Water Softning Plant
Water Softening Plant
We call water "hard" if it contains a lot of calcium or magnesium dissolved in it.
A water softener reduces the dissolved calcium, magnesium, and to some degree manganese and ferrous
iron ion concentration in hard water.
These "hardness ions" cause three major kinds of undesired effects.
Most visibly, metal ions react with soaps and calcium-sensitive detergents, hindering their ability to lather
and forming a precipitate-the familiar "bathtub ring". Presence of "hardness ions" also inhibits the
cleaning effect of detergent formulations
Second, calcium and magnesium carbonates tend to precipitate out as hard deposits to the surfaces of
pipes and heat exchanger surfaces. This is principally caused by thermal decomposition of bi-carbonate
ions but also happens to some extent even in the absence of such ions. The resulting build-up of scale
can restrict water flow in pipes. In boilers, the deposits act as an insulation that impairs the flow of heat
into water, reducing the heating efficiency and allowing the metal boiler components to overheat. In a
pressurized system, this can lead to failure of the boiler.
Third, the presence of ions in an electrolyte, in this case, hard Water, can also lead to galvanic corrosion,
in which one metal will preferentially corrode when in contact with another type of metal, when both are
in contact with an electrolyte. However the sodium (or potassium) ions released during conventional
Water softening are much more electrolytic ally active than the calcium or magnesium ions that they
replace and galvanic corrosion would be expected to be substantially increased by water softening and
not decreased. Similarly if any lead plumbing is in use, softened water is likely to be substantially more
plumbo-solvent than hard water
Ion-exchange resin devices
Conventional Water-softening devices intended for household use depend on an ion-exchange resin in
which "hardness" ions trade places with sodium ions that are electrostatically bound to the anionic
functional groups of the polymeric resin. A class of minerals called zeolites also exhibits ion-exchange
properties; these minerals were widely used in earlier water softeners. Water softeners may be desirable
when the source of water is a well, whether municipal or private.
How it works?
The water to be treated passes through a bed of the resin. Negatively-charged resins absorb and bind
metal ions, which are positively charged. The resins initially contain univalent hydrogen, sodium or
potassium ions, which exchange with divalent calcium and magnesium ions in the water. As the water
passes through the resin column, the hardness ions replace the hydrogen, sodium or potassium ions
which are released into the water. The "harder" the water, the more hydrogen, sodium or potassium ions
are released from the resin and into the water.
Resins are also available to remove carbonate, bi-carbonate and sulphate ions which are absorbed and
hydroxyl ions released from the resin. Both types of resin may be provided in a single water softener.
As these resins become loaded with undesirable captions and anions they gradually lose their
effectiveness and must be regenerated. If a cationic resin is used (to remove calcium and magnesium
ions) then regeneration is usually effected by passing concentrated brine, usually of sodium chloride or
potassium chloride, or hydrochloric acid solution through them.
For anionic resins a solution of sodium or potassium hydroxide (lye) is used. If potassium chloride is used
the same exchange process takes place except that potassium is exchanged for the calcium, magnesium
and iron instead of sodium. This is a more expensive option and may be unsuited for people on
In mixed-bed deionizers the caution-exchange and anion-exchange resins are intimately mixed and
contained in a single pressure vessel. The thorough mixture of caution-exchangers and anion-exchangers
in a single column makes a mixed-bed deionizer equivalent to a lengthy series of two-bed plants. As a
result, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by
a two-bed plant.
Although more efficient in purifying the incoming feed water, mixed-bed plants are more sensitive to
impurities in the water supply and involve a more complicated regeneration process. Mixed-bed
deionizers are normally used to 'polish' the water to higher levels of purity after it has been initially
treated by either a two-bed deionizer or a reverse osmosis unit.
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