WTRG18 Water Softening and 5/1/15
Hardness
Water Softening for
Hardness Removal
Hardness in Water
High concentration of calcium (Ca2+) and
magnesium (Mg2+) ions in water cause hardness
Generally, water containing more than 100 mg/l of
hardness expressed as calcium carbonate (CaCO3)
is considered to be hard
Excessive hardness is undesirable because it causes
the formation of soap curds, increased use of soap,
deposition of scale in boilers, pipelines and home
appliances, damage in industrial processes and can
cause objectionable tastes.
Methods of Removing
Hardness
Treatment Method Hardness Levels
Retained
Lime Softening Solubility Level of
(Chemical Precipitation) about 35 mg/l (CaCO3)
RO (Nanofiltration) 85 – 90% removal
(Membrane Filtration)
Ion Exchange Basically Zero
(Chemical Exchange) Water must be blended
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WTRG18 Water Softening and 5/1/15
Hardness
Hardness Descriptions
Description Hardness
(mg/l of CaCO3)
Extremely soft to soft 0 – 45
Soft to moderately hard 46-90
Moderately hard to hard 91-130
Hard to very hard 131-170
Very hard to excessively hard 171-250
Too hard for ordinary domestic Over 250
use
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Important Definitions in
Lime Softening Treatment
HARDNESS is caused mainly by the salts of calcium and magnesium, such as
bicarbonate, carbonate, sulfate, chloride, and nitrate
– CALCIUM HARDNESS is caused by calcium ions (Ca2+)
– MAGNESIUM HARDNESS is caused by magnesium ions (Mg2+)
TOTAL HARDNESS is commonly measured by titration and is described in two ways:
– The sum of the hardness caused by both calcium and magnesium ions, expressed as CaCO3
– The sum of the carbonate (temporary) and noncarbonate (permanent) hardness
CALCIUM CARBONATE (CaCO3) EQUIVALENT is an expression of the concentration of
a chemical in terms of their equivalent value to calcium carbonate. It allows us to
express different chemicals in “apples to apples” terms
CARBONATE HARDNESS is caused by alkalinity present in the water up to the total
hardness. It is the total measure of the waters alkalinity. (Carb hardness = alkalinity)
NONCARBONATE HARDNESS is that portion of the total hardness in excess of the
alkalinity. Requires use of both lime and soda ash to remove.
ALKALINITY is the capacity of water to neutralize acids. Alkalinity is a measure of how
much acid must be added to a liquid to lower the pH to 4.5. This capacity is caused
by the water’s content of bicarbonate, carbonate, hydroxide, and occasionally borate,
silicate, and phosphate. *These are loosely grouped in with CARBONATE HARDNESS 5
Categories of Hardness
Calcium Magnesium
Temporary = Alkalinity Temporary = Alkalinity
Carbonate Carbonate
(the scale you normally see – (the scale you normally see –
comes out by boiling). comes out by boiling).
Limestone contributes to this. Limestone contributes to this.
Noncarbonate Noncarbonate
(sulfate, chloride,
(sulfate, chloride,
nitrate)
nitrate)
Permanent Permanent
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WTRG18 Water Softening and 5/1/15
Hardness
Benefits of Lime Softening
Removal of Ca and Mg hardness
Removal of iron, manganese, arsenic
and uranium.
Reduction of solids, turbidity and TOC
Removal and inactivation of bacteria
and viruses due to high pH.
Raises pH and prevents Corrosion
Removal of excess fluoride.
Types of Lime Used in WTP
Quick Lime (CaO) (molecular wt. 56)
– 75 % - 99% purity (typically 85%)
– Dry powder and must be slaked for 15 -
30 minutes at cold temperature
– Slaking (agitating with water) produces
CaOH2 (Calcium Hydroxide)
– Used at large WTP because lower cost
Hydrated Lime Ca(OH)2 (molecular wt. 74)
– Made when Quick Lime is slaked
– CaO + H2O Ca(OH)2 + HEAT
– Very stable
– Small WTPs use directly due to
Volumetric Multiscrew Feeder convenience
– Hydrated Lime shipped 80% - 99%
purity (typically 95%)
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Chemical Precipitation
Hardness causing ions are converted from soluble to
insoluble forms (Ca and Mg) at high pH
Addition of lime:
– increases the hydroxide concentrations, increasing the
pH
– Converts alkalinity from the bicarbonate form to the
carbonate form which causes the calcium to precipitate
out as CaCO3
– If more lime is added the phenolphthalein (P) alkalinity
increases to a level where hydroxide becomes present
(excess causticity) allowing magnesium to precipitate
as magnesium hydroxide.
