Arsenic System Works All Too Well

Seated on the high desert mesa at the foot of the Sangre de Cristo Mountains, Taos, N.M. is rich with art and steeped in history. An artists’ haven for more than 100 years, the town is home to the influential Taos Art Colony and many art galleries and museums. Over the years, American Indians and Spaniards have had a significant impact on the culture of this part of northern New Mexico. Recreational activities like skiing, rafting and hiking combine with Taos’ natural beauty to make the area one of the Southwest’s most popular tourist attractions.

Taos is also home to some of the country’s most extensive arsenic deposits. Pyrite, or “fool’s gold,” silica-rich volcanic rocks and some sandstones and mudstones commonly found in the state also contain high amounts of arsenic.

In January 2005, Taos was awarded an arsenic removal treatment system as part of the U.S. Environmental Protection Agency’s (EPA) Arsenic Removal Demonstration Program. Nationwide, about 5% of municipalities must treat drinking water to meet the new EPA standard of 10 ppb.

In New Mexico, however, the percentage of municipalities is closer to 20%. Among Taos’ eight wells, which serve a population of fewer than 5,000, only the eighth and deepest well has elevated arsenic levels requiring treatment. Water is pumped from the well to the San Juan/Chama booster station, about five miles outside of Taos.

Given a choice of systems to install, Taos officials chose the SORB 33 arsenic removal system from Severn Trent Services over two other systems. According to Amos Torres, public utilities superintendent for the town, utilities in Arizona, Michigan, Maryland and New Hampshire had previously chosen the SORB 33 system and had reported on its effectiveness and ease of use. In February 2006, a SORB 33 arsenic package unit (APU) 450 was installed at Taos’ San Juan/Chama station to treat 450 gpm, all of the well pump’s capacity.

The SORB 33 APU is designed for arsenic removal at the wellhead of small water systems. The APU products are standard package units, engineered to remove arsenic contamination to less than 10 ppb across a complete range of drinking water systems up to 450 gpm. Mirroring the larger SORB 33 systems, the APU operates under the same principle of pumping source water through a vessel or series of vessels containing the new pelletized Bayoxide E33P granular ferric oxide media. As the contaminated water passes through the media, arsenic is adsorbed and removed to the required level until the media reaches its capacity. The spent media, which meets toxicity characteristic leaching procedure requirements, is then removed and disposed as non-hazardous waste. The SORB 33 APU system design and operation does not require the cleaning, regeneration or other complex steps required by competitive technologies.

Special design details
At the Taos facility, the skid-mounted SORB 33 APU unit consists of three FRP adsorbers and 195 ft of Bayoxide E33P iron oxide media. This was installed in four parts through the wall behind the system. The well water has a slightly elevated arsenic level of 14 micrograms per liter (µg/L) and is low in TDS, but has a high pH of 9.5. This pH level can hinder the performance of any metal-based, arsenic removal media, so the Taos system was designed with a pH reduction pretreatment process to lower the water’s alkalinity and improve the Bayoxide E33P media arsenic adsorption performance. Fifteen mg/L of carbon dioxide gas is injected into the water downstream to lower the water’s pH to 7.4—an optimum level that allows maximum adsorption without the need for pH readjustment. The gas is diffused into the water using a porous membrane housed in side stream piping that feeds well water to the adsorbers. Complete gas dissolution occurs within 10 ft of piping. Carbon dioxide liquid is supplied in 480 lb glass dewars delivered weekly to the well house, and the system has an emergency backup unit with two 50 lb high pressure CO2 cylinders. The dewars and cylinders are stored in a separate, ventilated room.

Effective results
The system’s results have been impressive. Arsenic levels have been reduced to less than 0.4 ppb at current bed volumes of 8,000. Arsenic levels are not expected to reach 10 ppb until 111,000 bed volumes have been treated, representing a media life of approximately 26 months.

According to Torres, everything he’d heard about the SORB system’s ease of operation was accurate.

“The system practically operates itself,” Torres said. “As for maintenance, we’ve had to make only a few minor adjustments to the CO2 injection system, and the arsenic system itself has required no maintenance at all.”

Patrick Vigil, Taos’ public utilities director, noted that the growing population in Taos will require additional water resources in coming months and the continued need for arsenic removal.

“We will drill an additional well this year and probably another one in the next few years to accommodate the expanding population,” Vigil explained. “In order to avoid depleting the shallow aquifers in the area, these wells will be deep—so arsenic contamination will be an issue.

“Based on its performance at the eighth well, the SORB system will be an excellent arsenic removal solution for our next well,” Vigil said.

Rich Dennis is a separation products manager for Severn Trent Services. He can be reached at 813/886-9331 or by e-mail at rdennis@severntrentservices.com.

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