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Advanced Water
WATER SOFTENING BY ION EXCHANGE
The presence of calcium (Ca) and/or magnesium (Mg) in water results in water being considered
"hard." Calcium and magnesium ions in water react with heat, metallic plumbing, and chemical
agents such as detergents to decrease the effectiveness of nearly any cleaning task. Hard water
can be softened using an ion exchange softening process. This article discusses the ion exchange
water softening process and related equipment used for household water treatment.

Contaminants removed by the water softening (ion exchange) process:
Calcium and magnesium ions are released into water as it dissolves rocks and minerals. An ion is
an atom or molecule that has a positive or negative electrical charge. These mineral ions in the
water can cause scale buildup in plumbing, fixtures and appliances and affect performance. In the
hot water heater, heat causes some removal of calcium carbonate and magnesium carbonate from
the water resulting in scale buildup. Scale buildup in the water heater can slow the heating process
and increase the energy usage. Cleaning agents used with hard water are not able to completely
remove dirt and grime. Clothes may become dingy and gray with time and feel harsh or scratchy,
and glassware may become spotted when dry. Films may be left on shower doors or curtains, walls,
and tubs, and hair washed in hard water may look dull and not feel clean. Hard water is considered
a nuisance problem. Removal of hardness ions is not necessary for health reasons.

The ion exchange water softening process can remove nearly all calcium and magnesium from
source water. Softeners may also remove as much as 5-10 ppm (parts per million; ppm is equal to
milligrams per liter, or mg/L) of iron and manganese. Consumers can check the manufacturer's
rating for removal of these contaminants. Water with iron and manganese concentrations greater
than the softener's removal rating may require pretreatment such as an iron/sulfur conditioner to
increase the lifespan of the softener.

Contaminants not removed by the water softening (ion exchange) process:
No one piece of treatment equipment manages all contaminants. All treatment methods have
limitations and often situations require a combination of treatment processes to effectively treat the
water. Water softening does not remove bacteria, hydrogen sulfide, silt or sand, lead, nitrate,
pesticides, and many other organic and inorganic compounds.

Water Testing
Regardless of the water treatment system being considered, water first should be tested to
determine which contaminants are present. Public water systems routinely are tested for
contaminants. Water utilities are required to publish Consumer Confidence Reports (CCRs), which
inform consumers on the source of the water, concentration of contaminants present, potential
health effects of those contaminants, and methods of treatment used by the utility. Depending on
the population served by the water utility, CCRs may be mailed, published in newspapers, or posted
on the Internet. Copies of the CCR can be obtained by contacting the local water utility. Public
supplies must conform to federal standards established by the Safe Drinking Water Act. If
contaminants exceed the Maximum Contaminant Level (MCL), the water must be treated to correct
the problem and/or another source of potable water must be provided.

In contrast, monitoring private water systems is the responsibility of the consumer. Therefore,
contamination is more likely to go undetected in a private water supply. Knowledge of which
contaminants may be present in the water should guide the testing, since it is not economically
feasible to test for all possible contaminants. It is essential to know which contaminants are present,
their concentrations, and reasons for removal (i.e., to reduce contaminants posing health risks, to
remove tastes or odors, etc.) prior to selecting treatment methods or equipment.

Testing laboratories, some city-county health departments, and some water treatment equipment
dealers can test water for hardness. It is also important to have water re-tested periodically after
installation of treatment devices to evaluate the effectiveness of the treatment equipment.

Water hardness is often expressed as grains of hardness per gallon of water (gpg) or milligrams of
hardness per liter of water (mg/L). The following table, adapted from the Water Quality Association
(WQA), shows hardness classifications. Hardness ions are typically combined with sulfate, chloride,
carbonate, or bicarbonate ions. For consistency, concentrations are generally converted to the
equivalent concentration as calcium carbonate (CaCO3) and expressed in terms of hardness as
calcium carbonate.

