Civil Engineering Reference
In-Depth Information
0
160 285 300
Ca
Mg
Na
?
HCO
3
SO
4
Cl
0 210 270 315
Concentrations are in mg/L as CaCO
3
Fig. 13-1.
Ion balance-water amenable to softening
graphically on the top bar, in the following order: Ca, Mg, Fe and Mn, Na. The anions
are plotted on the bottom bar, in the following order: HCO
3
,CO
3
,SO
4
, Cl. These will
represent the normal mineral constituents of most natural waters. The cations in the
top bar can be paired with the anions in the bottom to approximate the actual chemical
compounds making up the minerals dissolved in the water. In this example, they are:
Ca(HCO
3
)
2
160 mg / L as CaCO
3
Mg(HCO
3
)
2
50
MgSO
4
60
MgCl
2
15
NaCl
15
Unidentified
15
Total
315 mg / L as CaCO
3
In the above example and subsequent examples, a question mark indicates that the
anions and cations didn't exactly balance, and is assumed to represent the presence of
unidentified anions or cations.
Figure 13-2 shows what the chemical makeup of the same water might be after
softening, assuming that the calcium and magnesium have each been reduced to 40
mg / L. Note that the reduction of sulfates and chlorides by 25 mg / L (as CaCO
3
) has
resulted in a corresponding increase in sodium of 25 mg / L (as CaCO
3
) through the
addition of soda ash.
Figure 13-3 is an example of a water that will not benefit very much from lime-
soda softening. In this case, the sodium and sulfate concentrations are so high that
softening will not yield very acceptable finished water quality. Softening to reduce the
sulfate to 40 mg / L (as CaCO
3
) would require a dose of soda ash sufficient to raise
the sodium content of the water by 590 mg / L (as CaCO
3
). Figure 13-4 represents the
ion balance of this water as it might be after lime-soda softening. In this example,
0 40 80 12
0
Ca
Mg
Na
HCO
3
SO
4
+ Cl
Concentrations are in mg/L as CaCO
3
Fig. 13-2.
Ion balance after softening
