Drinking Water Treatment – Cation Exchange Units

EFFECTIVE AGAINST: Positively charged ions such as iron, magnesium, calcium or manganese.

NOT EFFECTIVE AGAINST: Negatively charged ions, such as nitrates, bicarbonate, sulfate, selenium, and some compounds of arsenic.


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 break down 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.

Water softening (cation exchange)

Water softeners are the most widely used home water treatment devices. They remove minerals that form scale on water heaters and soap film on sinks. Water softening is sometimes referred to as water conditioning.

Uses of water softening devices

Water softening devices have long been available in the water treatment industry. The technology, therefore, is highly developed and in most cases works well to reduce the hardness level. Softeners are in-line, point-of-entry devices that remove hard minerals. Softeners also remove barium, radium, and small amounts of dissolved iron and manganese.

How cation exchange works

Water softeners consist of a corrosion-resistant tank that is filled with resin beads that are saturated with sodium. The resin prefers calcium and magnesium (the principle components of hardness) over sodium. As water passes over the beads, sodium is released and calcium and magnesium are adsorbed to the resin beads. A distributor disperses the untreated water throughout the resin. This assures that all the untreated water contacts the exchange material instead of passing directly through the column without contact.

Capacity of water softeners

Water softeners can either treat all the water in the house or only the water leading to the hot water heater. Before installing a softener, consider the proposed uses of the treated water. In some cases, only the hot water should be softened to reduce scale formation in the water heater and hot water pipes. In other cases, water used only for cleaning, bathing, and laundering should be softened to prevent detergent films from forming. Water used for drinking and cooking and outdoor use should not need to be softened. Toilet water should be softened only if the softener is being used to remove low levels of iron, which may cause staining. A plumber or water treatment specialist can help determine the best installation scheme for a particular household.

The appropriate size of softener depends on the hardness level, daily water use, and flow rate.

Maintenance of water softening systems

When the softener has exchanged all of its available sodium, it needs to be regenerated. In this process, brine is passed through the bed to release the adsorbed calcium and magnesium ions from the resin and restore sodium to the resin. The released ions are carried away in the waste water. The distributor prevents resin from being washed out of the softener.

The regeneration process can be manual, semi-automatic or fully automatic. Manual systems require the homeowner to initiate the regeneration and rinsing cycles as well as the return to service. With semiautomatic systems, homeowners must initiate the regeneration cycle but the softener controls everything else. Fully automatic softeners are equipped with a timer that automatically starts the regeneration cycle and every step thereafter; regeneration is usually done during a period of low flow.

A fourth type of regeneration, demand-initiated regeneration (DIR), automatically regenerates the softener. The system initiates regeneration based on the gallons of water used, a change in the electrical conductivity of the resin bed, or a change in treated water hardness.

Potassium chloride can be used instead of sodium chloride to avoid elevated sodium levels in softened water. However, potassium chloride is more expensive and somewhat harder to find than sodium chloride. In addition, potassium adheres more strongly to the resin, so it may reduce the exchange efficiency. The water treatment industry views potassium chloride as a possible substitute for sodium chloride when the latter is not allowed for health-related or environmental reasons. Potassium chloride is not thought to be a feasible replacement for sodium chloride in all situations.

Regeneration does not completely restore the exchange capacity of the resin, so the softeners effectiveness will gradually decrease. Exhausted resin should be replaced. Another alternative is to ask water treatment professionals for guidance on improving exchange capacity with a strong acid or base solution.

Sediment can clog the exchange resin, nozzles, and orifices in softeners and reduce regeneration effectiveness. To prevent sediment buildup, pretreat water to remove turbidity, sediment, or particles of iron, manganese, or sulfur.

Iron fouling is a serious problem in water softeners. Water softeners remove dissolved iron, but once this iron is exposed to air or chlorine, it oxidizes and forms a solid that is trapped in the softener. If a softener becomes iron-fouled, commercially available products containing either sodium hydrosulfite or polyphosphates effectively clean the resin. It is important to note that phosphate use is banned in states bordering the Great Lakes and may be banned in other areas. Consult with a local health department for information on possible local regulations.

Because badly fouled exchange material is so difficult to clean, prevention is the best maintenance procedure. To lessen or help prevent iron fouling, prefilters can remove iron particles before softening. Another option is to use salt compounds that contain an iron-cleaning chemical. These compounds are commercially available and clean the exchange bed with each regeneration.

Slime, such as iron bacteria, sometimes forms in softeners. To prevent this buildup, disinfect water before it enters the softener. If using chlorine for disinfection, remember that chlorine oxidizes iron to a solid, which can lead to iron-fouling. If this happens, use filtration in addition to chlorination before water enters the softener. Chloramine also disinfects exchange material and has a lesser impact on the resin than chlorine bleach. Consult a water treatment professional for guidance.

Hydrogen sulfide, a noxious gas, is a problem in a softener only if the hydrogen sulfide mixes with iron, manganese, or copper. The resulting reaction creates a black deposit of metallic sulfide. If this occurs, remove any hydrogen sulfide before the water contacts the exchange material. Common softener cleaners will not clean a bed fouled by hydrogen sulfide; hydrochloric acid should be used instead. However, discuss this option with a water treatment professional before taking action to clean the hydrogen sulfide.

Special considerations for water softening

Regeneration places an additional hydraulic load on a septic system. It takes approximately 50 gallons of water to regenerate a softener. Certain states prohibit disposing of brine in an on-site sewage system. Exercise caution when disposing of regeneration water from a softener used for removing radium, as the water is radioactive. Check with local health officials for information on local regulations.

Whether to soften water is a matter of personal preference. While softening does reduce deposits in water heaters and other components of the water distribution system, it also adds sodium to water. The amount of sodium added is small when compared to the salt in a typical daily diet. People on low-sodium diets should, however, consult a physician before drinking softened water. If necessary, the added sodium can be removed at the kitchen tap for drinking and cooking using a reverse osmosis 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 cation exchange 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.