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CLARIFICATION 7.3

tors cause short-circuiting, disruption of flow patterns, breakup of floc, and scouring of
the settled sludge. The designer must learn as much as possible about the settling prop-
erties of the flocculated solids and then design basins to match these characteristics. When
the designer does not have access to source water data, it is best to select design criteria
known to have worked in similar applications, either from personal experience or from
regulatory guidelines.

Design Approach
The primary approach in designing conventional sedimentation basins is to select a de-
sign overflow rate for maximum expected plant flow. This rate may be chosen based on
all units being in service or on one unit being out of service, to allow for redundancy. Af-
ter selecting an overflow rate, the designer should determine the number of units needed
and select the type of sludge collection and removal equipment. The sludge removal equip-
ment may limit basin dimensions, which could establish the size and number of units.
With the number of basins selected, the designer should proceed to design inlet and out-
let conditions and finalize dimensions to suit all design parameters and site conditions.
The following are suggested guidelines for the various design parameters.
Overflow Rates. Hydraulic overflow rate is the primary design parameter for sizing sed-
imentation basins. This rate is defined as the rate of inflow Q divided by the tank surface
area A. Units are typically rated in gallons per day per square foot, gallons per minute per
square foot, or cubic meters per hour per square meter. Acceptable overflow rates vary
with the nature of the settling solids, water temperature, and hydraulic characteristics of
the settling basin.
Typical design overflow rates for sedimentation of solids produced through alum
coagulation/flocculation are shown in Table 7.1. For the ranges shown, higher rates are
typical for warmer waters with heavier suspended solids. Rates higher than these may be
applicable for warm waters greater than 20 ° C. Lower rates should be used for colder wa-
ters with lower turbidity or that are high in organic color or algae. After evaluation of
both cold and warm water loading rates, the design rate is based on whichever is more
critical. Plant flow variations between cold and warm water periods often allow selection
of higher rates for summer operation than the typical suggested loading rates.
Overflow rates can also be selected based on pilot studies. Piloting of conventional set-
tling basins is not especially reliable, but it is often done using tube settlers (see Chapter
28). Data from such studies, along with jar testing, are often useful in design. Pilot testing
of other types of settling, especially proprietar-y processes, is useful and is recommended.

TABLE 7.1 Typical Sedimentation Surface Loading Rates for
Long, Rectangular Tanks and Circular Tanks Using Alum
Coagulation
Application (L]day)/m 2 gpd/ft 2
Turbidity removal 32,592 to 48,888 800 to 1200
Color and taste removal 24,444 to 40,740 600 to 1000
High algae content 20,370 to 32,592 500 to 800