Nov 10, 2014

Adsorbing Arsenic

Texas town no longer burdened with elevated levels of arsenic, silica & vanadium

arsenic silica vanadium
arsenic silica vanadium

Although it is a small town with a population of 2,800, Little Freer, Texas, stakes claim to two distinguishing characteristics compared with other cities in Texas—one historical, the other tourist related. Freer is home to the country’s first drilled oil wells as well as a 7-ft-tall statue of a rattlesnake that stands outside the Freer Chamber of Commerce. The statue celebrates Freer’s annual rattlesnake roundup, one of the largest in the U.S. Although it has a unique background, Freer shares a common problem with hundreds of other communities in the Southwest: arsenic in its 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 parts per billion (ppb) and ranged as high as 45 ppb—both figures well above the U.S. Environmental Protection Agency (EPA) standard for drinking water, 10 ppb. The wells also contained elevated levels of phosphate, silica and vanadium, each of which can impact the effectiveness of an arsenic treatment system. 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.

Arsenic Removal Pilot Program 

The WCID and its consulting engineer Coym, Rehmet & Gutierrez Eng., conducted an arsenic removal pilot program in the spring of 2010 utilizing De Nora Water Technologies’ SORB 33 arsenic removal technology. The technology’s adsorption process employes Bayoxide E33 ferric oxide media in a fixed-bed process. The SORB system uses a 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. Bayoxide E33, however, 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 improved operational cost.

The 90-day pilot program was successfully completed on June 16, 2010. The pilot treated 29,600 bed volumes (BV) with arsenic breakthrough approaching 10 ppb. This was greater than the 27,700 BV predicted for arsenic breakthrough. Adjustment of the pH was effective in improving the media’s arsenic capacity and reducing the effects of high silica interference.

Based on the pilot test results, the low capital and operating costs, and ease of use, the SORB system and Bayoxide media were selected.

Full-Scale Operation

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 3.25 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 reclamation equipment; and treatment bypass of 10% to 16% of the system’s capacity. The bypass water then is 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. 

The treatment system was commissioned and began operation in April 2013. It 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. LP. “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.” 

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