Precipitation
Objective: To remove undesirable pollutants by a precipitation reaction.
M(aq) + N(aq)
MN(s)
Example removal of hardness by lime soda-ash and removal of iron and
manganese by oxidation.
The focus of this lecture is on removal of hardness by lime and soda-ash, but
before proceeding with the lime soda-ash method, we need to know how to
express concentration in units of meq/l and mg/l as CaCO 3?
meq Concentration (mg / L)
Concentration in
=
L
Eq. wt (mg / meq)
Eq. wt. =
Atomic or molecular wt
eq. #
Concentration of X (mg / L) × 50 ( mg CaCO3 / meq)
mg
of X as CaCO3 =
L
Eq. wt. of X ( mg / meq)
Symbol
At. or M. wt.
(mg/mmol)
Eq. #
Eq. wt.
(mg/meq)
Ca2+
40.1
2
20
Mg2+
24.3
2
12.2
K+
39.1
1
39.1
Na+
23
1
23
HCO3-
61
1
61
CO3 2-
60
2
30
Cl-
35.5
1
35.5
PO4 3-
95
3
31.7
SO42-
96
2
48
CaCO3
100
2
50
Lime Soda-Ash Softening
A precipitation method through which Ca is removed as
CaCO3 and Mg is removed as Mg(OH)2.
The first step is to remove CO2 from water:
CO2+H2O
H2CO3
Lime
H2CO3+Ca(OH)2
CaCO3 +2H2O
Now remove Ca or Mg that has alkalinity in the water:
Ca hardness
Lime
Ca(HCO3)2+Ca(OH)2
2CaCO3 +2H2O
Mg hardness
Lime
Mg(HCO3)2+2Ca(OH)2
2CaCO3 + Mg(OH)2+ 2H2O
Soda ash is needed if there is not enough alkalinity in the water:
Ca hardness
Soda ash
CaSO4+ Na2CO3
Mg hardness
Lime
CaCO3 + Na2SO4
Soda ash
MgSO4+ Ca(OH)2 +Na2CO3
CaCO3 + Mg(OH)2+ Na2SO4
Amount of lime and soda ash needed to remove hardness
1 meq/l of
meq/l needed of
lime
Soda ash
CO2
1
-
Ca(HCO3)2
1
-
Mg(HCO3)2
2
-
Ca with ions other than alkalinity
-
1
Mg with ions other than alkalinity
1
1
Excess
1.25
Practically, some Ca and Mg will be left in solution after treatment:
For Ca: 30 mg/l as CaCO3 =0.6 meq/l in equilibrium with CaCO3 is left
For Mg: 10 mg/l as CaCO3=0.2 meq/l in equilibrium with Mg(OH)2 is left
Example
A groundwater sample has the following characteristics. Find:
1.The total hardness
2.TDS
3.The amount of lime and soda ash needed to remove hardness
to practical limits.
4.The meq/l bar graph after lime soda addition.
Component
Concentration
pH (-)
6.3
CO2 (mg/l)
8.8
Ca2+ (mg/l)
40
Mg2+ (mg/l)
14.7
Na+ (mg/l)
13.7
HCO3- (mg/l)
164.7
SO42- (mg/l)
29
Cl- (mg/l)
17.8
Solution
1. Hardness: Convert the concentration of Ca and Mg to mg/l as CaCO3 using
Concentration of X (mg / L) × 50 (mg CaCO3 / meq)
mg
of X as CaCO3 =
L
Eq. wt. of X ( mg / meq)
Thus Ca= 100 mg/l as CaCO3, Mg=60 mg/l as CaCO3. Total Hardness is 160 mg/l as CaCO3
2. TDS= sum of all cations and anions in mg/l = 279.9 mg/l.
3. To determine the amount of lime and soda ash needed, we need first to convert
the concentration to meq/l, draw a meq/l bar graph and then use the table
Component
Concentration
(mg/l)
Equivalent weight
(mg/meq)
Concentration
(meq/l)
CO2
8.8
22
0.4
Ca2+
40
20
2.0
Mg2+
14.7
12.2
1.21
Na+
13.7
23
0.6
HCO3-
164.7
61
2.7
SO42-
29
48
0.6
17.8
35
0.51
Cl-
The meq/l bar graph of water before lime/soda-ash treatment
The amount of lime and soda ash needed to remove hardness to practical limits are
determined as shown in the table below
meq/l of
CO2 =0.4
Ca(HCO3)2=2.0
Mg(HCO3)2=0.7
CaSO4=0.0
MgSO4=0.51
Excess
Total
meq/l needed of
lime
Soda ash
0.4
2.0
1.4
0
0.51
0.51
1.25
5.56
0.51
The meq/l bar graph of water after lime/soda-ash treatment but before recarbonation
Note, the pH of the treated water is high (about 11), so CO2 is added (recarbonation)
to reduce the pH.