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11.12 CHAPTER ELEVEN

Approx. remaining alkalinity = 45 mg/L as CaCO3 (approx. minimum
solubility of Ca plus Mg as CaCo3)@
Approximate CO2 feed requirements remain the same as for Example 11.3.

Split-Treatment Excess Lime or Lime-Soda Ash Process
Split-treatment softening is a variation of the two-stage excess lime or lime-soda ash pro-
cess that can be advantageous under some conditions. A schematic of the process is shown
in Figure 11.3.
In split-treatment softening, a portion of the flow to the first-stage softening process
(generally 10% to 30%) is bypassed to the second stage. Carbon dioxide and bicarbonate
alkalinity in the bypassed flow react with excess lime leaving the primary softening pro-
cess, both to soften the bypassed flow with excess lime and to stabilize water entering the
secondary basin.
The net result of split treatment softening is a savings in lime and carbon dioxide feed
requirements and a reduction in residuals production. In addition, the first-stage soften-
ing unit may be smaller because it receives reduced flow. Chemical reactions involved
and calculation of chemical feed requirements and flow split are covered in detail in Wa-
ter Quality and Treatment.
Because a portion of the flow is not treated in the first stage of softening, it is im-
portant to determine whether adequate treatment of bypassed flow is provided in the
second stage. Split-treatment softening is most often used for groundwaters but has
also been used for some surface waters with pretreatment or coagulation of the by-
passed flow.

Soda Ash
f
_~" Filter Aid Polymer
|(if required)
L Polyphosphate
[-~Carbon Dioxide
_ ( Lime
[|(if required)
Carbon Dioxide
LCoagulant
L Coagulant
~ (if required)
1 ST-STAGE 2ND-STAGE
~
PRETREATMENT FILTRATION
SOFTENING SOFTENING
(Where Mixing and Mixing and
Required) Sedimentation Sedimentation
By-passed Flow
FIGURE 11.3 Split-treatment excess lime process.

tNote: Results are approximations based on theoretical equilibrium conditions. These conditions are seldom du-
plicated in practice and some variations should be expected at the particular plant.