ONLY DISSOLVED CONTAMINANTS
When contaminants dissolve in water, they typically form ions. Ions are electrically charged portions of a compound. There is a balance of positively and negatively charged ions in natural waters. When contaminants are dissolved in water, the water is typically crystal clear. If the water is cloudy or otherwise discolored, it is likely that some, or all, of the contaminants are in a solid form. Solid particles are not removed by IE and will clog the treatment media.

The Exchange of Ions
The electrical charge on an ion can be either positive (+) or negative (-). Valence is the term that describes the category of the electrical charge on a dissolved ion such as positive 2 or positive 3. If the contaminant has a positive charge, it would be called a cation, and would be removed by use of an IE media called a cation exchange resin. If the contaminant has a negative charge, it would be called an anion, and the appropriate treatment media would be called an anion exchange resin.

THE WATER SOFTENING PROCESS
A water softener at a private home typically has two tanks. The taller tank contains the purifying media called a "cation" exchange resin, while the smaller tank contains the sodium or potassium salt used to regenerate the resin media. During normal operations, raw water passes through the ion exchange resin media in the tall tank. The calcium (Ca⁺⁺), magnesium (Mg⁺⁺),
iron (Fe⁺⁺), or manganese (Mn⁺⁺) ions in the water are typically "exchanged" for sodium (Na⁺) or potassium (K⁺) ions, which have been temporarily stored in the "pores" of the resin during the previous regeneration cycle. In fact, any contaminant ion of valence positive 2 or greater will be removed in a water softener.

As the softener removes hardness minerals from the water, sodium or potassium will be given back proportionally. Shown below is the concentration of either sodium or potassium that would be added to the existing raw water concentration, if 10 mg/L of hardness is removed.
 

CATEGORIZING HARDNESS
Water treatment professionals use different terminology to categorize hardness in drinking water, as shown below. DES uses the terminology used by sanitary engineers.
 

EXPRESSING THE AMOUNT OF HARDNESS IN WATER
There are two terms which are used by drinking water professionals to identify the concentration of hardness in drinking water. They are:

To convert a hardness concentration from one set of units to the other, use one of the following formula:

• the concentration in milligrams per liter x ¹/_17.2 = the concentration in grains per gallon
• the concentration in grains per gallon x 17.2 = the concentration in milligrams per liter

Equivalent Concentration as CaCO₃
The concentration of hardness in water is normally reported as a equivalent concentration of calcium carbonate (CaCO₃). This laboratory calculation provides a common reference for the reactive power of various contaminants regardless of their atomic weight or valences. Thus, the typical laboratory units used for expressing hardness are "mg/L as calcium carbonate (CaCO₃)". The equivalent reactive concentrations for various cations are shown below.

REGENERATION
Eventually the removal capacity of the IE resin becomes exhausted and will need to be regenerated. The regeneration process typically begins by a rapid backwashing of the resin to remove fine particles that have been strained out of the water during the production (i.e. service) portion of the treatment cycle. This rapid backwash provides a physical cleaning of the outside of the media, but does not regenerate the resin's IE contaminant removal capability.

The backwash flow rate is then significantly reduced and brine (salt dissolved in water) is added. The sodium or potassium from the brine permeates the resin pores and displaces the previously removed contaminants. After approximately 20 minutes, the remaining brine, along with the concentrated, displaced contaminant ions are flushed out of the resin tank and disposed of into an approved dry well, septic tank, or sewer.

REDUCTION OF SALT USAGE
Being a good steward of the environment involves reducing excessive brine waste wherever possible. Salt brine can contaminate the general groundwater, and possibly that of your own well. Consequently, reducing salt usage while maintaining water softener effectiveness, should be an important goal of all users of water softeners. In areas without sewers, the more reduction of salt discharge, the less the possibility of contaminating groundwater.

Four methods can be used to reduce the amount of salt brine used when regenerating IE resin.

1. Initiation of the Regeneration Cycle
Older water softeners use a time clock to initiate the regeneration of the resin media. Modern softeners, however, regenerate by either of the following methods:

• A probe that measure the water's electrical conductivity.
• A meter that measures the volume of water already treated during the production cycle.

When using demand regeneration, the IE device can begin regeneration at any time of the day even when water is being actively used in the home. When this happens, the IE device goes into a bypass mode, and untreated water must be used within the home. This untreated condition, although a disadvantage of the demand mode, is of short duration. In response to the bypass of treatment, some manufactures are now producing softeners with dual media tanks. While one tank is being regenerated the other is available to produce treated water.

2. Strength of Brine Used to Regenerate
The regeneration of a water softener can be carried out using different strengths of brine solution. This is achieved by installing the properly rated venturi siphon insert at the time the system is installed. From an environmental view point, those IE devices with a higher efficiency of contaminants removed per pound of salt used, known as weaker brine regeneration, are the more appropriate to use.

The following summarizes the choice relative to the strength of brine versus the size of the treatment device:

a. The weak brine regeneration alternative; – recommended by DES. This method uses approximately 6-7 pounds of salt to regenerate each cubic foot of IE resin media.

4. Partial Treatment (Split Flow)
In this method a portion of the raw water bypasses the treatment process. This typically requires throttling valves and meters on both the treatment and the by-pass plumbing lines. A hardness target concentration could be approximately 50-75 mg/L in the blended treated water. This approach is not practical where iron or manganese are present in high concentrations. This process reduces both waste brine and sodium levels in the finished water.

