Lime softening involves a relatively
series of chemical reactions which will be discussed in depth
The goal of all of these reactions is to change the calcium and
magnesium compounds in water into calcium carbonate and magnesium
hydroxide. These are the least soluble calcium and magnesium
and thus will settle out of the water at the lowest
For example, calcium carbonate (which is essentially the same as
limestone) will settle out of water at concentrations greater than 40
In order to produce calcium carbonate and magnesium hydroxide, the pH
the water must be raised by the addition of lime. Calcium
water will be removed at a pH of about 9.0 to 9.5 while magnesium
compounds require a pH of 10.0 to 10.5. When soda ash is used to
remove noncarbonate hardness, an even higher pH is required - 10.0 to
10.5 for calcium compounds and 11.0 to 11.5 for magnesium
Carbon Dioxide Demand
The first step in lime softening is the addition of lime to water using
a typical dry feeder, either volumetric or gravimetric. As in the
chlorination process, lime reacts with substances in the water before
it can begin softening the water. Carbon dioxide is the primary
compound which creates the initial demand for lime. The following
reaction occurs, using up carbon dioxide and lime and creating calcium
carbonate and water:
Carbon dioxide + Lime → Calcium carbonate +
CO2 + Ca(OH)2 → CaCO3 + H2O
The resulting calcium carbonate precipitates out of solution.
When water, especially groundwater, has a high carbon dioxide
concentration, the water is often pretreated with aeration before
begins. Aeration removes the excess carbon dioxide and lowers the
Removal of Carbonate Hardness
Once the carbon dioxide demand has been met, the lime is free to react
with and remove carbonate hardness from the water. Calcium
react with lime in the reaction shown below.
Calcium bicarbonate +
→ Calcium carbonate + Water
Ca(HCO3)2 + Ca(OH)2 →
2CaCO3 + 2H2O
We have focussed on calcium bicarbonate since it is the most common
calcium compound in water, but other calcium-based hardness compounds
have similar reactions. In any case, the calcium carbonate
able to precipitate out of solution.
Magnesium compounds have a slightly different reaction. First,
magnesium bicarbonate reacts with lime and produces calcium carbonate
(which precipitates out of solution) and magnesium carbonate.
Magnesium bicarbonate +
Lime → Calcium carbonate +
Magnesium carbonate + Water
Mg(HCO3)2 + Ca(OH)2 → CaCO3
+ MgCO3 + 2H2O
Then the magnesium carbonate reacts with lime and creates more calcium
carbonate and magnesium hydroxide. Both of these compounds are
precipitate out of water.
Magnesium carbonate +
Lime → Calcium carbonate + Magnesium
MgCO3 + Ca(OH)2 → CaCO3 +
Removal of Noncarbonate Hardness
many cases, only the carbonate hardness needs to be removed, requiring
only the addition of lime. However, if noncarbonate hardness also
needs to be removed from water, then soda ash must be added to the
water along with lime.
Magnesium sulfate + Lime
→ Magnesium hydroxide + Calcium sulfate
Each noncarbonate hardness compound will have a slightly different
reaction. Here, we will consider the reactions of magnesium
The lime first reacts with the magnesium sulfate, as shown below:
MgSO4 + Ca(OH)2 → Mg(OH)2
The resulting compounds are magnesium hydroxide, which will precipitate
out of solution, and calcium sulfate. The calcium sulfate then
with soda ash:
Calcium sulfate + Soda Ash → Calcium
carbonate + Sodium
CaSO4 + Na2CO3 → CaCO3
The calcium carbonate resulting from this reaction will settle out of
the water. The sodium sulfate is not a hardness-causing compound,
it can remain in the water without causing problems.
The reactions which remove carbonate and noncarbonate hardness from
water require a high pH and produce water with a high concentration of
dissolved lime and calcium carbonate. If allowed to enter the
distribution system in this state, the high pH would cause corrosion of
pipes and the excess calcium carbonate would precipitate out, causing
scale. So the water must be recarbonated,
which is the process of stabilizing the water by lowering the pH and
precipitating out excess lime and calcium carbonate.
The goal of recarbonation is to produce stable water,
which is water in chemical balance, containing the concentration of
calcium carbonate in which it will neither tend to precipitate out of
the water (causing scale) nor dissolve into the water (causing
corrosion.) This goal is usually achieved by pumping carbon
into the water. Excess lime reacts with carbon dioxide in the
below, producing calcium carbonate:
Lime + Carbon dioxide
Calcium carbonate + Water
Ca(OH)2 + CO2 → CaCO3 + H2O
Recarbonation also lowers the pH, which encourages the
precipitation of calcium carbonate and
Recarbonation may occur in one step, in which the pH is lowered to
about 10.4 and carbonate hardness is precipitated out. In some
a second recarbonation step is used to lower the pH to 9.8 and
encourage yet more precipitation. In either case, the process
carefully controlled since carbon dioxide can react with calcium
carbonate and draw it back into solution as calcium bicarbonate,
the softening process.
Alternatively, recarbonation can be achieved through the addition of
acids such as
sulfuric or hydrochloric acids or through polyphosphate addition.
These types of recarbonation work differently from carbon dioxide
In The Treatment Process
Lime softening uses the equipment
already found in most treatment plants for turbidity removal. An
overview of the lime treatment process is shown below.
