Meeting Arsenic Standards
The southeastern Arizona city of Tucson depends on three sources for its water—groundwater, treated wastewater and the Colorado River, which flows along the state’s western border. Through the Central Arizona Project, water is pumped from Lake Havasu along the Colorado to three southwestern and southcentral Arizona counties through a 336-mile system of aqueducts, tunnels, pumping plants and pipelines.
Ensuring water quality is a major challenge, since arsenic and other minerals are abundant in the region. While groundwater arsenic levels in most parts of the U.S. rarely exceed 20 to 30 parts per billion (ppb), levels of 50 ppb and more are common in parts of Arizona.
The Flowing Wells Irrigation District on the north side of Tucson is one municipality that has taken steps toward arsenic reduction in its groundwater.
Led by Superintendent David Crockett, the district has more than 3,500 service connections serving the 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), which are about ? mile apart, were measured at 49 ppb and 38 ppb, respectively. When the U.S. Environmental Protection Agency (EPA) reduced the arsenic standard for drinking water from 50 ppb to 10 ppb in 2006, the district took steps to meet the new maximum contaminant level. Also, since 1987, the district and the Arizona State Department of Environmental Quality (ADEQ) began investigating high levels of volatile organic compounds (VOCs) in an upstream aquifer.
In 2002, the district and the ADEQ began investigating solutions for the arsenic and volatile organic compound problems.
After reviewing a variety of arsenic treatment alternatives, the district selected a granular iron media solution that was recommended by Malcolm Pirnie, the environmental consulting firm based in White Plains, N.Y. There were many possible system choices, but the district chose Severn Trent Services’ SORB 33 arsenic removal system and Bayoxide E33 media, which has been approved by health and environmental agencies in 24 states.
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 new arsenic removal system consisted of a 12-ft adsorber vessel preceded by a granular activated carbon system to treat the VOCs. The two wells had a combined flow of 900 gal per minute, and the district planned to treat the entire flow of 1.3 million gal per day to achieve a blended arsenic level under 8 ppb.
Both arsenic (III) and arsenic (V) were present in the groundwater. Since the arsenic removal process with the media is more effective at treating arsenic (V), the district also injected 0.5 part per million (ppm) of chlorine into the SORB vessel, effectively oxidizing the arsenic (III) and converting it to arsenic (V) for treatment.
In the contract with the district, Severn Trent guaranteed 53,900 bed volumes of media at 100% well utilization, which would provide a media life of more than six months.
The arsenic removal system and granular activated carbon VOC reduction system were dedicated and placed into operation on April 6, 2007. Arsenic in the treated water was immediately reduced to undetectable levels, only reaching about eight ppb after nine months.
Granular iron media typically must be periodically backwashed to keep it from compacting and losing its effectiveness. The district was advised to backwash when the pressure differential between the top and bottom layers reached 12 psi, but Crockett said the pressure differential did not change in the first few months. In fact, the media did not face compacting, or a need for backwashing, during the first 9 months of operation.
Arsenic levels remained below 8 ppb into January 2008, about three months beyond what was expected. “We would have been very pleased if we had performed the media changeout after six months, as the contract said,” Crockett said. “The extra three months were a welcome surprise.”
In December 2006, the EPA recognized the district with its Drinking Water State Revolving Fund Aquarius Award.