removal technology frees a town from drinking water classified as “challenging” because of its elevated levels of arsenic, silica & vanadium
Though a small town with a population of 2,800, Little Freer, Texas, stakes claim to two distinguishing characteristics compared to other cities in Texas—one historical, the other tourist-related. Freer is where one of the country’s first drilled oil wells were and also where tourists can view the world’s largest rattlesnake, a 7-ft-tall statue that stands outside of the Freer Chamber of Commerce. The statue celebrates Freer’s annual rattlesnake roundup, one of the largest in the United States. Despite its unique background, Freer shares a common problem with hundreds of other communities in the southwest United States: arsenic in the potable water supply.
In 2011, arsenic levels in the water provided by the Freer Water Control Improvement District (WCID) from its eight wells averaged 34 ppb and ranged as high as 45 ppb—both figures well above the U.S. Environmental Protection Agency (EPA) standard for drinking water of 10 ppb. The wells also contained elevated levels of phosphate, silica and vanadium, each of which can negatively impact an arsenic treatment system’s treatment capacity. Granted an extension by the Texas Commission on Environmental Quality (TCEQ) to remediate the problem after the EPA deadline for compliance, the Freer WCID began to investigate various arsenic treatment options.
The WCID and its consulting engineer, Coym, Rehmet & Gutierrez Eng., conducted an arsenic removal pilot program in spring, 2010, to validate Severn Trent Services’ SORB 33 arsenic removal technology. The technology’s adsorption process employed the Bayoxide E33 ferric oxide media in a fixed-bed process.
Pilot Testing the Arsenic Removal Technology
The SORB system employs a simple "pump and treat" process that flows pressurized well water through a fixed-bed pressure vessel containing the iron oxide media where the arsenic removal occurs through a combination of adsorption, adhesion and other physical/chemical mechanisms. Other contaminants common to groundwater—including phosphate, silica and vanadium—also have a high affinity for iron-based minerals. This affinity creates competition among ions, resulting in less arsenic being adsorbed per volume of treated water. But Bayoxide E33 is designed to adsorb arsenic while reducing competition with other ions, thus improving the arsenic-adsorbing potential of the media. These characteristics enable systems using the dry, crystalline granular media to achieve long operating cycles, reduced pressure drops and improve the operational cost.
The 90-day pilot program was completed successfully June 16, 2010. The pilot treated 29,600 bed volumes (BV) with arsenic breakthrough approaching 10 ppb, the EPA maximum contaminant level. This was greater than the 27,700 BV predicted for arsenic breakthrough. pH adjustment was effective in improving the media’s arsenic capacity and reducing the effects of high silica interference.
Based upon the pilot test results, the low capital and operating costs and ease of use, the SORB system and Bayoxide media were selected.
The Freer SORB system comprises three 10-ft-diameter adsorbers, each with a capacity of 260 cu ft of Bayoxide media at a depth of 3¼ ft. The 900-gal-per-minute system consists of three adsorbers in sequencing configuration with one operating in parallel and the other two in series flow mode, a pH adjustment system, backwash water reclaim equipment and treatment bypass of 10% to 16% of the system’s capacity. The bypass water is then blended with treated water. Sequencing reduces operating costs by increasing the treatment to 50,000 BV. The bypass process also allows the system to treat higher volumes of water, delaying media exhaustion and reducing operating costs. Severn Trent Services projects the treatment costs using the SORB system to be about 55 cents per 1,000 gal.
The new SORB treatment system with Bayoxide media was commissioned and began operation in April 2013 and has been meeting arsenic treatment guidelines since day one.
“The SORB system is working as planned," said Jeff Coym, P.E., of Coym, Rehmet & Gutierrez Eng. L.P. "The Bayoxide media life is exceeding our estimates and the TCEQ tests for the first three quarters of plant operations were between 0.005 and 0.007 mg/L, as designed.”
Steve Wood is global product manager - inorganics for Severn Trent Services. Wood can be reached at email@example.com or 831.659.3545.
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