Lime softening produces large quantities of sludge. In fact, for
every pound of lime used, about two pounds of sludge are formed.
The softening process usually requires two sedimentation basins, each
with a detention time of 1.5 to 3 hours, to deal with the large
quantities of sludge. One sedimentation basin handles the sludge
resulting from lime and soda ash softening and the other sedimentation
basin deals with the sludge resulting from recarbonation.
Disposal of lime sludge is the same as for sedimentation basin
sludge. Landfill disposal is the most common method, although
sludge may sometimes be sent to sanitary sewers. Lime sludge has
high pH and has increasingly been disposed of by applying it to
agricultural land to increase the pH of acidic soils.
If softening problems are discovered, the cause usually lies in either
chemical feeder malfunctions or source water quality changes. A
variety of water characteristics can influence lime-soda ash softening:
These four water characteristics should be monitored carefully when
softening water using lime. In addition, coagulants used to
influence the alkalinity or pH of the water, thus affecting the
softening process. After softening, the Langelier Index of
the water should be tested to ensure that the water is not
study the Langelier index and corrosive water in more depth in the next
- Water hardness
will determine the quantity of chemicals which must be added to soften
- pH influences the
chemical reactions in the softening process. A higher pH makes
the process more efficient.
- Alkalinity determines whether the
hardness in the water is carbonate or noncarbonate hardness.
influences the rate of the reaction and the amount of hardness which
the water will hold.
Softening is especially well-suited to treating groundwater since
groundwater characteristics tend to remain relatively constant.
Changing water conditions require a great deal of manipulating the
softening process to keep it efficient. In addition, the high
turbidity found in surface water sometimes requires presedimentation
prior to softening.
Chemicals Used in Lime Softening
Types of Lime
The lime used for softening comes in two forms - hydrated lime and
quicklime. Both types of lime soften water in the same way, but
equipment required for the two types of lime is different.
Hydrated lime (Ca(OH)2)
is also known as calcium hydroxide or slaked lime. Hydrated
be added to water as it is without requiring any special equipment, so
it is a popular choice for small water treatment plants.
In contrast, quicklime (CaO),
also known as calcium oxide or unslaked lime, must be slaked before it
is used. Slaking is the
process of converting quicklime to hydrated lime by adding water, as
Calcium oxide + Water
CaO + H2O → Ca (OH)2
Slaking requires specialized equipment. The cost of equipment and
the operator time required to run the equipment usually make quicklime
uneconomical in small plants. However, since the chemical cost of
quicklime is less than the cost of hydrated lime, quicklime is often
used in large plants.
The slaking process can also allow a large plant to reuse a large
quantity of the lime sludge produced in the softening process.
the sludge is heated, and the calcium carbonate in the sludge produces
Calcium carbonate → Calcium oxide + Carbon
CaCO3 → CaO + CO2
Then the calcium oxide can be slaked and reused in the plant.
lime sludge cuts down on both chemical purchase and sludge disposal
Lime Handling and Storage
Operators should observe safety procedures while handling both hydrated
lime and quicklime. Lime dust can be harmful when it comes in
with the eyes, nose, or mouth, and skin contact can cause burns.
result, operators should wear goggles and dust masks as well as
Both hydrated lime and quicklime can deteriorate in quality over time
while in storage. In addition, storing quicklime can cause safety
problems. If quicklime comes in contact with water, it begins to
slake, a process which produces a great deal of heat and can cause
explosions when uncontrolled. Quicklime should never be stored
alum since the quicklime will absorb water away from the alum and cause
Soda ash (Na2CO3) comes in only one form and does
not require any treatment before it is added to the water. Safety
issues resemble those for lime handling. Soda ash dust
irritates the eyes and mucous membranes of the nose, so the operator
should wear protective clothing, goggles, and a dust mask. In
areas in which soda ash is used should be equipped with a ventilation
system to deal with the dust.
Caustic soda (NaOH), also
as sodium hydroxide, can replace soda ash and some of the lime in the
treatment process. The treatment process using caustic soda
the same steps as that of lime-soda ash softening.
First, carbon dioxide reacts with the caustic soda to make sodium
carbonate and water.
Carbon dioxide +
→ Sodium Carbonate + Water
CO2 + 2NaOH → Na2CO3 + H2O
Then the remaining caustic soda can react with calcium bicarbonate
and magnesium bicarbonate.
Calcium bicarbonate +
→ Calcium carbonate + Soda ash + Water
Ca(HCO3)2 + 2NaOH → CaCO3
+ Na2CO3 + 2H2O
Magnesium bicarbonate + Caustic soda →
+ Soda ash + Water
Mg(HCO3)2 + 4NaOH → Mg(OH)2
+ 2Na2CO3 + 2H2O
Magnesium sulfate + Caustic soda → Magnesium
The caustic soda can also react with
magnesium noncarbonate hardness, as shown below. Also note
reactions between caustic soda and carbonate hardness produced soda
ash, which can react with noncarbonate hardness as well.
MgSO4 + 2NaOH → Mg(OH)2 + Na2SO4
Caustic soda has the advantages of stability in storage, lower sludge
formation, and easy handling. However, safety issues still
Caustic soda is dangerous to the operator and can cause severe
burns to the skin. As a result, rubber gloves, dusk masks,
and a rubber apron should be worn while handling the chemical.