AVOIDANCE OF SODIUM
Sodium is not regulated as a drinking water health contaminant, and thus its presence is only of strict importance to those on a doctor mandated no-salt diet.

If a source of untreated water with low sodium is desired in the home, untreated water can be provided to an additional faucet located at the kitchen sink. Sodium can also be avoided by use of potassium chloride to regenerate the unit.

DISPOSAL OF WASTES TO LEACHFIELD
There is often concern that septic tanks or disposal field will be harmed by brine waste. Studies by the Water Quality Association (WQA) indicate that waste brine and purged contaminants do not injure leach fields or septic tanks. This WQA report is available from DES. If concern remains, a separate dry well can be constructed on the user's property to dispose of the brine waste from the regeneration of the softener.

PREFERENCE SEQUENCE
1. Affinity Sequence For Cation and Anion Resins.
IE media have an affinity for certain contaminants over others. The strength of this selective affinity is governed by two factors:

a. The principal factor affecting the strength of the affinity is the valence of the contaminant. Thus, aluminum with a plus three valence will be more strongly held than calcium with a plus two valance. The higher the valence of the ion, the stronger the affinity of the media for that contaminant.

b. The second and less strong affinity factor is approximated by the weight/ size of the contaminant ion. The higher the weight of the ion, the higher the affinity of the resin for that contaminant.

3. Exception to the Affinity Sequence.
The affinity sequence holds for normal commercial grade IE resins at normal contaminant concentrations. When the contaminant is present at a much higher concentration, the affinity sequence will be effected, and the less preferred contaminant at high strength will be exchanged significantly. Regeneration, using sodium (Na+) or potassium (k+), is the ultimate example of how a high concentration of a low affinity ion can override natural selectivity sequence of ions.

4. Dumping
Dumping is a serious limitation of an IE process. Dumping is related to the affinity sequence and to the relative concentration of the contaminant in the raw water. To illustrate the importance of dumping, suppose there are two contaminants that are both initially exchanged in the treatment process. As treatment continues, the more highly favored contaminant will be better attracted to the exchange sites of the resin than the less preferred ions presently there.

As the production cycle continues, this highest preference contaminant would push off other low affinity contaminants. As a result, the lower preference contaminant will be dumped and would increase its concentration in the finished water above the raw water concentration. This concentration of the less preferred contaminant could exceed a water quality standard near the end of each production run.

Understanding the dumping phenomenon is critical when operating an IE system, and choosing which contaminants will govern the setting for the initiation of the regeneration of the treatment process.

OTHER ION EXCHANGE TOPICS
1. Broad Treatment. IE is a very broad treatment technology, where numerous other contaminants are removed, as well as the target contaminant. Thus, beneficial contaminants may be removed and excess treatment, other than intended, is carried out.

2. Solids Pretreatment. Solids will clog an IE treatment media preventing efficient exchange of ions. Prefiltration may be needed to remove solids.

3. Media Age. Every time the IE media is regenerated the resin is subject to great chemical stress caused by the high concentrated of brine. This constant compression and expansion weakens and destroys the IE resin with time. After many years, the resin may need to be replaced. The resin's remaining capability can be evaluated by having a water treatment professional determine the whole beed count and the percent of moisture of the IE media.

4. Loss of Media. If the rate of backwash of the device is too high, media may be washed out of the tank. This loss of media will reduce the effectiveness of the treatment. Temperature changes effects backwash rates. In some fiberglass tanks, the media depth can be identified by shining a strong light through the tank in an otherwise darkened room, and noting the shadow that represents the depth of the remaining IE media.

5. Anion Exchange Resin. Earlier in this document, we illustrated the process for cation exchange. Many other contaminants occur as anion, such as nitrates (NO₃), sulfates (SO₄), and arsenic compounds. To remove multivalent anions, an anion, rather than cation resin is used. Like the cation resin, the anion resin is regenerated by salt, although the actual regeneration is by the chloride ion rather than the sodium ion.

6. Deionization. In its more general form, IE can also be called deionization. This treatment is a form of IE which typically targets sodium and chloride and other single valence ions from water. In these cases acids (H+) and bases (OH -) are typically used as the regeneration chemicals. Any excess H+ and OH- converts to water. Deionization is used for medical and industrial situations requiring very pure water. Deionization is not necessarily for drinking water treatment, and would produce very flat tasting water. Strong acids and bases are dangerous chemicals to use in a residential home.

SUMMARY
We suggest that you discuss this brine strength issue and long term salt savings with your equipment supplier. In general, DES does not recommend the use of softeners to treat only iron and manganese in non sewered areas due to brine disposal concerns. Where hardness is above approximately 125-150 mg/l, or where there are multiple contaminants treatable by softening are present, an IE processes is supported. For detailed information concerning iron and manganese treatment, see fact sheet WD-WSEB-3-7.

FOR MORE INFORMATION
For further information concerning the layout of a water treatment system and its purchase, DES suggests reviewing the Fact Sheet entitled, "Considerations when Purchasing a Water Treatment System" WD-WSEB-2-5.

For more information please call the DES Water Supply Engineering Bureau at 271-3139. We would appreciate hearing from you concerning improvements to this fact sheet and your experiences when treating for hardness. For an overall listing of water supply related fact sheets, please request fact sheet WD-WSEB-15-2. Drinking water fact sheets are available through the DES web site at: www.des.state.nh.us/wseb then select: fact sheets. Please check the internet annually for any updates. 2/01