Drinking Water Treatment - Anion Exchange Units

Drinking Water and Human Health December 06, 2010 Print Friendly and PDF

EFFECTIVE AGAINST: Negatively charge ions, such as nitrates, bicarbonate, sulfate, selenium, and some compounds of arsenic.

NOT EFFECTIVE AGAINST: Positively charged ions such as iron, magnesium, calcium or manganese.

Contents

Ion exchange

Ion exchange replaces unwanted minerals in water with less objectionable ones. The exchange occurs in a tank filled with a special ion exchange material -– either a commercial resin, which is a petrochemical compound shaped into beads, or a synthetic zeolite, which is a crystalline formulation of aluminates and silicates.

The appropriate exchange material to use depends on the untreated water quality and the desired water quality. Certain exchange materials deteriorate in the presence of chlorine or other oxidants. Zeolites are better at reducing the concentration of iron and manganese than are commercial resins.

Typically, the exchange material is prepared by exposing the beads to a salt solution. As untreated water passes through the device, the undesired ions are exchanged for ions on the exchange material.

The two types of ion exchange units are water softeners and anion exchange devices. Water softeners remove cations (positively charged ions such as calcium and magnesium) and replace them with sodium. Anion exchange devices remove anions (negatively charged ions such as arsenic and nitrate) and replace them with chloride.

Mixed media ion exchange units remove both cations and anions. A typical mix would be 60 percent cation exchange material and 40 percent anion exchange material. The units usually must be regenerated at a central processing plant. Two-bed deionzers, which use separate cation and anion vessels, can be backwashed to remove trapped particles.

Uses of anion exchange devices

Anion exchange devices can treat either point-of-entry or point-of-use. They reduce the concentration of sulfate, nitrate, and arsenic in water. Point-of-use units are connected to a single tap and remove hazardous chemicals from drinking and cooking water only. Point of entry devices treat water for the whole household.

How anion exchange works

Ion exchange replaces (or exchanges) unwanted minerals in water with less objectionable ones. Chloride and hydroxide ions are the most commonly used materials on the resin beads. As water passes through the device, the resin adsorbs anions such as sulfate, nitrate, arsenic and bicarbonates and releases chloride into the water. The exchange occurs in a fiberglass tank or plastic-lined steel tank filled with either the resin or a synthetic zeolite.

Capacity of an anion exchange unit

Before installing an anion exchange unit, consider the requirements of the treatment system. Nitrate and sulfate are odorless, colorless and non-staining, so only drinking and cooking water may require treatment. The amount of water an anion exchange device can treat depends on the contaminant being removed. If removing nitrate, the removal capacity will depend on both the nitrate and the sulfate concentration in the water, as anion exchange resins preferentially adsorb sulfate. The sulfate will bump nitrate from the resin and into the treated water when all the exchange sites are filled. The size of the unit needed and the time between regeneration also depend on daily water use and flow rate.

Maintenance of anion exchange units

When all chloride has been exchanged, the resin beads are said to be exhausted. They are regenerated with brine. The brine solution releases the adsorbed contaminants and replaces them with chloride. The released anions are washed out with the waste brine. Maintaining an anion exchange unit also includes stocking the salt supply. Anion exchange treats water contaminants that can have negative health effects. It is crucial to monitor the treated water for these contaminants to verify that the unit is working properly. Home test kits are available to estimate nitrate and sulfate concentrations. If testing shows that contaminant concentrations exceed an acceptable level, regenerate the unit.

Special considerations

Anion exchange lowers the water pH, which can make the treated water more corrosive. It may be necessary to follow the anion exchange unit with a neutralizing system if the treated water is distributed through metal plumbing or used as drinking water for ruminant animals. Iron or turbidity may foul anion exchange units, so pretreatment to remove these contaminants may be necessary.

Use caution when disposing of the waste brine produced during regeneration. The brine contains the contaminants removed by the exchange resin. The contaminants may re-enter groundwater or surface water supplies if the brine is deposited on the ground, into a leaching pit, or into a septic system.

Questions to ask before you buy

Before purchasing a water treatment device, have your water tested at a state certified laboratory to determine the contaminants present. This will help you determine if an anion exchange unit is an effective treatment method for your situation. See Questions to Ask Before You Buy A Water Treatment System for more information.

Adapted from: Wagenet,L., K. Mancl, and M. Sailus. (1995). Home Water Treatment. Northeast Regional Agricultural Engineering Service, Cooperative Extension. NRAES-48. Ithaca, NY.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.