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6.20 CHAPTER SIX

Between each zone or stage of mechanical flocculation, baffles should be designed to
prevent short-circuiting. For these baffles, typical orifice areas should provide a velocity
of approximately 1.0 to 1.5 ft/s (30 to 46 cm/s). Baffles are normally constructed of wood
but may also be concrete, brick, or woven stainless steel strips.
Vertical flocculators are often higher-speed devices than horizontal shaft flocculators,
and the proportion of volume of the compartment that receives energy from the vertical
flocculators may be less. As a result, a wider range of energy is applied to the flow in
vertical flocculators, and for a portion of the time, some of the flow may be subjected to
a higher G. Vertical flocculators are more applicable to high-energy flocculation situa-
tions such as direct filtration.
Where uniform floc is required, low-tip-speed flocculators may be more suitable.
Equipment manufacturers should be consulted to ensure that appropriate paddle designs
are specified for large plants using vertical flocculators.
Vertical flocculators are often specified because they have no submerged beatings, are
usually higher-speed, and involve lower investments. High-speed flocculators, however,
may not provide floc suitable for high-rate horizontal flow basins. Improved clarification
may require increased coagulant doses or flocculent aids.
As a guide, for high-energy flocculators (G = 50 to 75 s-l), maximum tip speed of
mixer blades should not exceed 10 ft/s (30 m/s). For low-energy flocculators and paddle-
type flocculators (G = 20 to 45 s-l), blade tip speeds in the range of 1.0 to 2.5 ft/s (30
to 76 m/s) are appropriate. Some method of varying speed is normally provided. Vari-
able-speed drives or provisions to change pulleys or gears for different shaft speeds are
valuable features that should be provided on mechanical flocculators. As a rule, only the
upper 25% of the speed range requires adjustment, and such adjustment will provide a
variation of 65% to 100% of maximum G. The G output of a flocculator does not nor-
mally have to be varied frequently, but it may require adjustment after installation or on
a seasonal basis.

Hydraulic Flocculation. Hydraulic flocculation methods are simple and effective, es-
pecially if flows are relatively constant. The assumed flocculation volume is the total vol-
ume of each compartment, even though in some cases there may be reduced turbulence
in portions of the compartments. The disadvantage of hydraulic flocculators is that G val-
ues are a function of flow that cannot be easily adjusted.
Energy may be applied to water by means of maze-type baffles or cross-flow baffles,
as illustrated in Figure 6.11. For maze-type baffles, optimum plug flow conditions pre-
vail, and excellent results can be obtained. At velocities in the range of 0.7 to 1.4 ft/s (21
to 43 cm/s), adequate flocculation may be achieved from turbulence caused by the 180 °
turn at each end of the baffle. For lower channel velocities, it may be necessary to pro-
vide an orifice at the end of each channel to induce higher-energy input.
For cross-flow baffles, energy may be transmitted to the water in each compartment
from the head loss across orifices in the entrance baffle. The G value in each compart-
ment can be calculated from estimated head loss across baffles into each compartment
with the following equation:
G = 62.5 h v
t/x
where hv = head loss entering compartment, ft
t = detention time in compartment, s
/x = viscosity, lb • s/ft 2
The head loss through orifices in baffles may be computed from the square-edged sub-
merged orifice formula where the discharge coefficient may be assumed to be 0.8. Many