Tackling Arsenic With Adsorption

Jan. 16, 2015
Treatment system brings reservation’s water into EPA compliance

About the author: Martin Lawrence is process engineer - inorganic contaminants for Severn Trent Services. Lawrence can be reached at [email protected] or 813.886.9331

Supai, Ariz., is the capital of the Havasupai Indian Reservation in the western Grand Canyon. The reservation is the site of some of the country’s most beautiful waterfalls and is a popular destination for hikers and backpackers. In fact, hiking is one of the few ways to reach Supai, which is the only place in the U.S. where mail is still delivered by mules. The nearest surfaced road is 9 miles away.

When the U.S. Environmental Protection Agency’s (EPA) new, more stringent arsenic maximum contaminant level of 10 ppb took effect in 2006, the Havasupai Water Treatment Plant was found not compliant. Using only chlorination to treat the arsenic contamination so common in Arizona and surrounding states, the arsenic levels of the reservation’s water sources ranged from 10 to 12 ppb—lower than levels found in water elsewhere in the region, but still higher than EPA standards.

In response, the Havasupai Indian Reservation began a search for a treatment system that would help it meet the EPA standards. Ultimately, it chose Severn Trent Services’ SORB 33 arsenic removal system utilizing Bayoxide E33 arsenic removal media. The reservation had good reason to believe the system would be effective—approximately 150 SORB systems are currently in use at water treatment facilities throughout the West and Southwest.

Adsorption for Arsenic

The arsenic removal system is simple to operate and requires no cleaning, regeneration or complex, labor-intensive steps. It is a fixed-bed adsorption system, employing a simple “pump-and-treat” process that flows pressurized well or spring water through a fixed-bed pressure vessel containing iron oxide media. Arsenic has a high affinity for iron oxide-based minerals and can adsorb quickly to the surface of the media. This makes granular iron oxide media, such as Bayoxide, effective for arsenic removal. Both arsenite (arsenic III) and arsenate (arsenic V) oxyanions are removed from water via a combination of oxidation, adsorption, occlusion (adhesion) or solid-solution formation by reaction with ferric oxide ions.

Other contaminants common to groundwater also have a high affinity for iron-based minerals. This creates competition among ions, resulting in less arsenic being adsorbed per volume of treated water. Bayoxide E33 is specifically designed to adsorb arsenic while reducing competition with other ions, thus improving the arsenic-adsorbing potential of the media.

Although the media will need to be replaced after two to three years, the operation and maintenance costs of the new system will help the reservation save costs when compared with a coagulation/filtration process. Additionally, the system generates less waste than a coagulation/filtration system—an important factor because waste sludge cannot be removed by truck from the reservation’s location.

Because of the reservation’s remote location, the system was brought to Supai by helicopter. To accommodate the unusual delivery method, the SORB vessels were not mounted to a triplex structural steel skid, as the equipment typically is shipped. Instead, each vessel was shipped complete with individual face piping only. Piping headers for influent, effluent and rinse/waste were shipped loose for reassembly by Technologies Construction of Prescott, Ariz.

The system consists of three pressure vessels, each measuring 4 ft in diameter, capable of treating 200 gal per minute. Operating in parallel, the vessels each contain 3 ft of Bayoxide media. Sixty percent to 80% of the influent flow to the plant is bypassed. After going into operation in October 2014, water from the Havasupai plant has been fully compliant with EPA standards.

“We are very happy with the SORB system,” said Armando Marshall, Havasupai facilities and maintenance manager. “We heard of the high level of performance the system achieved at other treatment facilities in the region, and our system’s performance has certainly matched our high expectations.”

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About the Author

Martin Lawrence

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