Hard Water/ Soft Water
What is hard water?
As water moves through soil and rock, it dissolves
very small amounts of minerals and holds them in
solution. Calcium and magnesium dissolved in water
are the two most common minerals that make water
hard.
Where Hard Water is found in the U.S.
Effects of Hard Water
Soap scum
Scale in hot water pipes
Doesn’t rinse well
Doesn’t lather easily
Types of Hardness
• Temporary Salts that Cause Water Hardness:
Calcium Carbonate (CaCO3) - Known as limestone, rare in water
supplies. Causes alkalinity in water.
•
Calcium Bicarbonate [Ca(HCO3)2] - Forms when water containing CO2
comes in contact with limestone. Also causes alkalinity in water.
When heated CO2 is released and the calcium bicarbonate reverts
back to calcium carbonate thus forming scale.
•
Magnesium Carbonate (MgCO3) - Known as magnesite with
properties similar to calcium carbonate.
•
Magnesium Bicarbonate [Mg(HCO3)2] - Similar to calcium
bicarbonate in its properties.
Permanent Salts That Cause Water Hardness
Calcium Sulfate (CaSO4) - Know as gypsum, used to make plaster of paris.
Will precipitate and form scale in boilers when concentrated.
Calcium Chloride (CaCl2) - Reacts in boiler water to produce a low pH as
follows: CaCl2 + 2HOH ==> Ca(OH)2 + 2HCl
Magnesium Sulfate (MgSO4) - Commonly known as epsom salts, may have
laxative effect if great enough quantity is in the water.
Magnesium Chloride (MgCl2) - Similar in properties to calcium chloride.
Sodium salts are also found in household water supplies, but they are
considered harmless as long as they do not exist in large quantities. The US
EPA currently has no national policy with respect to the hard water levels
or soft water levels of public water supplies.
How to “Soften” Water
• Most conventional water-softening devices depend on a process
known as ion-exchange in which "hardness" ions trade places with
sodium and chloride ions that are loosely bound to an ion-exchange
resin or a zeolite (many zeolite minerals occur in nature, but
specialized ones are often made artificially.)
•
The illustration depicts a negatively-charged zeolite to which [positive] sodium
ions are attached. Calcium or magnesium ions in the water displace sodium
ions, which are released into the water. In a similar way, positively-charged
zeolites bind negatively-charged chloride ions (Cl–), which get displaced by
bicarbonate ions in the water. As the zeolites become converted to their Ca2+
and HCO3– forms they gradually lose their effectiveness and must be
regenerated. This is accomplished by passing a concentrated brine solution
though them, causing the above reaction to be reversed. Herein lies one of the
drawbacks of this process: most of the salt employed in the regeneration
process gets flushed out of the system and and is usually released into the soil
or drainage system— something that can have damaging consequences to the
environment, especially in arid regions. For this reason, many jurisdications
prohibit such release, and require users to dispose of the spent brine at an
approved site or to use a commercial service company.