ChE-413: Desalination and Water treatment
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Introduction
Hardness is an important water quality parameter in determining the
suitability of water for domestic and industrial uses
• Hard waters require considerable amounts of soap to produce foam
• Hard waters produce scale in hot-water pipers, heaters and boilers
•
Water Hardness and Softening
Ca2+ + 2HCO3- → CaCO3 (s) + CO2 (g) + H2O
Groundwater is generally harder than surface water
Principal cations causing hardness and the major anions associated with
them (in decreasing order of abundance in natural waters)
• Cations: Ca2+, Mg2+, Sr2+, Fe2+, Mn2+
• Anions: HCO3-, SO42-, Cl-, NO3-, SiO32-
Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
King Saud University
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Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
ChE-413: Desalination and Water treatment
Water Hardness & Softening
King Saud University
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Introduction
Hardness Definition
Total Hardness
• Technically - the sum of all polyvalent cations
• Practically - the amount of calcium and magnesium ions (the
predominant minerals in natural waters)
• It is divided into carbonate and noncarbonate hardness.
• It is divided into temporary and permanent hardness.
• It is divided into calcium and magnesium hardness.
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Hardness Units
milligrams per liter (mg/L) as calcium
carbonate (most common)
parts per million (ppm) as calcium carbonate
equivalents/liter (eq/L)
milliequivalents/liter (meq/L)
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King Saud University
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Water Hardness & Softening
Hardness Definition
Hardness Definition
Hard water - water that requires considerable amounts of soap
to produce foam or lather; also produces scale in hot water
pipes, etc.
Hardness Salt Common Name
Hardness Salt
Chemical Formula
Not a health concern, but $$$ concern
Calcium Bicarbonate
Ca(HCO3)2
1,620 (at 32 °F)
Magnesium Bicarbonate
Mg(HCO3)2
37,100 (at 32 °F)
Calcium Carbonate
CaCO3
13 (at 212 °F)
Magnesium Carbonate
MgCO3
75 (at 212 °F)
Noncarbonate Hardness
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Calcium Sulphate
CaSO4
1,246 (at 212 °F)
Calcium Chloride
CaCl2
554,000 (at 212 °F)
Magnesium Sulphate
MgSO4
356,000 (at 212 °F)
Magnesium Chloride
MgCl2
443,000 (at 212 °F)
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Water Hardness & Softening
King Saud University
Prof. Ibrahim S. Al-Mutaz
ChE-413: Desalination and Water treatment
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Hardness Level
Solubility at Indicated
Temperature (ppm CaCO3)
Carbonate Hardness
Caused by the presence of multivalent cations, mostly Ca2+
and Mg2+; (Fe2+, Mn2+, Sr2+, Al3+ may be present in much
smaller amounts).
Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Carbonate and Noncarbonate Hardness
Hardness expressed in mg/L as CaCO3
Water hardness is principally caused by:
- calcium ions
- magnesium ions
Total hardness = Carbonate hardness + Noncarbonate hardness
Carbonate hardness = (temporary hardness); eliminated at
elevated temperatures in boilers
Methods of determination
• Calculation:
Hardness (mg/L) as CaCO3 = [Ca++]+ [Mg++]
• EDTA (Ethylenediaminetertraacetic) titrimetric method
• Ca2+ + 2HCO3- → CaCO3 + CO2 + H2O
• Ca2+ + 2HCO3- + Ca(OH)2 → 2CaCO3 + 2H2O
Noncarbonate hardness = permanent hardness; can not be
removed or precipitated by boiling. Noncarbonate hardness
cations are associated with SO42-, Cl- and NO3-.
Water softening is needed when hardness is above 150-200 mg/L;
Hardness 50-80 is acceptable in treated water
Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
King Saud University
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Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
King Saud University
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Carbonate and Noncarbonate Hardness
Example: Hardness Calculation
Total Hardness (TH) - for practical purposes, this is the sum of the
calcium hardness and the magnesium hardness.
Calculate the hardness of a water sample with the following
analysis:
Total Hardness = Mg2+ hardness + Ca2+ hardness
UNITS
mg/l as CaCO3
Carbonate hardness - the portion of total hardness that is
chemically equivalent to the CO32- and HCO3- alkalinity present in
the water.
