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LIME SOFTENING 11.13

SOFTENING PROCESS BASINS

Conventional Softening Basins
Conventional softening basins are similar to conventional basins used for coagulation and
clarification. They consist of rapid mixing, flocculation, and sedimentation.
Lime, soda ash, coagulant, and a polymer coagulant aid (if required) are mixed with
the water prior to flocculation. Often the mixing is done hydraulically in a flume. Me-
chanical mixers may be problematic due to encrustation with calcium carbonate.
Flocculation typically consists of horizontal paddle wheel or vertical turbine floccula-
tors with total detention times of 30 to 45 min and up to three-stage tapered flocculation.
Flocculators are usually equipped with variable-speed drives. The flocculation basin should
be designed to facilitate periodic cleaning because of residuals buildup.
Sedimentation loading rates are in the range of 0.4 to 1.0 gpm/ft 2 (0.98 to 2.4 m/h)
with detention times of 2 to 4 h. The higher loading rates are generally used for waters
not requiring coagulation for turbidity removal. Continuous residuals collection equip-
ment should be provided, and there must also be a means of draining the basin for peri-
odic cleaning.
Recycling previously formed calcium carbonate residuals from the sedimentation basin
to the mixing zone prior to flocculation is beneficial in the softening process. Recycling
accelerates the precipitation reactions, and the process more closely approaches true sol-
ubility when the mix is seeded with these previously formed crystals. Recycling residu-
als also allow precipitation to occur on the recycled residuals that serve as nuclei, reduc-
ing precipitation on the mechanical equipment.
In addition, recycling calcium carbonate residuals promotes growth of larger calcium
carbonate crystals that settle and dewater more rapidly. A study has indicated that 50%
to 100% residuals recycle, based on solids produced, controls particle size optimally for
this type of process (Bums et al., 1976). This same study found that particle growth ap-
proached an equilibrium value after about 4 cycles.
Figure 11.4 illustrates the effect of residuals recycle on particle growth. Curve A is a
sample of residuals taken before recycling was practiced. Curve B represents a sample of
sludge taken after several cycles at 25% recycling based on solids produced. Curve C rep-
resents 300% sludge recycling. Curve D was a sample of sludge taken from a solids con-
tact clarifier with an extremely high recycle rate operating at a high solids concentration.
Residuals recycle for conventional basins may be accomplished by a separate basin sump
and solids handling pumps that recycle sedimentation residuals to the mixing zone.
Conventional softening basins are mostly found at older facilities. They provide a high
degree of process stability, but the relative size and number of the basins, when compared
with high-rate solids contact processes, substantially increase the initial cost. In addition,
the high-rate solids contact processes are generally more effective in driving the soften-
ing reaction to completion and reducing chemical costs.

Solids Contact Softening Basins
Solids contact softening basins combine mixing, residuals recirculation, and sedimenta-
tion functions in one basin. Rapid mixing may be provided ahead of the solids contact
unit but generally is not needed. Lime and other chemicals are applied directly to the mix-
ing zone of the solids contact unit. This type of unit provides a high degree of continu-
ous sludge recirculation and contact, and it produces more stable water and larger cal-