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LIME SOFTENING 11.7

f Filter Aid Polymer
_ ~ Lime
Carbon Dioxide
F\ Polyphosphate
L Coagulant
PRETREATMENT SOFTENING RECARBONATION I FILTRATION --~
Mixing and
(Where
Required) Sedimentation

FIGURE 11.1 Single-stage lime process.

Recarbonation. Recarbonation with carbon dioxide is required to convert carbonate al-
kalinity to bicarbonate alkalinity and stabilize the water before filtration. An often used
guideline is 40 mg/L of bicarbonate alkalinity in the finished water, although this could
vary depending on stability requirements for the particular water. The reaction of car-
bonate alkalinity with carbon dioxide to produce bicarbonate alkalinity is
(11.19)
CO2 + CaCO3 + H20 = Ca(HCO3)2
The dosage of carbon dioxide required to react with the carbonate alkalinity to pro-
duce bicarbonate alkalinity may be estimated as follows:
CO2 (lb/mil gal) = 3.7 × carbonate alkalinity (mg/L) (11.20)

where carbonate alkalinity is that amount to be converted to bicarbonate alkalinity.
The straight lime or lime-soda ash process is typically a single-stage softening pro-
cess where a single basin is used for softening followed by a recarbonation basin prior to
filtration. A flow diagram for a single-stage lime or lime-soda ash process is shown in
Figure 11.1.
Straight Lime or Lime-Soda Ash Process Examples. The following examples demon-
strate the use of the above equations to estimate chemical feed requirements.
Given:
Surface water source (carbon dioxide is negligible for this source)
Total hardness = 270 mg/L as CaCO3
Alkalinity = 180 mg/L as CaCO3
Calcium hardness = 240 mg/L as CaCO3
Magnesium hardness = 30 mg/L as CaCO3
Noncarbonate hardness (NCH) = 270 - 180 = 90 mg/L as CaCO3
Available lime is 90% CaO
Available soda ash is very close to 100% Na2CO3

EXAMPLE 11.1 Reduce calcium carbonate hardness as much as possible, using lime only.
(Note: In this example, the magnesium hardness is less than the selected target maximum
of 40. Therefore, lime addition for magnesium hardness removal is not required.)