Arsenic Treatment Options for the Southwest

Arizona Water Quality Association

Arsenic occurs naturally in the environment  as a heavy metal in two different forms, arsenite  (arsenic III) and arsenate (arsenic V). Arsenic is released into water supplies from erosion of rocks and soil. The distribution of arsenic in soil, groundwater and surface water has extensively been investigated during the past two decades.

Long-term exposure to arsenic is proven to result in health effects such as cancer, cardiovascular disease, diabetes and reproductive problems. Nationally, about 3,000 (or 5.5 percent) of the nation's 54,000 community water systems and 1,100 (or 5.5 percent) of the 20,000 non-transient non-community water systems will need to take measures to lower arsenic in their drinking water. Of the affected systems, 97 percent serve fewer than 10,000 people. While high concentrations of arsenic are found mostly in the Western region of the United States, parts of the Midwest and New England show levels of arsenic

that exceed the newly approved U.S. Environmental Protection Agency (EPA) standard of 10 parts per billion (ppb). The Western states have more systems with arsenic levels greater than 10 ppb as compared to the national average. Some systems in parts of the Midwest and New England have current arsenic levels that are greater than 10 ppb, but most systems have arsenic levels that range from 2 to 10 ppb of arsenic.

Treatment Options

During the EPA initial review of treatment options for arsenic removal, ion exchange, activated alumina, reverse osmosis, enhanced coagulation/filtration and oxidation/filtration were identified as best available technologies (BAT). These technologies, along with other industry emerging technologies, can be divided into three categories: sorption treatment processes, membrane treatment processes and precipitation/filtration processes.

Through ion exchange (IX), arsenic is removed by continuously passing water under pressure through column(s) packed with exchange resin. As a low-cost treatment option when used under specific operating criteria, IX has problems operating effectively with the presence of high levels of sulfate (SO4-2) and total dissolved solids (TDS) in process water.

Activated alumina (AA) is a porous, granular material with properties similar to IX and commonly used for the removal of silica, natural organic matter and fluoride. To remove arsenic using this process, water under pressure is continuously passed through one or more beds of AA media. AA is pH sensitive and its selectivity requires As III to be preoxidized, converting it to As V before treatment. Currently, modified AA media are emerging to provide drinking water systems with media that has greater overall adsorption capacities, arsenic selectivity and operational flexibility than traditional AA.

Reverse osmosis (RO), is an attractive treatment process because it can address various water quality problems through a simple and easy-to-use operation. RO is a pressure-driven membrane separation process that removes dissolved solutes and greater than 90 percent of arsenic from water. The RO treatment process is relatively insensitive to pH and has water recovery rates ranging from 60 to 80 percent.

Enhanced coagulation/filtration, both conventional and pressurized methods, can be used to remove inorganic arsenic from water. During the treatment process, arsenic is adsorbed onto an aluminum, ferric hydroxide, ferric sulfate or ferric chloride precipitate depending on application-specific parameters. The economics and efficiency of this treatment system rely on coagulant type and dosage, mixing frequency and pH levels. If optimized, this treatment process can effectively remove greater than 90 percent of arsenic from water.

Oxidation/filtration is a pressurized granular-media filtration process that uses manganese-oxide media because of its adsorptive and catalytic qualities. Under optimized conditions, this process and be a cost-effective treatment option that yields an efficiency rate between 80 and 95 percent.

Granular ferric oxide/hydroxide media is an arsenic treatment technology not initially included in the EPA's evaluation of treatment processes. This adsorption process can be applied in fixed-bed pressure column(s) similar to those for AA. Granular ferric oxide/hydroxide media is not as pH sensitive as AA, can treat larger bed volumes and has higher surface areas. Although this adsorption process has not been designated as a BAT by the EPA, evaluation of the technology is underway.

Area of Concern

The EPA estimates that approximately 97 percent of community water systems serving fewer than 10,000 people will be impacted by the 10 ppb maximum contaminant level. This poses a problem, considering most small water systems have small customer bases, few community assets and little income. Funding to comply with the arsenic standard is available through the EPA's drinking water state revolving fund (DWSRF). Capital projects that include new technology and upgrading systems are eligible under the DWSRF. However, as one might imagine, if a public water system applies for funding closer to the compliance date of January 2006, the surge in applications coming in at that time will impede the process to effectively meet all systems' needs.

Nonetheless, all systems will be required to comply with the new standard, and consolidating or restructuring the water systems or using point-of-use (POU) devices might be the most cost-effective options for these small water systems.

Under the final EPA ruling, POU devices are approved as small system compliance technologies (SSCT). SSCTs must be owned, controlled and maintained by the public water system or by an agency under contract with the water system (i.e., responsibility to operate and maintain these systems cannot be passed along to the customer). While small system use of POU devices will result in lower capital and treatment costs. Administrative and monitoring costs will be higher. The EPA notes that previous studies show this to be an economically viable treatment alternative for systems treating 50 to 250 people. Adsorption (AA or granular ferric oxide/hydroxide) and RO probably are the industry's two most recognized treatment technologies for POU arsenic removal. These technologies should be applied based on performance and cost for effective arsenic removal.

What Dealers Should Know

Cost-effective and commercially proven arsenic removal technologies currently are available to treat arsenic contamination. Individuals not willing to wait for their water system's compliance with the arsenic standard currently are looking for treatment systems to use in their homes. POU and even point-of-entry (POE) treatment systems are an attractive solution for these individuals. A water treatment dealer can address these concerns by offering POU and POE systems for installation. The process should begin with a basic understanding of arsenic contamination and the element's chemistry, a complete water quality analysis of the application-specific water and the knowledge of available technologies. When combined, water treatment dealers then can present individual customers with the appropriate treatment option for arsenic removal.        

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