Arsenic-Free in Arizona
The cities of Litchfield Park and Goodyear, Ariz., share more than their proximity to Phoenix. Both are located approximately 20 miles west of Arizona's capital city. Both owe their names to the world famous Goodyear Tire and Rubber Co., for it was a Goodyear junior executive, Paul Litchfield, who was assigned the task in 1916 of establishing a cotton growing business west of Phoenix to supply fibers needed for tire cords. Today the two communities are cooperating to bring potable water to their residents.
Litchfield Park-based Adaman Mutual Water Co. (AMWC) serves a population of 1,000. The company is a small water system, composed of two groundwater source wells, #6A and #1B, which have a combined treatment output of 1,700 gal per minute (gpm). Arsenic contamination varied between 10 ppb and 12 ppb at Well #6A and between 14 ppb and 16 ppb at Well #1B.
Before the new arsenic maximum contaminant level (MCL) of 10 ppb took effect in January 2006, the AMWC obtained an extension from the Arizona Department of Environmental Quality. The extension allowed the utility to review commercially available treatment technologies and obtain the capital funding required to procure the system.
At the onset of the arsenic treatment system technology evaluation process, Dave Schofield, AMWC's general manager, decided to consult with other local water systems in Arizona about the arsenic removal treatment technologies they had in place. In discussions with officials in Goodyear, Schofield learned the city had been using an adsorptive arsenic removal process, SORB 33, and arsenic removal media, Bayoxide E33P, from Severn Trent Services. The Goodyear representatives were pleased with the SORB technology and Bayoxide media, which had been in use since 2006 to treat arsenic levels of 38 ppb.
In addition, the officials mentioned that Goodyear was experiencing a period of growth and lacked the capacity to meet its current potable water demands. Schofield advised the officials that AMWC had a 1,200-gpm output capacity for Well #1B but was only running the site at 500 gpm. As a result of these discussions, AMWC and Goodyear soon entered into an agreement for bulk water sales that would allow AMWC to divert extra potable water capacity to Goodyear to meet its increased demand. The agreement included the purchase by Goodyear of an arsenic removal treatment system, which AMWC could install at Well #1B to provide potable water for both systems to meet the arsenic MCL.
The bulk water agreement between AMWC and the city of Goodyear began in 2007. Because of the Goodyear facilities' positive experience with Severn Trent Services' SORB 33 systems and Bayoxide arsenic removal media, the city's officials recommended and purchased the same technology for installation at AMWC. A SORB 33 arsenic removal treatment system with a treatment capacity of 550 gpm was contracted in April 2008 and installed by Hennesy Mechanical Sales LLC in Phoenix. The system was commissioned and operational by December 2008.
The AMWC arsenic removal system included two 8-ft vessels containing 236 cu ft of Bayoxide E33P media designed to treat 550 gpm (250 gpm per vessel with two adsorbers running in parallel) with an empty bed contact time of 3.2 minutes. The treatment objective for the arsenic effluent was set at less than 8 ppm. Based upon Severn Trent Services' review of the water quality for Well #1B, the company issued a 175,000-bed volume performance guarantee for the Bayoxide media (the equivalent of 337 million gal) over a 26-month period.
Water from Well #6A is piped to Well #1B, where the combined capacity is treated for arsenic removal. The treated water is then cross-utilized to meet domestic and agricultural demands. Well #6A, with an average output of 500 gpm, is the primary well, used day in and day out to meet the potable water needs of the Adaman Water District. Well #1B, with an average output of 1,200 gpm, is primarily used for irrigation purposes for the local agricultural market of rose growers and vegetable farmers.
Source water is first pH adjusted using carbon dioxide (CO2) to reduce pH from 8.3 to 7.2. The CO2 is flow-pace controlled and monitored by a pH analyzer downstream. The pH-adjusted water is chlorinated and flows into the two adsorber vessels where a pump-and-treat process sends pressurized water through the filter vessels containing the arsenic removal media. As water is forced through the fixed bed, arsenic is attracted to the media, and the water is reduced to 8 µg/L of arsenic or less. The media—a dry, crystalline pellet—is designed to adsorb a large amount of arsenic to achieve long operating cycles, reduce pressure drops and improve the operating cost.
Pressure differential is monitored through each adsorber and when pressure differential on either vessel exceeds the high pressure differential setpoint (normally 10 psi), an alarm sounds indicating high pressure differential on that adsorber. After a 15-minute backwash is conducted to reclassify the compacted media, the adsorber is returned to service. During the backwash process, 100% of the water is captured and reused in the system.
The SORB 33 arsenic removal system completed the first year of full-scale commercial operation in December 2009. During that time, the AMWC staff undertook monthly sampling of raw water, samples from each vessel and finished water. After a year of operation, monthly testing of finished water samples showed arsenic levels at non-detect.
As a result, AMWC staff will be looking to undertake quarterly sampling during 2010. Prior to the installation of the arsenic removal system, the AMWC was equipped with a very basic chlorination disinfection system that required no day-to-day monitoring. With the new SORB 33 treatment system, the staff spends approximately 15 to 20 minutes per day logging meter readings and reviewing other parameters such as pressure differential and pH. This monitoring activity, while minimal, has nevertheless afforded the AMWC staff the opportunity to learn more about managing a process-based treatment system.
According to Schofield, the best thing about the SORB system and Bayoxide media is their reliability. "The arsenic removal process has performed just as we expected," he said. "We had been monitoring arsenic levels on a monthly basis, but after a year we switched to quarterly monitoring because the SORB system has so consistently hit our arsenic reduction targets. We're seeing the same excellent results experienced by Goodyear for the last four years."
The bulk water agreement between AMWC and the city of Goodyear is still in place. In fact, the city of Goodyear has recently completed construction of its pipeline into the Adaman Mutual Water District, and the first of two new wells has been drilled. AMWC will own and operate the two new well sites, which are completely dedicated to supplying the city with water to meet its ever-growing demand. The first well is scheduled to deliver water to Goodyear in the spring of 2010.
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