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Did
you know?
Reverse osmosis is the finest water filtration method known. This process will
allow the removal of particles as small as ions from a solution. It is used
to purify water and remove salts and other impurities in order to improve the
color, taste or properties of the fluid. R.O. uses a membrane that is semi-permeable,
allowing the fluid that is being purified to pass through it, while rejecting
other ions and contaminants from passing.
This technology uses a process
known as crossflow to allow the r.o. membrane to continually clean itself. This
is the reason of why an r.o. element can last many years before clogging or
need replacement. This
water purification process requires a driving force to push the fluid through
the membrane, and the most common force is household water pressure or pressure
from a booster pump. The higher the pressure, the larger the driving force and
efficiency.
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LIME-SODA
ASH WATER TREATMENT METHOD
Lime-soda
ash treatment for the reduction of hardness involves the addition of slake
lime [Ca(OH)2] to a hard water supply to remove the carbonate
hardness by precipitation with the precipitation being removed by filtration.
Non-carbonate hardness is in turn reduced by the addition of soda ash
(Na2C03) to form insoluble precipitate which is
also removed by filtration.
This
particular method of removing hardness a sometimes used by municipal water
plants to reduce the amount of calcium and magnesium in a water supply.
While it is quite effective in reducing hardness, it is not a complete
removal treatment.
Often when a city has
a raw water source that has 35 to 40 grain hard water, the local water
system will use the lime-soda ash treatment to reduce hardness to between
5 and 10 grains.
Lime-soda ash treatment
is especially effective if a water contains bicarbonate (temporary) hardness.
Where calcium and magnesium are primarily in chloride or sulfate compounds,
this treatment is noticeably less effective.
Slaked lime is used
to remove calcium bicarbonate from water. In the water to be treated,
the slaked lime ions react with the calcium bicarbonate to form the very
slightly soluble calcium carbonate. This precipitated material is usually
removed by first settling and then filtering.
Ca(OH) 2+ Ca(HC03
) 2 --> 2 CaCO3
¥ + 2 H20
Calcium
hydroxide plus calcium bicarbonate reacts to form calcium carbonate plus
water
NOTE: The arrow pointing
down (¥) indicates the formation of an insoluble compound.
To remove the magnesium,
additional lime is used. The reaction for this process is:
Ca(OH) 2 + Mg -->
Mg(OH)2 ¥ + Ca++
Calcium
hydroxide plus magnesium ions react to form magnesium hydroxide plus calcium
ions
This step has simply
replaced the magnesium with calcium. If soda ash is then fed into the
water, the calcium will precipitate as calcium carbonate:
Ca++ + Na 2CO3 --> CaCO3 ¥
+ Na+
Calcium
ions plus sodium carbonate react to form calcium carbonate plus sodium
ions
There are many variants
possible under this general heading. Their discussion here, however, is
not essential to our course of study.
Lime-soda ash treatment
becomes increasingly costly when the hardness of the water must be reduced
to less than 5 grains. Municipally, the complete elimination of hardness
is rarely attemped as less than 5% of a municipality's water is used for
home consumption. The use of soda ash for the reduction of non-carbonate
hardness increases the sodium in the effluent water in the same proportion
as ion exchange softening.
The use of the lime-soda
ash treatment is impractical for individual home softening of supplies.
For one thing, there are difficulties in feeding lime and soda ash into
raw water. Further, close control of the operation is required both while
the settling and filtering occurs.
An additional deterrent to
home use of the lime-soda ash treatment is the size of the equipment necessary,
together with the high cost of this method of treatment.
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