Cations
mg/L
Anions
mg/L
Na+
Ca2+
Mg2+
Sr2+
20
15
10
2
ClSO42NO3Alkalinity
40
16
1
50
Non-carbonate hardness - that hardness which is in excess of
carbonate hardness; will only occur in water where TH > alkalinity
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Chemical Engineering Dep.
King Saud University
Prof. Ibrahim S. Al-Mutaz
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Example: Hardness Calculation
A sample of water having a pH of 7.2 has the following
concentrations of ions
Ca2+
40 mg/L
Mg2+
10 mg/L
Na+
11.8 mg/L
K+
7.0 mg/L
HCO3110 mg/L
SO4267.2 mg/L
Cl11 mg/L
Calculate the TH, CH, NCH, Alkalinity
Prof. Ibrahim S. Al-Mutaz
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Example: Hardness Calculation
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Ion
Conc.
mg/L
M.W.
mg/mmol
Ca2+
Mg2+
Na+
K+
HCO3SO42Cl-
40.0
10.0
11.8
7.0
110.0
67.2
11.0
40.1
24.3
23.0
39.1
61.0
96.1
35.5
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Eq. Wt.
mg/meq
Conc.
meq/L
Conc.
mg/L as
CaCO3
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Periodical Table
Example: Hardness Calculation
Ion
Conc.
M.W.
mg/L mg/mmol
2+
Ca
40.0
2+
Mg
10.0
+
Na
11.8
+
K
7.0
HCO3 110.0
-
2-
SO4
Cl
67.2
11.0
n
40.1
24.3
23.0
39.1
61.0
2
2
1
1
1
96.1
35.5
2
1
Eq. Wt. Conc. Conc.
mg/meq meq/L mg/L as
CaCO3
20.05
12.15
23.0
39.1
61.0
48.05
35.5
Sample Calculation: Equivalent Weight of Ca2+ = M.W. / |n|
= 40.1/2 = 20.05
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Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Conc.
M.W.
mg/L mg/mmol
2+
Ca
40.0
2+
Mg
10.0
+
Na
11.8
+
K
7.0
HCO3 110.0
-
40.1
24.3
23.0
39.1
61.0
ChE-413: Desalination and Water treatment
Example: Hardness Calculation
n
2
2
1
1
1
Ion
Eq. Wt. Conc. Conc.
mg/meq meq/L mg/L as
CaCO3
20.05
1.995
12.15
0.823
23.0
0.51
39.1
0.179
61.0
1.80
Conc.
M.W.
mg/L mg/mmol
Ca2+ 40.0
2+
Mg
10.0
Na+ 11.8
K+
7.0
HCO3 110.0
-
2-
2-
SO4
Cl
67.2
11.0
96.1
35.5
2
1
48.05
35.5
SO4
Cl-
1.40
0.031
Prof. Ibrahim S. Al-Mutaz
King Saud University
67.2
11.0
n
40.1
24.3
23.0
39.1
61.0
2
2
1
1
1
96.1
35.5
2
1
Eq. Wt. Conc. Conc.
mg/meq meq/L mg/L as
CaCO3
20.05
1.995
99.8
12.15
.823
41.2
23.0
.51
25.7
39.1
.179
8.95
61.0
1.80
90.2
48.05
35.5
1.40
.031
69.9
15.5
Sample Calculation:
(Ca2+ in meq/L)* (EW of CaCO3) = Ca2+ in mg/L as CaCO3
(1.995 meq/L)*(50 mg/meq) = 99.8 mg/L as CaCO3
Sample Calculation: Concentration of Ca2+ =
(Concentration in mg/L) / (Equivalent Weight in mg/meq) =
(40.0 mg/L) / (20.05 mg/meq) = 1.995 meq/L
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Water Hardness & Softening
Example: Hardness Calculation
Ion
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Prof. Ibrahim S. Al-Mutaz
King Saud University
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Water Hardness & Softening
Example: Hardness Calculation
Example: Hardness Calculation
Alkalinity = (HCO32-) + (CO32-) + (OH-) - (H+)
Check Solution
Σ(cations) = Σ(anions)
175.6 =
175.6
Since pH = 7.2 → [H+] = 10-7.2 M … (H+) = 10-7.2 eq/L
to within ± 10%
mg/L as CaCO3
[OH-] = 10-6.8 M …. (OH-) = 10-6.8 eq/L
From before we see that (HCO3-) = 1.80 meq/L, since n = 1 then [HCO3-]
= 1.80 x 10-3 M
(Can check using concentrations in meq/L or mg/L as CaCO3)
Using the equilibrium expression for HCO3-/CO32-, we find that [CO32-] =
10-10.33[HCO3-]/[H+] = 1.33 x 10-6 M …. Since n = 2, then (CO32-) = 2.66 x
10-6 eq/L
Total Hardness = Σ of multivalent cations
•
ChE-413: Desalination and Water treatment
= (Ca2+) + (Mg2+) = 99.8 + 41.2 =
•
141 mg/L as CaCO3
Alkalinity = (1.80 x 10-3) + (2.55 x 10-6) + 10-6.8 - 10-7.2 = 1.801 x 10-3
eq/L
* Alkalinity = 1.801 x 10-3 x 1000 x 50 =90.1 mg/L as CaCO3
Chemical Engineering Dep.
