Civil Engineering Reference
In-Depth Information
This equation states that the pure soda ash required, expressed as itself, in pounds per
million gallons softened, is equal to the amount required to precipitate the desired
amount of noncarbonate hardness, expressed as CaCO
3
.
The chemicals are added to the water, normally through a rapid-mix process, then
the precipitates are flocculated, settled, and removed as sludge. The addition of 1 pound
(0.45 kg) of lime to hard water can lead to the precipitation of up to 3
1
⁄
2
pounds (1.59
kg) of mineral solids. The flocculation process may be aided by the use of a coagulant
such as aluminum or iron salts, and / or polymer.
To determine how the chemical makeup of the water can be altered by lime-soda
softening, it is often useful to prepare a bar chart of the cations and anions in the raw
water. Since hardness and alkalinity are always expressed as mg / L CaCO
3
, it is con-
venient to express all the other components this way also. Table 13-2 lists conversion
factors that can be applied to express several common substances as CaCO
3
.
Figure 13-1 is a graphical representation of the ion balance of an actual well water
that would be amenable to softening. In preparing such a chart, the cations are plotted
TABLE 13-2. CaCO
3
Equivalent of Common Substances
Molecular
Weight
Equivalent
Weight
CaCO
3
Conv. Factor
Multiply by:
Substance
Formula
Aluminum
Al
3
27.0
9.0
5.56
Aluminum hydroxide
Al(OH)
3
78.0
26.0
1.92
Aluminum sulfate (filter
alum)
Al
2
(SO
4
)
3
666.4
111.1
0.45
18H
2
O
Bicarbonate
HCO
3
61.0
61.0
0.82
Calcium
Ca
2
40.1
20.0
2.50
Calcium bicarbonate
Ca(HCO
3
)
2
162.1
81.1
0.62
Calcium carbonate
CaCO
3
100.1
50.0
1.00
Calcium chloride
CaCl
2
111.0
55.5
0.90
Calcium hydroxide
Ca(OH)
2
74.1
37.0
1.35
Calcium oxide
CaO
56.1
28.0
1.78
Calcium sulfate
CaSO
4
136.1
68.1
0.73
Carbon dioxide
CO
2
44.0
22.0
2.27
Carbonate
CO
3
2
60.0
30.0
1.67
Chloride
Cl
35.5
35.5
1.41
Ferric chloride
FeCl
3
162.2
54.1
0.93
Ferric iron
Fe
3
55.8
18.6
2.69
Ferric sulfate
Fe
2
(SO
4
)
3
399.9
66.6
0.75
Ferrous iron
Fe
2
55.8
27.9
1.79
Magnesium
Mg
2
24.3
12.2
4.12
Magnesium bicarbonate
Mg(HCO
3
)
2
136.4
73.2
0.68
Magnesium carbonate
MgCO
3
84.3
42.2
1.18
Magnesium hydroxide
Mg(OH)
2
58.3
29.2
1.71
Magnesium sulfate
MgSO
4
120.4
60.2
0.83
Manganese
Mn
2
54.9
27.5
1.82
Sodium
Na
23.0
23.0
2.18
Sodium carbonate
Na
2
CO
3
106.0
53.0
0.94
Sodium chloride
NaCl
58.5
58.5
0.85
Sodium sulfate
Na
2
SO
4
132.1
71.0
0.70
Sulfate
SO
4
2
96.1
48.0
1.04
