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7.4 CHAPTER SEVEN

Detention Time. Detention time (i.e., flow rate divided by tank volume) is usually not
an important design parameter. Many regulatory agencies (e.g., Great Lakes, 2003), how-
ever, still have a requirement for detention periods of 4 h. It is likely that this detention
requirement is a carryover from the days of manually cleaned basins designed to provide
a sludge storage zone. These basins were often 15 to 16 ft (4.6 to 4.9 m) deep or greater
and operated so that more than one-half the volume could be filled with sludge before be-
ing cleaned. Real detention time could vary from 4 h when clean to less than 2 h just be-
fore cleaning.
Modem designs with mechanical sludge removal equipment need not provide a sludge
storage zone, and deep basins with long detention times are no longer required. Conven-
tional basins with detention times of 1.5 to 2.0 h provide excellent treatment.
Basin Depth and Velocities. In theory, basin depth should not be an important param-
eter either, because settling is based on overflow rates. However, in practice, basin depth
is important because it affects flow-through velocity. Flow-through velocities must be low
enough to minimize scouring of the settled floc blanket. Velocities of 2 to 4 ft/min (0.6
to 1.2 rn/min) usually are acceptable for basin depths of 7 to 14 ft (2.1 to 4.3 m), the shal-
lower depths often used with multiple-tray basins. Single-pass basins are generally deeper,
to offset the effects of short-circuiting from density and wind currents.
Basin depth may also play a role in allowing greater opportunity for flocculent parti-
cle contact. Additional flocculation that takes place as particles settle allows for growth
of heavier floc and the formation of a sludge blanket that may be less susceptible to re-
suspension. The formation of this blanket helps increase the solids content of the residu-
als withdrawn by removal equipment. The blanket can, however, also contribute to the
creation of a density current along the bottom of the tank, causing floc carryover to the
effluent.
Number of Tanks. One important choice to be made is the number of basins. The min-
imum, and by far the least costly, plant would have only a single settling basin. However,
that would make for poor operation, because tanks must periodically be taken out of ser-
vice for maintenance. Two tanks would partially offset this problem, but unless plant flow
can be reduced, the load on one tank could be excessive when the other is out of service.
A minimum practical number of tanks would be three, allowing for a 50% increase on
two tanks when one is out of service.
If the design overflow rate is conservative, the three-tank approach is acceptable. In
general, however, a minimum of four tanks is preferred. The number of tanks may also
depend on the maximum size tank that can accommodate the selected sludge removal
equipment or on other factors, such as site constraints.
Factors to consider in selecting the number of tanks are their relationships to the floc-
culation basins and the filters. Are units to be lined up as consecutive processes, or is each
process to be a separate unit? When processes are to be consecutive, a decision on the
minimum or maximum number of filters or floc basins may determine the number of set-
tling basins. The designer should refer to the chapters on flocculation and filtration de-
sign and should approach design of these units as a common process.

Rectangular Basins
Long, narrow basins have been used for sedimentation for many years and will be in op-
eration for years to come. Such basins are not as affected by wind and density currents
as are square or circular basins.