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Arizona college selects tried-and-true arsenic treatment system
What happens when the largest community college district in the United States measures arsenic at levels averaging 15 parts per billion (ppb) in its well? It selects an arsenic treatment system used by various utilities throughout its home state of Arizona.
Comprising 10 college campuses throughout south-central Arizona, the Maricopa County Community College District (MCCCD) is the largest community college district in the United States. Located in one of the largest and fastest growing counties in the country, MCCCD is also located where arsenic is a common problem.
One of the schools, Scottsdale Community College, not only offers its nearly 12,000 students an excellent education, it also provides them with clean drinking water through the only water treatment system among MCCCD schools. The system's water comes from a single well with a capacity of 2.44 million gal per day (mgd), and serves a student population of more than 10,000.
Scottsdale Community College’s well showed arsenic levels that averaged 15 ppb. While far below levels found in many other Arizona communities, treatment is still required to meet the revised, and more stringent, federal arsenic MCL of 10 ppb. The college received a three-year waiver to delay compliance with the January 2006 arsenic MCL and used the time to investigate and determine the best possible arsenic removal treatment technology to meet its application-specific needs.
During the waiver period, Frank Staley, the Scottsdale Community College water system manager; general contractor Garney Construction and civil/environmental consulting firm Damon S. Williams Associates investigated arsenic treatment systems in use by other water utilities throughout Arizona. Among the technologies they considered were reverse osmosis, coagulation filtration, ion exchange and adsorption.
Each technology having its own distinct advantages, Staley was impressed with an adsorptive arsenic removal process being used by a number of Arizona utilities, including eight nearby sites run by the city of Chandler. One other company, the Flowing Wells Irrigation District located just north of Tucson, had pre-treatment arsenic levels measured at 38 ppb and 49 ppb in two of its wells. David Crockett, superintendent of the District, said he had been pleased with performance of the SORB 33 arsenic treatment system and Bayoxide E33 arsenic removal media from Severn Trent Services .
When it came time to select an arsenic treatment solution, Scottsdale Community College chose the SORB 33 system and the Bayoxide media, provided by Hennesy Mechanical Sales. "When we amortized the cost of iron oxide-based adsorption vs. competing technologies, the SORB system was the clear choice," Staley said. Construction of the facility began in July 2007 and was complete in early 2008, going online in March 2008.
The Adsorption Process
A fixed-bed adsorption system, SORB 33 uses Bayoxide media, a granular ferric oxide media for the adsorption of dissolved arsenic. The system employs a simple "pump and treat" process that flows pressurized well or spring water through a fixed-bed pressure vessel that contains the iron oxide media where the arsenic removal occurs. With a high affinity for iron oxide-based minerals, arsenic can adsorb quickly to the surface of the media. This makes granular iron oxide media, such as Bayoxide, excellent 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. Specifically designed to adsorb arsenic while reducing competition with other ions, Bayoxide E33 improves the arsenic-adsorbing potential of the media.
At the Scottsdale Community College facility, water from the source well is pre-chlorinated before it flows into the SORB 33 system. Three 12-ft-diameter SORB adsorption vessels, containing a total of 1,029 ft? Bayoxide E33 media, are designed to handle an anticipated system flow of 2,300 gal per minute. Approximately 39% of the flow bypasses the SORB process and is blended with the treated water with an arsenic level reduced to 4 ppb. When combined, arsenic levels average 7 ppb. The three SORB vessels run in parallel with either two or all three adsorbers in service at once
After two years of operation, Staley reports that the system is "running perfectly—and it has justified our decision to use the adsorptive media vs. other technologies."
As part of the project, Severn Trent Services provided a bed volume guarantee of 69,000 bed volumes, equivalent to 531 million gal through 1,029 ft? of media. "A little more than two years into the media's life, we're right on target with the guarantee," Staley said. "In fact, it looks like the media might surpass the guarantee.
"I like to kid that the SORB system is 'boring,'" Staley concluded. "Nothing goes wrong; it always does what it's supposed to do."
Rich Dennis is separation process manager with Severn Trent Services.
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