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11.6 CHAPTER ELEVEN

Aeration is often used to oxidize iron in iron and manganese removal plants, but the
elevated pH of the softening process together with chemical oxidation, if needed, can re-
move iron and manganese without the need for aeration.

Presedimentation
Presedimentation is used primarily by those plants treating high-turbidity surface waters,
such as sources on the Missouri and Mississippi rivers. Some of these plants use metal
salt or polymer coagulants to enhance suspended-solids removal.
Presedimentation provides a more uniform water quality at the treatment plant, re-
moves a major portion of the suspended solids with little chemical cost, and offers po-
tential cost savings in residuals treatment and disposal. Because of the cost of dewater-
ing and land disposal of these solids and the minor impact on highly turbid rivers, state
and federal agencies have often allowed return of presedimentation residuals directly to
the river.
Presedimentation also provides an opportunity for pretreatment and removal of tastes
and odors and other organic compounds with powdered activated carbon and oxidizing
agents before the elevated pH associated with the softening process. In some cases, this
provides more efficient and effective treatment.

LIME SOFTENING PROCESSES

The common lime softening processes are (1) the straight lime or lime-soda ash process,
(2) the excess lime or excess lime-soda ash process, and (3) the split-treatment excess
lime or excess lime-soda ash process. The following paragraphs present formulas that can
be used to estimate chemical feed requirements for these processes. A design factor of at
least 10% is typically added to the chemical feed dosages indicated by the formulas for
design of the feed systems. A more detailed presentation can be found in AWWA's Wa-
ter Quality and Treatment.

Straight Lime or Lime-Soda Ash Process
For the straight lime softening process, only carbonate hardness in the form of calcium is
to be removed, and only lime is added to the softening stage. The lime dosage required
to react with calcium hardness may be estimated by using part of Equation (11.9) as
follows:
CaO (lb/mil gal) = 10.6[CO2 (mgFL)] + 4.7[alkalinity (mg/L)] (11.18)
where alkalinity is equal to the portion of carbonate hardness associated with calcium to
be removed.
For the straight lime-soda ash process, both carbonate and noncarbonate hardness are
removed, and soda ash is added to the softening stage. The dosage of lime required to re-
act with calcium hardness may be estimated by using Equation (11.18), where alkalinity
is the portion of carbonate hardness associated with calcium to be removed. The dosage
of soda ash required may be estimated using Equation (11.10).
The straight lime or lime-soda ash process removes only calcium hardness down to a
minimum of about 35 mg/L.