CE 428 Water and Wastewater Treatment Design
Lime Softening
 Dr. S.K. Ong
Hardness in water is caused by the presence of polyvalent metallic cations such as calcium, magnesium, aluminum,
iron and strontium. Most prevalent cations are calcium and magnesium. Usually expressed as mg/L as CaCO3 or
meq/L
In most waters
Total Hardness = __________________________________
Hardness classification scale
Hardness range (mg/L as CaCO3)
0 – 75
75 – 150
150 – 300
> 300
Water Description
_________________ Hardness (temporary hardness) – hardness that are associated with bicarbonate and carbonate
ions or alkalinity
_______________ Hardness (permanent hardness) - hardness that are associated with sulfate, chloride and silicates
or the difference between total hardness and carbonate hardness
i.e.,
Noncarbonate Hardness = ____________________________________
If total hardness is equal or less than alkalinity then
and
Total hardness = _________________
Noncarbonate Hardness = ___________________
Example:
Water Analysis
Ca2+
Mg2+
Na+
= 75 mg/L
= 40 mg/L
= 10 mg/L
HCO3CO32ClSO42-
= 300 mg/L
= 0 mg/L
= 10 mg/L
= 109 mg/L
Draw a bar chart and estimate the total hardness, carbonate hardness and noncarbonate hardness in mg/L as CaCO3
Conc.
(mg/L)
Ca2+
Mg2+
Na+
75
40
10
HCO3ClSO42-
300
10
109
Equivalent wt
Conc
(meq/l)
Bar Diagram
(Cations on the top row – start with Ca2+ followed by Mg2+, Na+
Anions on the bottom row – start with OH-, CO32-, HCO3-, SO42-, Cl-)
Conc
(mg/L as CaCO3)
Total hardness = _____________ = ________________mg/L as CaCO3
Alkalinity = bicarbonate alkalinity = ______________ mg/L as CaCO3
Carbonate Hardness = alkalinity = ________________ mg/L as CaCO3
Noncarbonate Hardness = _____________ = _________ mg/L as CaCO3
Removal of Hardness
(i) chemical precipitation using lime and soda ash
(ii) ion exchange – exchange of ions in solution for other ions on the ion exchange resin
(iii) membrane processes – separate dissolved constituents from water using pressure (reverse osmosis), electrical
gradient with a membrane (electrodialysis)
Chemical Precipitation - Lime Softening Process
- removal of calcium and magnesium by precipitation
- examples of solubility of various calcium and magnesium compounds (solids) at 25o C.
CaCO3
Ca(OH)2
CaSO4
CaCl2
mg/L
18
770
1620
745,000
Mg(OH)2
MgCO3
MgSO4
mg/L
40
727,000
738,000
See Attached chart for chemical reactions
With lime softening, hardness cannot be removed completely because of the solubility of calcium carbonate and
magnesium hydroxide. The minimum practical limits are calcium = 30 mg/L of CaCO 3 and magnesium = 10 mg/L
as CaCO3.
3 Basic Schemes to produce finished water with desired hardness:
(i) ________________________ – providing enough lime to precipitate the calcium only, usually done when the
Mg is < 40 mg/L as CaCO3, single stage system
(ii) ___________________________
- excess lime added, can be one or two stages
(iii) __________________________
Excess Lime Treatment
- addition of surplus lime, normally about 35 mg/L of CaCO3 above stoichiometric requirements, will bring the pH
up to 11.0
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________________________________________________________________________________________________________________________________
Species
Conc.
Reactions
Comments
Lime and Soda Ash Needs and Solids Produced
(meq/L)
_________________________________________________________________________________________________________________________________
Lime needed
Soda Ash
Solids Produced
_______________________________________________________________________________________(meq/L)________(meq/L)_________(meq/L)_____
CO2
X1
CO2 + Ca(OH)2 => CaCO3 +H2O
Ca(HCO3)2
X2
Mg(HCO3)2
X3
MgSO4
X4
CaSO4
X5
CO2 not part of hardness
but consumes Ca(OH)2
X1
0
X1 (CaCO3)
Ca(HCO3)2 + Ca(OH)2 =>
2CaCO3 + 2 H2O
X2
0
2X2 (CaCO3)
Mg(HCO3)2 + Ca(OH)2 =>
CaCO3 + MgCO3 + H2O
X3
0
X3 (CaCO3)
MgCO3 + Ca(OH)2 = >
Mg(OH)2 + CaCO3
X3
0
X3 (CaCO3)
X3 (Mg(OH)2)
MgSO4 + Ca(OH)2 =>
Mg(OH)2 + CaSO4
X4
0
X4 (Mg(OH)2)
CaSO4 + Na2CO3 =>
0
X 4 + X5
X5 (CaCO3)
CaCO3 + Na2SO4
_____________________________________________________________________________________________________________________________ ___
Total Lime needed = X1 + X2 + 2X3 + X4 + excess (if any)
Total Soda Ash = X4 + X5
Sludge produced = X1 + 2 X2 + 2 X3 + X5 of CaCO3 and X3 + X4 of Mg(OH)2
Lime Softening Example
Determine the amount of lime (CaO) and soda ash (Na2CO3) needed to soften the following water. Assume excess
lime is used. Estimate the approximate water quality after softening and recarbonation. Flow rate = 10 5 m3/day
- practical limits : Ca2+ at 30 mg/L as CaCO3
Mg2+ at 10 mg/L as CaCO3
- excess lime added - 35 mg/L as CaCO3 or 0.7 meq/L
- assume recarbonation converts one-half of the remaining alkalinity to bicarbonate ions.
Useful information
Water Analysis
Components
CO2
Ca2+
Mg2+
Na+
Alkalinity
SO42Cl-
mg/L
8.8
70
9.7
6.9
115 mg/L as CaCO3
96
10.6
Equiv. Wt.
Plot Bar Graph
meq/L
Lime
Soda ash
CO2
Ca(HCO3)2
CaSO4
MgSO4
__________________________________________________________
_____________________________________
Lime required
= ____________________________________
Soda ash required
= _________________________________
4
meq/L
Ions Removal by Ion Exchange
● Use of naturally occurring porous materials (___________________) or synthetic resins (ion exchange resins)
with functional groups which the capacity of preferentially sorbing certain ions in solutions and at the same time
release replacement ions back into the water
● Ion exchange resins are used for ________________ and __________________ of water (producing pure water).
● Most resins have a structure of polymer (styrene or divinylbenzene) with functional groups :
● Cation exchange resins
Strong acid cation exchange (functional groups – SO3-)
Weak acid cation exchange (functional groups – COO-)
Anion exchange resins
Strong base anion exchange (functional groups – R N+)
Weak base anion exchange (functional groups – R NH3+)
where R is the polymer base
Softening
Regeneration:
R2Ca + 2 Na +
= > 2 R- Na + Ca2+
R2 Mg + 2 Na+
= > 2 R – Na + Mg2+
Certain ions (Fe2+ and Fe3+) cannot be easily removed and eventually the resin cannot be fully regenerated.
Demineralization
Use strong acid cation and strong base anion resin
Cation
x R - SO3 H + Mx+ == > (R SO3)x M + x H+
Anion
z R - OH + Az - == > Rz A + z OH-
Where Mx+ and Az - are cations and anions. Final water in terms of H+ and OH-. Regenerate the cation resin with
strong acid (HCl or H2SO4) and anion resin with a strong base (NaOH).
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