The U.S Environmental Protection Agency (EPA) set a maximum contaminant level (MCL) of 10 ppb for drinking water, and reducing high levels of arsenic in a community’s water supply had been a challenge for Eureka County, Nev. Read on to learn how a community that once searched for silver hunted down a way to remove high levels of arsenic from its drinking water.
Project Background
For years, Eureka County had been finding something in the drinking water supply of one of its communities, Crescent Valley, something that is quite common to water supplies in the western U.S.: arsenic. With levels of 13.25 and 13.5 ppb in the system’s two wells, the Crescent Valley Water System needed to lower the levels to 10 ppb in order to meet the MCL. The county contracted with Day Eng. of Fallon and Ely, Nev., to determine the best way to reduce the arsenic.
De Nora’s SORB 33 arsenic removal system was selected as the arsenic removal method following a successful pilot test. The SORB 33 arsenic removal system is a fixed-bed adsorption system that 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. 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 SORB 33 arsenic removal media, suitable for arsenic removal. Both arsenite (arsenic III) and arsenate (arsenic V) oxyanions are removed from water via a combination of oxidation, adsorption, occlusion (adhesion) and/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. SORB 33 arsenic removal media is specifically designed to adsorb arsenic while reducing competition with other ions, thus improving the arsenic-adsorbing potential of the media.
Pilot Testing Arsenic Treatment Solutions
Day Eng. coordinated a 30-day pilot study with three companies, with technologies including coagulation/filtration and adsorption. According to Martin Ugalde, the company’s project manager, coagulation/filtration had been shown to pose operational challenges in other demonstrations.
“There are more chemicals required with coagulation/filtration, and the sludge byproduct from backwash must be managed,” he said. “The chemical, typically including ferric chloride, is also highly corrosive and can damage chemical feed pumps in as little as a year.”
Upon evaluating the three bids, Day Eng. recommended the adsorptive technology provided by De Nora’s SORB 33 arsenic removal system. According to Ugalde, there were two major determining factors in selecting the adsorptive process: ease of operation and simplicity in modifying operating set points, and operations and maintenance costs.
Arsenic Treatment System Operation
To meet the drinking water needs of approximately 265 residential and commercial customers in Crescent Valley, Day Eng.’s plan called for the construction of a 350-gal per minute (gpm) arsenic treatment plant. Two 8 ft in diameter SORB 33 adsorption vessels, each containing 169 cu ft of SORB 33 arsenic removal media and able to treat 175 gpm, run in parallel. Crescent Valley is currently blending the treated 13-ppm Well #5 with untreated 8-ppm Well # 4, successfully maintaining an average result of 5 ppm.
Following seamless installation, “Everything is working excellently,” said Bruce Harland, the water system operator plant’s operator. “I haven’t had to backwash the system for a month because the differential pressures haven’t approached 10 psi. We are very happy with the way the plant is running.”
Years later, Harland is pleased to share that the SORB 33 arsenic removal media lasted longer than expected.
“Our arsenic removal system has proven to be outstanding these past several years,” Harland said.