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WTRG18 Water Softening and 5/1/15
Hardness
SUPERSATURATED
Following the softening process the
– pH is high
– Water is Supersaturated with excess caustic
alkalinity in either the hydroxide or
carbonate form
– Carbon dioxide can be used to decrease the
causticity and scale-forming tendencies of
the water prior to filtration
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Relationships among pH,
Alkalinity and Indicators
0% 100%
Bicarbonate and
Bicarbonate, HCO3- Carbonate
Carbonate and
CO2 Hydroxide
Total Alkalinity
T=0
Phenolphthalein Alkalinity
P=0
CaCO3 Mg(OH)2
pH 4.5 100% >10 >11
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Types of Alkalinity that can be
Present at pH Values
Addition of lime to water increases the hydroxide
concentrations, thus increasing the pH. It is important to
control pH in the finished water in a lime softening plant to
prevent scaling or corrosion.
Below 4.5 only CO2 present, no Alkalinity
Between 4.5 to 8.3, CO2 and Bicarbonate present
Above 8.3 alkalinity may consist of Bicarbonate,
Carbonate, and Hydroxide (no CO2 present)
When pH is greater than 8.3, the amount of titrant used
to reach pH 8.3 is the phenolphalein alkalinity
Between 10.2 to 11.3 Carbonate & Hydroxide
At 9.4 Calcium Carbonate becomes insoluble and
precipitates
At 10.6 Magnesium Hydroxide becomes insoluble and
precipitates 12
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WTRG18 Water Softening and 5/1/15
Hardness
Chemical Titration with Methyl
Orange (T) and Phenolphthalein (P)
Methyl Orange is used to Phenolphthalein is used to
determine the combination determine the carbonate
of alkalinity provided by and hydroxide alkalinity
carbonate, bicarbonate and present.
hydroxide or Total Alkalinity. A sample of the water is
A sample of the water is titrated by adding the
titrated by adding the Methyl Phenolphthalein color
Orange color indicator and indicator and adding
adding measured amount of measured amount of acid
acid until the color is absent. until the color is absent.
The Total Alkalinity (T) is The Hydroxide and
then computed. Carbonate Alkalinity (P) is
then computed
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Calculation of Alkalinity Constituents in Drinking Water
based on Methyl Orange and Phenolphthalein Titrations
Equation Titration Bicarbonate Carbonate Hydroxide
# ph 4.5 to 8.3 >PH 8.3 >11.3
Result
1 P=0 T 0 0
2 P<½T T – 2P 2P 0
3 P=½T 0 2P 0
4 P>½T 0 2T-2P 2P-T
5 P=T 0 0 T
Key: P – phenolphthalein alkalinity; T – total alkalinity
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Hardness Relationship to Alkalinity
TH = CH + NCH (each expressed as mg/l as CaCO3)
The amount of carbonate and noncarbonate hardness
depends on the alkalinity of the water
– Alkalinity > Total Hardness (all hardness is in
carbonate form)
TH = CH
– Alkalinity < Total Hardness (both Carbonate Hardness
and Noncarbonate Hardness are present)
CH = Alkalinity
NCH = TH - CH = TH - Alkalinity
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WTRG18 Water Softening and 5/1/15
Hardness
Calculate the total hardness in grains per
gallon for a water with a carbonate
hardness of 250 mgl/ and a non-
carbonate hardness of 160 mg/l.
From formula sheet
NCH = TH – CH
Therefore, TH = NCH + CH
TH = 160 + 250 = 410 mg/l
Change to grains per gallon
410 mg/l x 1 gr/gal = 24 gpg
17.1 mg/l
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Lime Softening Process Limitations
Unable to remove all carbonate (~ 30 mg/l) and no
Non-Carbonate hardness.
