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11.10 CHAPTER ELEVEN

The second stage provides additional process stability before filtration and is
equipped with residuals collection equipment to eliminate residuals buildup. A coag-
ulant added at the second stage further clarifies the water before filtration. The second
stage also provides an opportunity for further treatment by adsorption or oxidation af-
ter primary softening and before filtration. After the second stage, carbon dioxide is
added to convert carbonate alkalinity to bicarbonate alkalinity for stability, similar to
single-stage treatment.

Recarbonation. Recarbonation is required for the excess lime process to convert excess
hydroxide alkalinity to carbonate alkalinity and also to convert carbonate alkalinity to bi-
carbonate alkalinity for stabilizing the treated water.
The reaction of carbon dioxide with hydroxide alkalinity is shown in Equation (11.6).
The dosage of carbon dioxide can be estimated as follows:
CO2 (lb/mil gal) = 3.7 × [hydroxide alkalinity (mg/L)] (11.21)
where hydroxide alkalinity is the excess hydroxide alkalinity required for magnesium
removal.
In addition, carbon dioxide is required to convert carbonate alkalinity to bicarbonate
alkalinity for stabilizing treated water in accordance with Equation (11.20).

Excess Lime or Excess Lime-Soda Ash Process Examples. The following examples
demonstrate the use of the above equations to estimate chemical feed requirements.
Given:

Groundwater where carbon dioxide is present
COa = 20 mg/L as COE (mg/L)
Total hardness = 305 mg/L as CaCO3
Alkalinity = 224 mg/L as CaCO3
Calcium hardness = 235 mg/L as CaCO3
Magnesium hardness = 70 as CaCO3
Noncarbonate hardness (NCH) = 305 - 224 = 81 as CaCO3
Available lime is 90% CaO
Available soda ash is very close to 100% Na2CO3
EXAMPLE 11.3 Reduce hardness as much as possible, using lime only. (Note: In this ex-
ample, the magnesium hardness is greater than the selected target maximum of 40. There-
fore, lime addition for magnesium hardness removal is required.)
Using Equation (11.9) for approximate lime feed requiremments, we have
CaO (lb/mil gal) = 10.6CO2 (mg/L) + 4.7 [alkalinity (mg/L)
+ magnesium hardness (rag/L) + x]
Assume 65 mg/L of excess hydroxide alkalinity (expressed as CaCO3) is required to re-
duce Mg hardness to --10 mg/L.
CaO (lb/mil gal) = 10.6(20) + 4.7(224 + 70 + 65) = 1,899 lb/mil gal,
as 100% pure CaO