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CHAPTER 11

LIME SOFTENING

Michael B. Horsley
Black & Veatch
Kansas City, Missouri

Doug B. Elder, P.E.
Black & Veatch
Olathe, Kansas

Leland L. Harms
Black & Veatch
Kansas City, Missouri

Cold lime softening uses chemical precipitation with lime and other chemicals to reduce
a water's hardness and, in some cases, to enhance clarification before filtration. Hot-
process softening is predominantly used in industrial applications and is not discussed
here. Ion exchange softening is covered in Chapter 12, and membrane processes that may
be used to soften water are covered in Chapter 13.
Designing plants with lime softening processes is somewhat different from the design
of other types of water treatment plants. Factors responsible for these differences include
types of chemicals used, the relatively large quantity of some of these chemicals, the spe-
cial chemical handling considerations, and the nature of the corresponding chemical
reactions. These factors influence process and equipment design and selection, plant
layout, and other design considerations.

WATER HARDNESS AND SOFTENING TREATMENT

Hardness in water is the sum of the concentrations of multivalent ions, principally cal-
cium and magnesium. Other ions that produce hardness include iron, manganese, stron-
tium, barium, zinc, and aluminum; but these ions are generally not present in significant
quantities. Hardness is generally expressed in terms of equivalent milligrams per liter
(mg/L) of calcium carbonate. The sum can also be expressed in milliequivalents per liter.
Another expression, used commonly in the past, is grains per gallon, where 17.1 mg/L
(as calcium carbonate) is equal to 1 gr/gal.

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