Civil Engineering Reference
In-Depth Information
0 610 1000 1130 120
0
Ca
Mg
Na
?
HCO
3
SO
4
0 370 1200
Concentrations are in mg/L as CaCO
3
Fig. 13-3.
Water with limited softening benefit
the hardness has been reduced to 120 mg / L. The initial sodium plus the added sodium
has resulted in a total sodium content of 720 mg / L as CaCO
3
, or 330 mg / L as sodium.
The sulfate is unchanged at 830 mg / L as CaCO
3
, or 798 mg / L as sulfate. The water
is relatively soft, but its total mineral content is still very high. To produce a water of
overall better quality, other processes such as reverse osmosis should be considered.
Pretreatment
Prior to softening, some pretreatment may be advisable if any of the following con-
ditions exist:
Raw water turbidities exceed 1,000 NTU at times.
•
Raw water has a high concentration of free carbon dioxide (more than 10
mg / L). This much CO
2
is usually found only in ground water. CO
2
can be re-
moved by stripping.
•
The raw water is high in organic colloids of a type that impedes crystallization
of calcium carbonate.
•
Raw water quality is highly variable over short periods of time.
•
Recalcining of sludge is to be practiced, in which case the sludge purity is im-
portant.
•
If none of the above is true, the clarification and softening process trains can usually
be combined. Basically, the applicable design standards for mixing, flocculation, and
sedimentation are the same for the lime-soda process as for conventional clarification.
Use of Caustic Soda
Caustic soda (NaOH) can be used as the primary softening chemical in place of both
lime and soda ash. The reactions of caustic soda with carbon dioxide, and carbonate
and noncarbonate hardness, are:
0 80 120 840 91
0
Ca
Mg
Na
?
HCO
3
SO
4
0 80 910
Concentrations are in mg/L as CaCO
3
Fig. 13-4.
Ion balance after softening
