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LIME SOFTENING 11.5

CO2 + 2NaOH = Na2CO3 + H20 (11.11)
Ca(HCO3)2 + 2NaOH = CaCO 3 + Na2CO3 + 2H20 (11.12)
Mg(HCO3)2 + 2NaOH = Mg(OH) a + 2Na2CO3 + 2H20 (11.13)
MgSO4 + 2NaOH = Mg(OH) 2 + Na2SO4 (11.14)
CaSO4 + Na2CO3 = CaCO a + Na2SO4 (11.15)

The sodium carbonate formed in Equations (11.11) through (11.13) is available to pre-
cipitate calcium noncarbonate hardness, as shown in Equation (11.15). A combination of
lime and caustic soda can be used, the ratio depending on the calcium noncarbonate re-
moval required. This combination provides some savings in chemical cost compared with
the use of caustic soda alone because caustic soda is more expensive than lime. Using
caustic soda may be a good option for low-alkalinity water, because alkalinity reduction
with caustic soda is one-half that of lime softening. A disadvantage of using caustic soda
is the increase in finished water sodium concentration.

PRETREA TMENT

The principal types of pretreatment used before lime softening are aeration and presedi-
mentation.

Aeration
Aeration may be used to remove carbon dioxide from the source water before softening.
This is usually only applicable to groundwaters where carbon dioxide concentrations are
relatively high. Lime removal of carbon dioxide in source water adds to operations costs
because of chemical expenses and increased calcium carbonate residuals, in accordance
with Equation (11.2).
Induced-draft or open-tray aeration is often used and may reduce the carbon dioxide
level to 10 mg/L or less. Aeration also oxidizes iron that may be present. For some ground-
waters containing substantial iron, clogging of aeration trays is a problem. The aerator
should be designed to minimize clogging and provide ready access for periodic cleaning.
The lime dosage required to react with carbon dioxide may be estimated by using part of
Equation (11.9), as follows:

CaO (lb/mil gal) = 10.6 × CO 2 (mg/L) (11.16)
where CO2 is expressed as CO2 (mg/L).
Reduced lime consumption and residuals production associated with aeration must be
weighed against the capital cost, as well as the operating and maintenance costs, of aer-
ation equipment. Aeration is primarily used where carbon dioxide levels are high enough
to justify the cost. Residuals produced by the reaction of lime and carbon dioxide are in
accordance with Equation (11.2) and may be estimated as
Dry weight CaCO3 residuals (Ib/mil gal) = 19.0 x CO2 (mg/L) (11.17)
where CO2 is expressed as CO2 (mg/L).