The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the...
Incorporating 10 college campuses throughout south-central Arizona, the Maricopa County Community College District (MCCCD) is the largest community college district in the U.S. The colleges, which offer approximately 1,000 occupational programs, 37 academic associate degrees and more than 10,000 courses, are located in one of the largest and fastest growing counties in the country.
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).
Arsenic, a common problem in water throughout much of Arizona, has been measured at levels averaging 15 parts per billion (ppb) in the well. This is far below levels found in many other Arizona communities, but still requires treatment to meet the revised, and more stringent, federal arsenic Maximum Contaminant Level (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 being used by other water utilities in the state. Among the technologies they considered were reverse osmosis, coagulation filtration, ion exchange and adsorption.
While each technology had its 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 pretreatment 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 completed in early 2008, with plant operation beginning in March 2008.
The Adsorption Process
The SORB 33 arsenic removal system is a fixed-bed adsorption system using 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 containing the iron oxide media where the arsenic removal occurs. 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, 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. Bayoxide E33 is specifically designed to adsorb arsenic while reducing competition with other ions, thus improving its arsenic-adsorbing potential.
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 cu ft Bayoxide E33 media, are designed to handle an anticipated system flow of 2,300 gpm. Approximately 39% of the flow bypasses the SORB process and is blended with the treated water, whose arsenic level is 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 versus 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 cu 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.”