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Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Water Hardness & Softening
King Saud University
Prof. Ibrahim S. Al-Mutaz
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Example: Hardness Calculation
Hardness Measurement – titration method
Carbonate Hardness (the portion of the hardness associated
EDTA solution in the burette
with carbonate or bicarbonate)
Take approx. 100 mL of the EDTA solution.
Alkalinity = 90.1 mg/L as CaCO3
Dissolve the indicator tablet fully before starting
the titration.
TH = 141 mg/L as CaCO3
CH = 90.1 mg/L as CaCO3
Titrate water with EDTA until colour changes from
red to blue.
(Note: if TH < Alk then CH = Alkalinity; and NCH = 0 )
Non-carbonate Hardness
Water sample in the
conical flask
NCH = TH - CH = 141 - 90.1 =
50.9 mg/L as CaCO3
EDTA: Ethylenediamine-tetraacetic acid
Chemical Engineering Dep.
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Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
King Saud University
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Hardness Measurement – titration method
Hardness Measurement – titration method - example
Eriochrome Black
50.0 mL tap water sample.
• Indicator
13.75 mL EDTA used in the titration.
Chemical Engineering Dep.
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a)
Calculate the moles of EDTA used.
b)
Calculate the molarity of metal ion present in the tap water
sample (Remember: EDTA binds to metal ions on a 1:1 molar
ratio).
c)
Find the ppm calcium ion concentration in the sample based
on your results from question b).
Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
ChE-413: Desalination and Water treatment
Water Hardness & Softening
ChE-413: Desalination and Water treatment
Water Softening Methods
Calculate the moles of EDTA used
Objective: to reduce the hardness and thus prevent scaling
Molarity of EDTA = 0.005 mol/L
Ion exchange
Moles of EDTA = (Molarity) (Liters of EDTA used)
b)
22
Water Hardness & Softening
Hardness Measurement – titration method - example
a)
King Saud University
Zeolite or Permutit process
Calculate the molarity of metal ion present (Remember EDTA binds
to metal ions on a 1:1 ratio)
Mixed bed deionizezer
Chemical process
Molarity of metal ion = moles of EDTA / liters of water sample
c) Calculate concentration of CaCO3 in ppm
Lime Soda process: was patented in 1841. 1st municipal softening
plant was installed in 1854 in England. The first lime-soda softening
plant was in 1903.
CaCO3 (ppm) = (Molarity of metal ion)(MW of Ca2+)(1000 mg/g)
CaCO3 (ppm) =
50.0 mL tap water sample.
13.75 mL EDTA used in the titration.
Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
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Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
King Saud University
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Lime-Soda Process for Water Softening: Chemical Process
Lime-Soda Process for Water Softening: Chemical Process
Calcium & Magnesium Salts
Chemical Formula
Solubility at 0 oC
(ppm CaCO3)
Calcium Salts
Calcium Bicarbonate
Ca(HCO3)2
1,620
Calcium Carbonate
CaCO3
15
Calcium Chloride
CaCl2
336,000
Calcium Sulphate
CaSO4
1,290
Calcium Hydroxide
Ca(OH)2
2,390
Mg(HCO3)2
37,000
Magnesium Salts
Magnesium Bicarbonate
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Magnesium Carbonate
MgCO3
101
Magnesium Chloride
MgCl2
362,000
Magnesium Sulphate
MgSO4
170,000
Magnesium Hydroxide
Mg(OH)2
Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Lime-Soda Process for Water Softening: Chemical Process
Calcium
Bicarbonate –
HCO3
Carbonate –
CO3
Hydroxide OH
Solubility
Very Soluble
Insoluble
Somewhat
Soluble
Magnesium
Bicarbonate –
HCO3
Carbonate –
CO3
Hydroxide OH
Solubility
Very soluble
Soluble
Insoluble
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Water Hardness & Softening
Lime-Soda Process for Water Softening: Chemical Process
Chemical Engineering Dep.
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
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Pre-treatment unit
Coagulation /flocculation (most commonly used):
Ca2+, Mg2+ → CaCO3 (s), Mg(OH)2 (s)
• lime-only process: when Ca2+ is present primarily as
“bicarbonate hardness”
• lime-soda [Ca(OH)2-Na2CO3] process: when bicarbonate is
not present at substantial level
27
Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
King Saud University
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Lime-Soda Process for Water Softening: Chemical Process
Lime-Soda Process for Water Softening: Chemical Process
Common Chemical Names
Baking Soda
– NaHCO3, Sodium Bicarbonate
Caustic Soda
– NaOH, Sodium Hydroxide
Soda Ash
– Na2CO3, Sodium Hydroxide
Washing Soda
– Na2CO3, Sodium Hydroxide
Quick Lime
- CaO, Calcium Oxide
Active Lime
- CaO, Calcium Oxide
Hydrated Lime
– Ca(OH)2, calcium Hydroxide
Lime Stone
–CaCO3 , calcium carbonate
Schematic Flow Diagram of Salbukh Water treatment plant
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Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Water Hardness & Softening
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Lime-Soda Process for Water Softening: Chemical Process
Lime is commercially available in the forms of:
Lime-Soda Process for Water Softening: Chemical Process
Lime to remove Ca2+ in the form of natural alkalinity
- quicklime
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3↓ + 2H2O
- hydrated lime
Lime to remove Mg2+ in the form of natural alkalinity
Mg(HCO3)2 + Ca(OH)2 → MgCO3 (soluble) + CaCO3↓ + 2H2O
Quicklime
additional lime must be added to remove MgCO3
- available in granular form
MgCO3 + Ca(OH)2 → CaCO3↓ + Mg(OH)2↓
- contains minimum of 90% CaO
Mg2+ hardness in the form of a sulfate requires both lime and
soda ash
- magnesium oxide is the primary impurity
MgSO4 + Ca(OH)2 → CaSO4 + Mg(OH)2↓
Hydrated Lime
CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4↓
- contains about 68% CaO
CO2 in the water will also consume lime
Slurry lime is written as Ca(OH)2.
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Prof. Ibrahim S. Al-Mutaz
CO2 + Ca(OH)2 → CaCO3 ↓ + H2O
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Chemical Engineering Dep.
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King Saud University
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Lime-Soda Process for Water Softening: Chemical Process
Lime-Soda Process for Water Softening: Chemical Process
Photo of a Precipitator
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Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Water Hardness & Softening
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Lime-Soda Process for Water Softening: Chemical Process
Rules of Thumb
Lime Requirements, Ca(OH)2
- Add 1 meq/L per meq/L CO2
-Add 1 meq/L per meq/L alkalinity associated with hardness
- Add 1 meq/L per meq/L Mg++ that is to be removed (Eq. d and e)
- In practice, when it is desired to remove Mg++ , one meq/L Ca(OH)2 is
added in excess of the stoichiometric amounts designated above to insure
adequate and rapid precipitation of the Mg(OH)2.
Soda Ash Requirements, Na2CO3
- Add 1 meq/L per meq/L of non-carbonate hardness
Schematic Diagram of a Precipitator
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Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Water Hardness & Softening
Selective Calcium Carbonate Removal
Alkalinity and Hardness
If the water to be treated contains low concentration of magnesium (<40
mg/l as CaCO3), selective calcium carbonate removal can be used.
