Arsenic in Drinking Water
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Arsenic can be removed by passing untreated water though adsorptive granular media contained in a pressure vessel. As the water passes through the media, the negatively charged arsenic V ions are adsorbed onto the surfaces of the positively charged media particles. There are currently several adsorption media available: activated alumina (AA), titanium based media, zirconium based media, and iron based sorbents. The most common media include modified activated alumina and iron-based materials.
In addition to strong arsenic removal efficiency, several factors make adsorptive media treatment attractive to systems installing treatment for the first time.
The recommended empty bed contact time (EBCT) for adsorptive media systems is 3 to 10 minutes, with a median around 5. The bed depth is generally 3 to 6 feet, depending on the size of the vessel, with space left for bed expansion of 15 to 50% during backwash. Most adsorptive media systems require backwash to remove particulates and redistribute the bed material. However, it is important to follow the vendor's recommendations for optimal use.
For all adsorption media, liquid residuals from the filter backwashing step contain low concentrations of arsenic. Spent media will also need to be disposed of as either a solid or hazardous waste. See water.epa.gov/lawsregs/rulesregs/sdwa/arsenic/index.cfm for more information on waste disposal options.
Iron Based Sorbents
Adsorption of arsenic onto iron based adsorptive granular media is applied in fixed bed pressure vessels like those for AA. Due to limited performance research at the time the Arsenic Rule was promulgated, it was not listed in the Rule as a best available technology (BAT) or small system compliance technology (SSCT). For additional information on BATs and SSCTs see www.federalregister.gov/articles/2001/01/22/01-1668/national-primary-drinking-water-regulations-arsenic-and-clarifications-to-compliance-and-new-source.
Recent studies, including the EPA Arsenic Treatment Technology Demonstration Program, have shown iron based media to be effective for arsenic removal. For information on the Demonstration Program visit http://epa.gov/nrmrl/wswrd/dw/arsenic/.
The affinity of iron media for arsenic is strong under natural pH conditions, relative to AA. This feature allows iron based sorbents to treat more bed volumes without the need for pH adjustment. However, similar to AA, optimal performance is obtained at lower pH values. It is important to weigh the advantages and disadvantages of pH adjustment, if deemed optimal. Chemical addition for pH adjustment often increases the required operator skill level and introduces additional concerns such as the handling of hazardous chemicals and possible effects on a system's ability to comply with the lead and copper rule.
As with the selection of any treatment strategy, it is important to consider raw water characteristics for potential interference. Phosphate and silica have been shown to compete aggressively with As(V) for adsorption sites. Each 0.5 mg/L increase in phosphate above 0.2 mg/L will reduce adsorption capacity by roughly 30%.
Iron-based sorbents are typically used in the throw-away mode. Thus, solid wastes will be produced as well as backwash water. The backwash water can easily be recycled through the treatment plant. Exhausted iron based sorbents media have not exceeded Resource Conservation and Recovery Act (RCRA) toxicity characteristics, enabling it to be disposed of in a municipal solid waste landfill.
Activated alumina (AA) is a sorption process that uses porous, granular aluminum-based material. In drinking water treatment, packed-bed AA adsorption is used for removal of natural organic matter and fluoride. The removal of As(V) by adsorption can be accomplished by continuously passing water under pressure through one or more beds. The efficiency and economics of the system are contingent upon factors such as: water quality characteristics, pre-oxidation of As(III) to As(V), and presence of suspended solids. Although AA has a long history as adsorptive media for arsenic removal, it is rarely used because the emerging media, including modified AA, treat much higher bed volumes of water without the need to adjust pH.
Several constituents of water can interfere with the arsenic removal process by competing for adsorption sites or clogging the media with particulates. Table 1 below shows some of the most widely recognized water quality characteristics that interfere with arsenic removal.
Table 1: Water Quality Interferences with Activated Alumina Adsorption
There are several varieties of these proprietary activated alumina media, which are commonly referred to as "modified AA." Modified AA systems typically contain iron and can have greater overall adsorptive capacities, enhanced selectivity towards arsenic, and/or greater overall operational flexibility than conventional AA, thus making them more cost effective. Many of these media can achieve high arsenic removal in the presence of interfering substances and in higher pH conditions.
The capacity for arsenic removal by AA is pH dependent. If the pH range deviates from the range pH 5.0 - 6.0, the adsorption capacity for arsenic decreases at an increasing rate.
If operating in a regeneration mode, liquid waste is produced from the backwash, caustic regeneration, neutralization, and rinse steps. Also, the media loses adsorptive capacity with each regeneration, and will eventually have to be disposed of. If using throw away media, solid waste will be produced and will need to be tested to determine if it can be disposed of in a municipal landfill.