Classification of water hardness (hardness as calcium carbonate).












Treatment Principles
Household water softeners are ion exchange devices. Ion exchange involves the removal of the
hardness ions calcium and magnesium and replacing them with non-hardness ions. The non-
hardness ion is typically sodium supplied by dissolved sodium chloride salt, or brine. The softener
contains a microporous exchange resin, usually sulfonated polystyrene beads that are
supersaturated with sodium to cover the bead surfaces. As water passes through this bed, calcium
and magnesium ions attach to the resin beads and the loosely held sodium is released from the
resin into the water. The softening process is illustrated in Figure 1.

After softening a large quantity of hard water the beads become saturated with calcium and
magnesium ions. When this occurs, the exchange resin must be regenerated, or recharged. To
regenerate, the ion exchange resin is flushed with a salt brine solution (Figure 1). The sodium ions
in the salt brine solution are exchanged with the calcium and magnesium ions on the resin and
excess calcium and magnesium is flushed out with waste water.

Frequency of the regeneration or recharge cycle depends on the hardness of the water, the
amount of water used, size of the softener and capacity of the resins.  60 to 120 minutes generally
are required for the brine to pass through the unit and flush the tank before soft water is available
again.

Through the softening process, sodium is added to water at a rate of about 8 mg/L for each grain
per gallon (gpg) of hardness. After treatment, water that was 10 gpg of hardness will have about 80
mg/L of sodium. This means that for every liter (0.26 gallon) of water intake, there would be 80 mg
of sodium intake (By contrast, 1 cup of skim milk has about 125 mg sodium.). People on restricted
sodium diets due to health reasons should account for increased intake through softened water
and consult their physician. Drinking and cooking with softened water is often avoided by using a
reverse osmosis device that provides purified water to a dedicated faucet at the kitchen sink for
drinking, cooking and other uses.

It is not recommended to repeatedly use softened water for plants, lawns or gardens due to the
sodium content.

Equipment
Ion exchange water softeners can be classified as one of four different types:
  1. Semi-automatic: Semi-automatic units require the operator to initiate the regeneration cycle.
    The necessary steps to complete regeneration and return to service are done automatically
    by the softener controls.
  2. Automatic: Fully automatic softeners are typically equipped with a timer that automatically
    initiates the regeneration cycle and the required steps in that process. The operator only
    sets the timer and adds salt as needed. Regeneration is generally done during periods of low
    water usage, such as between midnight and 4 a.m.
  3. Demand initiated regeneration (DIR): Demand initiated regeneration units initiate and handle
    regeneration operations automatically in response to the demand for treated water. Need for
    regeneration is determined by measuring gallons of water used.  DIR units may have two
    softening tanks and a brine tank so that one tank can be softening while the other is
    recharging.  DIR units usually lead to less overall usage of salt and water since regeneration
    is only done when necessary.
  4. Off-site regeneration: Rental units are available in which the resin tank is exchanged in the
    home and then recharged at a central location.

All types of softeners must be correctly installed and monitored for proper operation. If an automatic
or DIR unit appears to be using more salt than expected or water is not softened, the unit settings
should be checked by a service provider. The amount of salt used for softening depends on the
number of people in the household, daily water usage, softener capacity, and water hardness. The
appropriate size of water softener depends on several factors including the water hardness level,
daily water use and water flow rate.

Types of Salt
There are different types of salt available for water softeners. The choice depends upon the type of
water softener and manufacturer design. Softeners are designed to use specific types of salt so it is
important to follow manufacturer's recommendations regarding the salt type.
Compressed salt has undergone an evaporation and compression process that yields small
particles referred to as pellets, nuggets, or beads. These are sized according to manufacturer's
specifications. The majority of softeners are designed for this type of salt.

Salt blocks for use in some softener brine tanks are similar to those used for livestock feeding.
Blocks for livestock feeding, however, may contain additives incompatible with water softener units.
Block salt should be used only in those units specifically designed for this form of salt. It should be
high grade evaporated salt without additives.

Rock salt may have high levels of soluble or insoluble impurities. If the impurities are calcium or
magnesium salts, they may significantly reduce the effectiveness of the regeneration process and
plug valves. Rock salt is only an option if suggested by the manufacturer and only those averaging
less than 1% impurities should be used.

In certain situations, potassium chloride may be used for softener regeneration instead of sodium
chloride. Potassium chloride can be more costly and more difficult to obtain than sodium chloride,
however. Potassium chloride also adheres more strongly to the resin. This can reduce the
exchange efficiency compared to sodium chloride and require more potassium chloride.
Substituting potassium chloride for sodium chloride may be appropriate if health or environmental
reasons necessitate restricting sodium; a water treatment professional should be consulted
regarding this option.

Maintenance
Maintenance of water softening equipment is somewhat dependent on the type of softener. Some
degree of monitoring or managing the regeneration process is generally required. The softener
must be kept regenerated to avoid hard water flowing into pipes and appliances. Regeneration
does place additional load on a septic system. Approximately 50 gallons of water (roughly equal to
that required for a load of laundry) is used to regenerate a water softener. Also, certain situations
such as high concentrations of iron or manganese in the water can affect the exchange capacity of
the resin. In this case the resin may eventually need to be cleaned or replaced. A water treatment
professional should be consulted for guidance on cleaning or replacing the resin.

The brine tank requires periodic checking and cleaning. Frequency of this cleaning depends on the
type and amount of salt and characteristics of the water being treated. The tank should be
inspected for scale build-up. The brine valve and float assembly, if used, should also be cleaned
and inspected at least once a year. Adequate backwashing of the bed is important to ensure
efficient regeneration of the unit. If backwashing is to be done manually or is semiautomatic, the
backwash should be continued until the water runs clear. If the unit is fully automatic and backwash
time is adjustable, adjust the time so the backwash is long enough to produce clear water in the
drain.

Excess iron (above 5 ppm) or hydrogen sulfide in the water can reduce the effectiveness of the
water softener. The water should be tested for these contaminants and proper pre-softening
equipment installed if required.  Sediment in the water can also reduce the effectiveness of the
softener.

Selection Requirements
Federal, state, or local laws do not regulate home water softening systems. The industry is self-
regulated. The NSF (formerly the National Sanitation Foundation) and the Water Quality
Association (WQA) evaluate performance, construction, advertising, and operation manual
information. The NSF program establishes performance standards that must be met for
endorsement and certification. The WQA program uses the same NSF standards and provides
equivalent American National Standards Institute (ANSI) accredited product certifications. WQA
certified products carry the Water Quality Association Gold Seal. Though these certifications and
validations should not be the only criteria for choosing a water softening system, they are helpful to
ensure effectiveness of the system.

In some situations when water hardness is low, hard water nuisance problems in the laundry may
be reduced by use of packaged water conditioners rather than by ion exchange softening.

Summary
Drinking water treatment by water softening (ion exchange) is one option for the homeowner in
treating water problems. Calcium and magnesium ions in water decrease the effectiveness of most
cleaning tasks and can cause film and scale buildup in and on plumbing and fixtures. Water
softening is an effective method for reducing the calcium and magnesium mineral ions from the
water. It can also reduce iron and manganese concentrations. Selection of a water softening unit
should be based on water analysis and assessment of the individual homeowner's needs and
situation. Regular maintenance of the unit is a critical factor in maintaining effectiveness of the unit.
NSF and the WQA test and certify products and this certification can help guide selection.
Classification   
mg/l or ppm   
grains/gal
Soft  
0 - 17.1  
0 - 1
Slightly hard   
17.1 - 60  
1 - 3.5
Moderately hard  
60 - 120
3.5 - 7.0
Hard
120 - 180  
7.0 - 10.5
Very Hard  
180 & over  
10.5 & over