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Arsenic in Drinking Water

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Treatment Options


Overview

The main considerations when selecting a treatment technology include:

  • Water quality characteristics (including pH levels and initial concentrations of iron, As(III), and As(V) present in the water);
  • Ease of implementation with current system;
  • Residual management; and,
  • Cost.

Arsenic occurs in natural waters in both inorganic and organic forms. Inorganic species such as arsenite [As(III)] and arsenate [As(V)] however, are predominant in natural waters. The relative concentration of inorganic arsenic species found in source waters is unpredictable and site specific. It is important to know the form of arsenic in your source water because studies have shown that As(V) is more effectively removed from source waters than As(III). Consequently, if the arsenic in the source water is predominately As(III), then oxidizing As(III) to As(V) with a chemical oxidant (such as chlorine) will result in a higher arsenic removal efficiency. A chemical oxidant should be selected as aeration is ineffective at oxidizing arsenic. Addition of pre-treatment such as oxidation, like any modifications to an existing treatment train, should be evaluated for potential distribution system effects.

Given various regional water quality parameters, the presence of iron (Fe) will likely play a prominent role in technology selection and the ability to treat a given water source. Because of the unique role that iron can play in facilitating arsenic removal, the level of iron in the source water is a primary consideration in the selection of an optimal treatment technology. Below is a more detailed description of the range of iron concentrations relative to arsenic concentrations and how the Fe:As ratio can influence the treatment technology chosen.

  • High iron levels (> 0.3 mg/L) and High Iron to Arsenic ratio (20:1). Iron removal processes can be used to promote arsenic removal from drinking water via adsorption and co-precipitation. Source waters with this ratio are potential candidates for arsenic removal by iron removal. (Section A of Figure 1)
  • High iron levels (> 0.3 mg/L) and Low Iron to Arsenic ratio (< 20:1). If the iron to arsenic ratio in the source water is less than 20:1, then a modified treatment process such as coagulation/filtration with the addition of iron salts should be selected. (Section B of Figure 1)
  • Low iron levels (< 0.3 mg/L). Technologies such as adsorptive media and ion exchange are best suited to sites with relatively low iron levels in their source waters (less than 0.3 mg/L, the secondary MCL for iron). Above this level, taste, odor, and color problems can occur in the treated water, along with increased potential for fouling of system components with iron particulates. (Section C of Figure 1)

This concept, illustrated in Figure 1 below, is meant to be used as a general 'rule of thumb.' Additional guidance for treatment selection is provided under the Decide section of this site.

Figure 1: Arsenic Treatment Basic Selection Guide
Figure 1: Arsenic Treatment Basic Selection Guide (Treatment Technologies for Arsenic Removal, EPA Office of Research and Development, National Risk Management Research Laboratory)

Additional Resources:

Laboratory Study on the Oxidation of Arsenic III to Arsenic V (EPA 600/R-01/021)

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