Tucson, Arizona depends upon three water sources – groundwater, treated wastewater and water from the Colorado River, which flows along the state’s western boarder. Through the Central Arizona Project, water is pumped from Lake Havasu along the Colorado to three Arizona counties through a 336-mile long system of aqueducts, pumping plants and pipelines.
Arsenic, among other minerals, is found in abundance in the region. While groundwater arsenic levels in most parts of the United States rarely exceed 20 to 30 parts per billion (ppb), levels of 40 ppb and more in Arizona are common.
The Flowing Wells Irrigation District in Tucson, under the leadership of Superintendent David Crockett, has more than 3,500 service connections providing the domestic and fire protection needs of 16,000 residents and numerous businesses.
Flowing Wells draws all its water from eight wells scattered throughout the district. Arsenic levels in two of its wells (#70 and #75) were measured at 49 ppb and 38 ppb, respectively. Since 1987, the district and the Arizona State Department of Environmental Quality (ADEQ) had been investigating high levels of volatile organic compounds (VOCs) in an upstream aquifer. In cooperation with the ADEQ, which would pay for the needed VOC equipment, the district began to investigate solutions for the arsenic and VOC problems in 2002. When the USEPA reduced the arsenic standard for drinking water from 50 ppb to 10ppb, district staff took steps to meet the new maximum contaminant level (MCL).
After reviewing several arsenic treatment alternatives – including granular iron media, ion exchange, coagulation microfiltration and reverse osmosis – the district selected a granular iron media solution, as recommended by environmental consulting firm Malcolm Pirnie. Of the various granular iron media solutions considered, the district chose Severn Trent Services’ SORB 33® arsenic removal system and Bayoxide® E33 media.
In order to meet the new EPA standard, the district took wells #70 and #75 offline until the new treatment system was operational. System installation began in May 2006 and was completed in November 2006. The six remaining wells were able to handle the district’s water needs during the installation period.
The new arsenic removal system consisted of a single 12-foot SORB 33 adsorber vessel preceded by a granular activated carbon system to treat the VOCs. The two wells had a combined flow of 900 gallons per minute (GPM), and the district planned to treat the entire flow - 1.3 million gallons per day - to achieve a blended arsenic level of less than eight ppb.
Both arsenic (III) and arsenic (V) were present in the groundwater. Since the arsenic removal process utilizing Bayoxide media is more effective at treating arsenic (V), the district also injected ½ part per million of chlorine into the SORB vessel, effectively oxidizing the arsenic (III), converting it to arsenic (V) for treatment by the SORB 33 system.
As part of the contract with the district, Severn Trent Services guaranteed 53,900 bed volumes of media at 100 percent well utilization, which would provide for a media life of over six months.
The arsenic removal system and granular activated carbon VOC reduction system were placed into operation on April 6, 2007. Arsenic levels in the treated water were immediately reduced to undetectable levels and remained at very low levels for several months, only reaching approximately eight ppb after nine months. (VOCs were not present in the water at the beginning of the system’s operation, and have only recently been drawn into the well system water as the wells have been in operation.)
Granular iron media typically must be periodically backwashed in order to keep it from compacting and reducing its effectiveness. The district was advised to backwash when the pressure differential between the top and bottom layers reached 12 pounds per square inch (psi). But Crockett discovered that the media did not become compacted through the first 9 months of system operation, and no backwashing was performed during this period.
Arsenic levels remained below eight ppb into January 2008, about three months beyond the contracted guarantee. “We would have been very pleased if we had performed the media change-out after six months, as the contract said,” said Crockett. “The extra three months were a welcome surprise.
“The SORB 33 system with the Bayoxide media has met our expectations. It has been so easy to operate and maintain that we thought we might be missing something. Except for the media change-out, we really haven’t had to do anything.”
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