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CLARIFICATION 7.29

PLAN VIEW

ILLUSTRATIVE SECTION
FIGURE 7.17 Typical solids contact slurry recirculafion unit. (Courtesy of Eimco Water Technologies.)

Surface Loading Rate. The surface loading rate is the primary basis for sizing the clar-
ification zone. Loading rates may range from 0.5 to 1.5 gpm/ft 2 (1.2 to 3.7 m/h) for color
and turbidity removal, with even higher rates for turbid warm waters and lower rates for
colored colder waters. Rates for softening are often higher. Surface settling rates used for
solids contact units typically are higher, by as much as 2 to 4 times, than for conventional
sedimentation in similar applications.
A number of factors affect settling efficiency, and loading rates should be adjusted ac-
cordingly. Specific gravity of particles in the settling zone often ranges from as low as
1.02 for alum floc to as high as 2.65 for sand particles. The designer should consider the
nature of the flocculated material in selecting the loading rate.
Temperature is also an important consideration. Higher-temperature water allows for
more efficient chemical coagulation and ftocculation and has a lower viscosity, exerting
less drag on settling particles. For these reasons, higher surface loading rates may be used
for warm waters.

Flocculation/Contact Zone. A detention time of 30 min or more is provided in the re-
action wells for units designed to remove turbidity to ensure complete chemical reactions
and adequate time for flocculation. Shorter or longer times may be appropriate depend-