Civil Engineering Reference
In-Depth Information
Paddle or reel-type devices
•
Reciprocating units (walking beam flocculator)
•
Flat-blade turbines
•
Axial flow propellers or turbines
•
Typical units are shown in Figure 10-13. The spatial distributions of velocity gra-
dients that the units produce are shown in Figure 10-14 for some of the mixers.
The paddle or reel-type devices are mounted horizontally or vertically and rotate
at low speeds (2 to 15 rpm). Design is based on limiting the tip speed of the paddle
farthest from the center axis to 1 to 2 ft / sec. Argaman and Kaufman used a stake and
stator device for their studies.
50
This unit is similar to reel-type units but is mounted
vertically and was found to be superior to a turbine.
Walking beam flocculators are driven in a vertical direction in a reciprocating fash-
ion. The unit contains a series of cone-shaped devices on a vertical rod. The cone
devices impart energy to the water as they move up and down, thereby creating velocity
gradients. The manufacturer's literature should be consulted to design this unit.
Turbines are flat-bladed units connected to a disc or shaft. The flat blades are in
the same plane as the drive shaft. The blades can be mounted vertically or horizontally
and typically operate at 10 to 15 rpm. Walker found that plate turbines are effective
up to a
G
-value of 40 sec
1
, but produced high-velocity currents at
G
-values greater
than 45 sec
1
.
58
His suggested design criterion is to limit the maximum peripheral
velocity to 2 ft / sec for weak floc and 4 ft / sec for strong floc. Other investigators have
found that turbines are the least effective units for flocculation.
42,50,58
The axial flow unit ''pumps'' liquid because the impeller has pitched blades. This
unit may be installed vertically or horizontally. Typically, these units are high-energy
flocculation devices operating at 150 to 1,500 rpm, and there is no limitation on the
tip speed. Hudson and Walker favor these units because they are simple to install and
maintain, and they produce uniform turbulence in the flocculator.
42,58
DESIGN EXAMPLES—FLOCCULATION
The design examples presented will be continuations of the examples presented for
the rapid-mix design.
Example 1 (In-Line Blender)
Determine the optimum velocity gradient, based on alum doses of 15 to 25 mg / L.
The advantages of compartmentalization have been shown before. Assume a four-
compartment flocculator. Using Equation 10-26:
44
10
5
44
10
5
(
G
*)
2.8
T
to
15
25
2.933
10
5
to
1.76
10
5
Using a flocculation time of 30 minutes because longer times do not increase ef-
ficiency significantly:
