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7-6 WATER AND WASTEWATER ENGINEERING
Total hardness (TH)
Noncarbonate
Carbonate hardness (CH) hardness (NCH)
0 250
Ca 2 Mg 2
HCO 3 Cl
0 200 250
(a)
Total hardness (TH)
Carbonate hardness (CH)
0 250 275
2 2
Ca Mg Na
HCO
3
0 275
(b)
FIGURE 7-4
Relationships between total hardness, carbonate hardness, and noncarbonate
hardness. (Source: Davis and Cornwell, 2008.)
7-2 LIME-SODA SOFTENING
Objectives
Prior to the mid-twentieth century, the primary purpose of lime-soda softening by municipal
water treatment systems was to satisfy domestic consumer desire to reduce the aesthetic and
economic impact of soap precipitation. The importance of this objective has been reduced by the
introduction of synthetic detergents and home water softeners. Other benefits of lime-soda soft-
ening systems have been shown to be quite substantial. These include removal of heavy metals,
NOM, turbidity, and pathogens as well as improving the water quality that reduces costs for
distribution system corrosion, boiler and cooling water feed, and home water heater systems.
The concurrent removal of arsenic, chromium, iron, lead, manganese, and mercury provides an
additional benefit to the removal of hardness and, in some cases, may be the overriding reason for
selection of the technology (Kawamura, 2000).
Lime-Soda Softening Chemistry
Solubility Product. Because all solids are soluble to some degree, there is an equilibrium
between the ions in solution and the solid. This equilibrium can be expressed as
b
A B ()s aA bB a (7-5)
ab
where (s) solid precipitate.