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LIME SOFTENING 11.3

• Removing radium 226 and 228
• Removing heavy metals, including arsenic
• Removing certain organic compounds and reducing total organic carbon (TOC)
• Removing silica and fluoride
• Removing iron and manganese
• Reducing turbidity of surface waters in conjunction with the hardness precipitation
process
The degree of removal of constituents usually depends on the treatment process used.
Benefits to the consumer depend on source water quality and user requirements.

Softening Plants in the United States
Currently more than 1,000 domestic-use water plants in the United States use lime soft-
ening processes. Although water plants using lime softening are found in most sections
of the United States, the majority of large plants are located in the midwestern states and
in Florida. These plants range in size from less than 1 mgd to more than 100 mgd.

Chemistry of Lime Softening
Water softening involves a number of complex and dynamic chemical interactions. The
chemical reactions involved and calculation of chemical feed requirements are discussed
in detail in AWWA's Water Quality and Treatment. The discussion that follows in this
text simplifies the chemistry involved, highlighting only the predominant reactions.
Lime, the primary chemical used for water softening, reacts with carbon dioxide and
carbonate hardness to precipitate calcium carbonate and magnesium hydroxide. Quick-
lime, CaO, is first slaked to produce calcium hydroxide:
CaO + H20 = Ca(OH)2 (11.1)
Chemical Reactions. Reactions between calcium hydroxide and carbon dioxide and bi-
carbonate alkalinity are shown in Equations (11.2) and (11.3). The reactions convert the
bicarbonate alkalinity present to carbonate alkalinity, which precipitates as insoluble cal-
cium carbonate.
CO2 + Ca(OH)2 = CaCO 3 + H20 (11.2)
Ca(HCO3)2 + Ca(OH)2 = 2CACO3 + 2H20 (11.3)
The optimum pH to produce minimum soluble calcium carbonate is about 10.3, de-
pending on water temperature, total dissolved solids, and other factors affecting the sol-
ubility of calcium carbonate. In precipitating the calcium ion, 2 mol of calcium carbon-
ate is formed for every 1 tool of calcium ion removed from the water, as shown in Equation
(11.3).
Magnesium hardness, present as magnesium bicarbonate, is removed in a stepwise
fashion, as shown in Equations (11.4) and (11.5).
(11.4)
Mg(HCO3)2 + Ca(OH)2 = CaCO3 + MgCO3 + 2H20
MgCO3 + Ca(OH)2 = CaCO 3 + Mg(OH) 2 (11.5)