High degree of operator control.
Color removal for highly colored waters may be
hindered due to high pH.
Important to maintain proper pH to prevent scaling or
corrosion.
Sludge handling
and disposal are
costly (~ 2.5 mg/l
per mg/l lime added)
. 17
Primary Coagulants Sometimes
Used in Lime Softening
Acidic compounds: aluminum sulfate (alum),
ferrous sulfate, ferric sulfate, and ferric chloride.
– Increase lime demand
– Highly colored waters are best treated at low pH values
Basic compounds: sodium aluminate
– Lime required will be less
– High pH values will tend to set color
Cationic polymers
– Not very pH sensitive and often used in softening
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WTRG18 Water Softening and 5/1/15
Hardness
Secondary Coagulant Aids
Sometimes Used in Lime Softening
Coagulant aids often added to help stimulate
the production of floc.
They include sodium aluminate, bentonite or
clay, sodium silicate and various synthetic
cationic and non-ionic polymers.
Bentonite is often used in waters with high
color and low turbidity to bind with small floc
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Types of Lime Treatment
Excess Lime Treatment
– Removes Ca & Mg
– Produces an excessive amount of sludge
Partial Lime Softening
– Used for low Mg FL waters
– Only enough lime to remove calcium is added
– Sludge amounts reduced
Split Treatment
– Split Treatment is sometimes used when non-carbonate hardness is low and Mg
concentration are high
– CO2 requirements can be reduced or eliminated by blending the softened water
with an unsoftened flow stream
– Generally 80% treated and 20% raw
Non-carbonate Hardness removal using Lime-Soda
– When the water contains non-carbonate hardness (calcium sulfate) it requires the
addition of soda ash (sodium carbonate, Na2CO3)
– An alternative to the lime-soda ash process is the use of caustic soda (sodium
hydroxide, NaOH)
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Types of When water cannot be
softened to the desired level,
Treatment what action do you take?
STRAIGHT LIME TREATMENT Add soda ash or caustic soda
COAGULANT
LIME CO2
CLEAR
SOURCE MIX SETTLE SETTLE FILTER WELL
SODA ASH PO4
SPLIT LIME TREATMENT (OPTIONAL)
LIME CO2
SODA ASH
MIX CLEAR
SOURCE SETTLE MIX SETTLE FILTER
WELL
BYPASS
PO4
COAGULANT (OPTIONAL)
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WTRG18 Water Softening and 5/1/15
Hardness
Non-Carbonate Removal
First Identify the Amount of Non-Carbonate
Hardness
If Alkalinity > TH then CH = TH; NCH = 0
If Alkalinity < TH then CH = Alkalinity
Therefore NCH = TH - Alkalinity
The amount of Soda Ash (Na2CO3) required is
Equivalent to the Non-Carbonate Hardness
adjusted to its Calcium Carbonate Equivalent
Soda ash = NCH x (106/100)
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Recarbonation in
Lime Softening
Lime softened water is supersaturated
If not recarbonated, Ca and Mg carbonate will form on the
filters and distribution piping
Unused lime (calcium hydroxide and magnesium hydroxide) in
solution at high pH (11), must be converted to a stable forms.
First, CO2 is added to reduce Ca(OH)2 to CaCO3 which
precipitates at about pH 10
Additional CO2 is added to convert Mg(OH)2 to soluble
Mg(HCO3)2 which occurs at a pH of 8.4.
Reaction must be completed before filtration so that calcium
carbonate will not precipitate in the filters or carry into
distribution system.
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Summary
Hardness is from Calcium and
Magnesium ions in the water
(Ca2++Mg2+)
100 mg/L expressed as CaCO3 is
considered to be hard
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WTRG18 Water Softening and 5/1/15
Hardness
Summary
3 types of treatment for hardness
– Lime Softening
– Reverse Osmosis / Nanofiltration
– Ion Exchange
Carbonate hardness = alkalinity.
Noncarbonate hardness = excess over
alkalinity
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Summary
Alkalinity is the capacity of the water to
neutralize acid.
(The more alkalinity, the more acid you
have to add to lower the pH).
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