Magnesium hardness of more than 40 mg/l as CaCO3 is not recommended
due to the possible formation of hard magnesium silicate in high
temperature waters (85° C)
enough lime is added but not in excess
soda ash may be used depending on the extent of noncarbonate hardness
if precipitation of CaCO3 is not satisfactory, alum or a polymer can be
used to aid flocculation
recarbonation is used to reduce scale formation on the filter and to
produce stable water
Chemical Engineering Dep.
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ChE-413: Desalination and Water treatment
King Saud University
Condition
CH, lime
NC, lime+soda
TH < alkalinity
TH
0
TH = alkalinity
TH
0
TH > alkalinity
Alk
TH-Alk
TH: total hardness, CH: carbonate hardness, NC: noncarbonate hardness
37
Chemical Engineering Dep.
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
King Saud University
38
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Alkalinity and Hardness
Alkalinity and Hardness
Solution
Chemical Engineering Dep.
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King Saud University
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Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
King Saud University
40
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Water Hardness & Softening
Lime-Soda Process for Water Softening: Chemical Process
Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Lime-Soda Process for Water Softening: Chemical Process
King Saud University
Prof. Ibrahim S. Al-Mutaz
41
Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Prof. Ibrahim S. Al-Mutaz
42
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Lime-Soda Process for Water Softening: Chemical Process
Chemical Engineering Dep.
King Saud University
Prof. Ibrahim S. Al-Mutaz
Lime-Soda Process for Water Softening: Chemical Process
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Chemical Engineering Dep.
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Recarbonation by bubbling CO2 after softening
Recarbonation by bubbling CO2 after softening
Recarbonation is usually required after lime-soda process
Recrabonation is used to stabilize lime-treated water, thus
reducing its scale-forming potential.
Why?
• To prevent super-saturated CaCO3 (s) and Mg(OH)2 (s) from
forming harmful deposits or undesirable cloudiness in water
at a later time
Carbon dioxide is used for the recarbonation process. It converts
lime to calcium carbonate. Further recarbonation will convert
carbonate to bicarbonate.
CaCO3 (s) + CO2 + H2O ⌫ Ca2+ + 2HCO3-
CO2 + Ca (OH ) 2 → CaCO3 ↓ + H 2 O
MgCO3 (s) + CO2 + H2O ⌫ Ca2+ + 2HCO3• To neutralize excessively high pH caused by Na2CO3
CO2 + Mg (OH ) 2 → MgCO3 + H 2 O
OH- + CO2 ⌫ HCO3-
Chemical Engineering Dep.
CO2 + CaCO3 + H 2 O → Ca ( HCO3 ) 2
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Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Water Hardness & Softening
46
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Stabilization
Stabilization
Caustic soda is commonly used
Unstable water: red water, lead and copper corrosion
problems
Orthophosphates, silicates used to prevent lead & copper
corrosion, sequester Ca & carbonate
Final pH should be selected by looking at scaling indices:
Process of making water less corrosive and less depositing
Increasing Ca hardness, alkalinity or pH:
• Increase scaling and decrease corrosive tendency
Increasing temperature:
• Increase scaling and corrosive tendency
•
•
•
•
TDS can affect scaling and corrosivity
Chemical Engineering Dep.
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LSI: Langlier Saturation index
CCPP: Calcium Carbonate Precipitation Potential
Ryznar index
Lead solubility
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Water Hardness & Softening
Bar-graph: Hypothetical chemical composition
Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Bar-graph: Hypothetical chemical composition
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Chemical Engineering Dep.
ChE-413: Desalination and Water treatment
Water Hardness & Softening
Prof. Ibrahim S. Al-Mutaz
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Bar-graph: Hypothetical chemical composition
Chemical Engineering Dep.
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Prof. Ibrahim S. Al-Mutaz
Bar-graph: Hypothetical chemical composition
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Chemical Engineering Dep.
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ChE-413: Desalination and Water treatment
Water Hardness & Softening
Water Hardness & Softening
Bar-graph: Hypothetical chemical composition
Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
Bar-graph: Hypothetical chemical composition
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53
ChE-413: Desalination and Water treatment
Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
ChE-413: Desalination and Water treatment
King Saud University
55
Chemical Engineering Dep.
Prof. Ibrahim S. Al-Mutaz
King Saud University
54