|Design Manual: Removal of Arsenic From Drinking Water by Ion Exchange (71 pp, 1.04 MB) (EPA/600/R-03/080) June 2003
This design manual is an in-depth presentation of the steps required to design and operate a water treatment plant for removing arsenic (V) from drinking water using the anion exchange process. Because arsenic (III) occurs as an uncharged anion in ground water in the pH range of 6.5 to 8, the process will not remove arsenic (III) unless it is first oxidized to arsenic (V).
The anion exchange process is a proven efficient and cost-effective treatment method for removing arsenic (V) from water supplies with low sulfate levels. This process removes arsenic using a strong base anion exchange resin in either the chloride or hydroxide form (chloride is the preferred form because salt can be used as the regenerant). The process preferentially removes sulfate over arsenic. However, as the sulfate increases in the raw water, the process becomes less efficient and more costly. Furthermore, because sulfate occurs in significantly higher concentrations than arsenic, treatment run lengths are dependent almost entirely on the sulfate concentration of the raw water.
The configuration of an anion exchange system for arsenic (V) removal can take several forms. The method presented in this design manual uses three vertical cylindrical pressure vessels operating in a downflow mode. Two of the three treatment vessels are piped in parallel to form the primary arsenic removal stage. The third treatment vessel is piped in series in the lag position.
In the primary stage, raw water flows through one of the two treatment vessels while the second vessel is held in the standby position. When the treatment capacity of the first vessel approaches exhaustion, it is removed from service and replaced by the second primary-stage vessel. While out of service, the first vessel is regenerated and placed in the standby position. The role of the third treatment vessel in the lag position is to ensure that any arsenic that breaks through one of the first two vessels does not enter the distribution system. Although this design results in higher capital costs, it prevents high arsenic concentrations in the treated water, if applied properly.
The manual also discusses the capital and operating costs, including many of the variables that can raise or lower costs for identical treatment systems.
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