Ion Exchange

The use of a water softener unit is considered a satisfactory way to remove limited amounts of iron from water supplies. No hard and fast rules can be given on the amount of iron that can be treated. The answer in each case depends upon the design of the softener as well as a number of other variables. Softener manufacturers normally set the limits of tolerance for their equipment based on experience with the product design.

Ion exchange materials remove ferrous ions just as they do calcium and magnesium ions. Ion exchange techniques and materials can remove ferrous iron from water. Some experts in the water-conditioning field feel that this type of equipment will effectively treat ferrous iron in amounts comparable to the amounts of hardness cations that can be removed from the water. Others believe that the use of such equipment should be considered only where amounts of iron are extremely small, for example, less than 2 ppm.

(1)

(2)

(3)

(4)

(5)

(6)

Na+

›

Na+

+ Fe +

- R -

Na+

›

+

+

+

Na+

Na+

Na+

Fe++

2NaR

FeRNa

2Na+

FeR

2Na+

Ferrous ions

Sodium cation exchanger

Partially exhausted cation exchanger

Sodium ions

Exhausted cation exchanger

Sodium ions from salt

(7)

+

(8)

Na+

Na+

2NaR

Fe++

Sodium cation exchanger (partially regenerated)

Ferrous ions

The reactions involved in the removal of iron during a softening cycle and in the salt regeneration cycle are exactly the same as in the softening process.

NOTE: When the regeneration frequency of a softener is calculated and a significant amount of iron is pre­sent in the water, you must take this factor into account (usually add 3 to 5 grains per gallon of hardness for each part per million of iron). In no case should you allow the softener to become completely exhausted? In some cases, only one-half to three-fourths of the normal hardness capacity should be used before regeneration is begun to prevent iron fouling of the ion exchange resin.

NOTE: Some manufacturers set separate capacity ratings for iron. In such cases, you then base the regeneration frequency on the lower number of gallons calculated from the hardness and iron capacities.

If iron present in a water supply remained only in the soluble ferrous state prior to its passage through the softener (as in the case of the hardness producing magnesium and calcium ions), treatment could be more positive, and probably more satisfactory.

As it is, most iron-bearing waters contain amounts of insoluble ferric iron along with soluble ferrous iron. This oxidation of the iron may be caused by contact of the water with air during the pumping process or while it is stored in pressure tanks with air cushions. When water contains iron, it is essential to use a bladder-type pressure tank to prevent contact of air and water.

In any event, some of the iron is apt to have become insoluble before it enters the water softener. Now this insoluble ferric hydroxide can be removed by simple filtration as the softening process goes on. But its removal presents one serious problem. Trouble sets in when this ferric hydroxide begins to precipitate in the interstices of the resin bed. Even worse, ferric hydroxide (a jelly-like substance) may coat the resinous beads. If present in sufficient quantity, this coating may completely stop the interchange between the hardness minerals and the sodium ions.

Some of this ferric hydroxide is removed from the unit when the softener is backwashed. At the same time, some of the precipitated iron collects to form large particles which may sift down to the bed support during the backwashing! Such a situation can lead to pressure drop complications.

The better the filtration possible with a unit, the more effectively it removes precipitated iron. Good filtration depends on the particle size (grading of the resin), the resin bed depth, resin bed volume, and the operating rate of the flow through the bed. The smaller the crevices between the particles of the ion exchanger and the deeper the bed (within limits), the better the unit will filter precipitated iron, but the higher will be the pressure drop.

Assuming that all these conditions are at their best to provide for good filtration, the second requirement is the need to keep the resin bed clean. A clean ion exchange bed largely depends on frequent and thorough backwashing. The advent of the fully automatic units has brought a marked improvement in the treatment of certain iron-bearing waters because of their frequency of backwashing and regeneration. (Note: Not all fully automatic softeners are designed for use on iron-bearing water.) If the regeneration program is properly regulated, precipitated iron may not have time to "set" permanently within the resin bed.

As a safety precaution in the maintenance of a clean ion exchange bed, the periodic application of a chemical cleaner, along with the salt at the time of regeneration, is helpful. Some softener manufacturers recommend "preventative maintenance" with each regeneration. To accomplish this they suggest a bed-cleaning agent with salt. Polyphosphates, sterilizing agents, sodium bisulfite, sodium hydrosulfite, and other materials have been used to remove iron deposits. However, indiscriminate use of some of these materials may cause other problems. In each case, the softener manufacturer should be consulted for his recommendation. If a bed of a softener is fouled with large quantities of iron, professional servicing of the unit is recommended.

How much iron will a softener remove? There are differences of opinion here which you will have to evaluate on the basis of your equipment and local conditions. The ability of a softener to resist the fouling action of iron, especially in its ferric hydroxide state, varies with the design, type of exchange medium, and operating conditions, including pH, turbidity, organic matter, and other factors in relation to the water.

When iron content exceeds the maximum established by the individual softener manufacturer, some other type of corrective treatment must be brought into use.