How Do Water Softeners Work?

How Do Water Softeners Work?
Most water softeners on the market operate on an “ion exchange” system. This is where hardness
ions (calcium and magnesium) are exchanged for sodium (salt) or potassium (salt) ions. This takes
place within the resin tank of the water softener. When water flows through the tank it comes in
contact with small resin beads that are covered with sodium or potassium ions. As the water flows
through the resin beads, the calcium and magnesium ions trade places with the sodium ions. That
is why softened water becomes higher in salt content.
Eventually as larger volumes of water is softened the beads become exhausted and contain
nothing but calcium and magnesium, thus requiring recharge. That is why you need to put bags of
salt in the brine tank. The brine tank is either beside the resin tank or is packaged together with the
resin tank as a single unit. Recharging the mineral tank works exactly the same way as softening
the water except in reverse. The calcium and magnesium again swap with sodium or potassium
rich solution from the brine tank, and the excess minerals are rinsed into the wastewater drain.
A “demand initiated regeneration” or DIR water softener is the most common ion exchange softener
being sold locally. This system meters the water usage over time and only regenerates when
needed. Some softeners operate on a timer or schedule that regenerates at set increments. This
older technology can be very wasteful in terms of salt and water usage as the softener will
regenerate even if it is not required due to low consumption in the home over a given time period.
At the same time, these models could leave you short of soft water if you have higher consumption
due to house guests, etc.
Water Softener Components
1) Hard Water Supply
2) Soft Water to House
3) Drain
4) Timer-and-Valve Assembly
5) Mineral Tank
6) Outlet Manifold
7) Plastic Beads
8) Brine Tank
9) Salt
10) Float-and-Valve Assembly
1
2
4
8
3
7
9
5
6
10
What size of softener do I need?
Household water softeners are rated by the number of grains of hardness they can remove through
a specific volume of resin in the tank and the amount of salt needed to regenerate the resin.
Many retailers and product websites provide conflicting information about how to calculate the
appropriate size of softener for your household needs. Note that most, if not all, retailers in North
America quote U.S. measures for weights and volumes.
To know the size of softener you need, it is important that you define two things:
1.
2.
Your average daily household water use (not including outdoor watering)
The average grains per U.S. gallon of hardness in your tap water
An average person in the City of Guelph and Waterloo Region consumes about 186 and 227 litres
(49 and 60 US gallons) per day of water, respectively. To calculate your daily household
consumption, simply multiply the gallons used per day per person by the number of people in your
household. A more accurate way to measure your household water consumption is to use
fall/winter water bills to calculate your average indoor water usage per month. Then divide by 30 to
get your average daily water use. Most Ontario city water bills measure water consumption in cubic
metres (m3). A cubic metre of water is 1,000 litres, and one litre equals 0.264 U.S. gallons.
Note: don’t be fooled by salespeople that tell you if you have a dishwasher you should “add a
person” when calculating your daily water usage. Dishwashers in fact use less water than the
average household washing dishes by hand!
The next step is to know how hard your water is in grains per U.S. gallon (1 grain = 17.1 milligrams
per litre). City of Guelph and Waterloo Region residents connected to municipal water supply can
find the average water hardness in their neighbourhoods by clicking here.
People on private well need to have their hardness and iron tested separately to calculate their
hardness rating. Households receiving municipal drinking water will not be receiving high levels of
iron in their water, but people on private wells may need to increase their grain hardness rating to
compensate for iron in the water. For every 1 mg/l (milligrams per litre) of iron, 3 grains per gallon
(gpg) of hardness needs to be added to the total hardness value.
The formula to calculate your daily hardness removal requirement is detailed below.
Number of
people in your
home
Average indoor
daily water
consumption
per person
Grains of
hardness in
your water
Grains of
hardness to be
removed daily
Most suppliers recommend people buy a water softener that could potentially remove hardness
from the water for seven days before needing to regenerate. So you should take your daily
hardness removal requirement and multiply by seven to get the water softener capacity needed.
Using the formula above, a family of three using 60 gallons per person of water with a hardness of
23 grains per gallon will need a softener that can remove 4,140 grains of hardness per day. Over
seven days, the softener would need to remove 28,980 grains of hardness.
Although water softener sizing can vary, the table below gives some of the sizes of softener
available for a range of daily grain requirements.
Daily Grain Removal Requirements
Suggested Softener Size
0 - 3,500 grains per day
24,000 grain capacity
3,501 - 4,500 grains per day
32,000 grain capacity
4,501 - 5,750 grains per day
40,000 grain capacity
5,751 - 6,850 grains per day
48,000 grain capacity
The Region of Waterloo and City of Guelph are continuing to investigate the capacity and
performance of water softeners through local testing. What is clear is that knowledgeable suppliers
will set up a water softener to regenerate before resin tanks are fully exhausted to maximize salt
efficiency, and to make sure household water stays soft until the evening regeneration time (units
with single resin tanks). An example would be that a 32,000 grain capacity water softener, which
comes with 1 cubic foot of resin, will work more efficiently if it regenerates after 20,000 grains of
hardness has been removed. One supplier states that it takes 5-7 pounds of salt to regenerate at
20,000 grains removed, compared with 36 pounds of salt needed to regenerate the system at
30,000 grains removed!
What are the differences between water softener salts?
There are generally three different types of water softener salts available:
ROCK SALT
SOLAR SALT
EVAPORATED SALT
Salt produced by the
evaporation of seawater
Refined and completely
purified salt, such as table salt
Rock salt is cheaper than solar salt or evaporated salts but contains more insoluble material, as a
result solar and evaporated salts keep your water softener cleaner. The type of salt you choose to
buy depends on the frequency of regeneration. If your water softener regenerates a lot you will
experience quicker build-up of non-soluble matter if you use rock salt rather than solar or
evaporated salt. However, for the average homeowner this will not be a concern as the excess
material is washed away during regeneration.
Potassium chloride is an alternative to sodium chloride for those concerned about the sodium
content in their drinking water. Potassium chloride is slightly more expensive to buy but works in the
same way as regular water softening salt. Potassium chloride is safe for most water softeners; but
consult your owner’s manual before switching.
Are there alternative technologies for treating hard water?
Some retailers sell technologies that are supposed to prevent scale building up in household
appliances without the use of salt and sometimes water. Most of these alternative technologies are
not designed to “soften” water by removing calcium and magnesium, but may change the
properties of suspended solids in water to stop scale from forming on water heater elements, taps,
etc. In some cases the technologies will still require backwashing.
There is currently very little information published by independent agencies to verify the
effectiveness of new technologies offered as alternatives to the traditional ion exchange softeners.
It is hoped that results from a study to be published this fall by the U.S. Water Reuse Research
Foundation (WRRF-08-06) will clarify the effectiveness of the four technologies listed below.
•
•
•
•
Electrically Induced Precipitation
Electromagnetic Water Treatment
Capacitive Deionization (CDI)
Template Assisted Crystallization
For more details and to get study results when available, go to www.watereuse.org/foundation