Civil Engineering Reference
In-Depth Information
RAPID MIXING SYSTEMS
There are several types of rapid-mix systems used in water treatment:
Mechanical mixers
•
In-line blenders
•
Jet injection
•
Hydraulic mixing
•
Mechanical Mixing
Mechanical mixing is the most commonly used system for rapid mixers. This system
is effective, has little headloss, and is unaffected by the volume of flows or flow
variations. Typical design practice provides a contact time in the range of 10 to 30
sec and a
G
-value in the range of 700 to 1,000 sec
1
.
Gemmell recommends a 20-sec or less detention time and a power input of 1 to 2
hp / cu ft / sec (26.36 to 52.72 kW / m
3
s) of flow.
36
He further states there are specific
guidelines for determining detention time or power dissipation required to disperse
chemicals. Examples of typical rapid-mix units are shown in Figures 10-6 and 10-7.
Camp conducted studies on Boston tap water using ferric sulfate.
37
These studies
indicated that a maximum floc volume was obtained using a flat-bladed mixer with a
velocity gradient of 700 to 1,000 sec
1
, and a detention time of 2 to 2.5 minutes. The
detention time is significant because precipitation of ferric hydroxide occurred within
8 sec. The dosage of 15 mg / L created sweep-floc coagulation.
Another study states that the
G
-value ''does not provide a complete characterization
of the mixing in the rapid-mix operation.''
38
This study found that the optimum period
of rapid mix was 2 minutes at a
G
-value of 1,000 sec
1
.
TeKippe and Ham demonstrated that a rapid mixer design, which has a detention
time of long enough duration to allow floc particles to reach near-equilibrium sizes,
provides for optimum sedimentation.
39
The rapid mixer should be followed by a ta-
pered flocculation velocity gradient. Visible floc formation ranged in time from 2 to
6 minutes.
Finally, Vrale and Jorden tested five different types of rapid mix units and con-
cluded that the ''backmix reactor is very inefficient for rapid mixing.''
40
They further
concluded that ''a tubular reactor appears to be the most efficient type.''
Amirtharajah summarized and recommended guidelines for designing a mechanical
rapid mix unit:
41
A square vessel is superior in performance to a cylindrical vessel.
•
Stator baffles are advantageous.
•
A flat-bladed impeller performs better than a fan or propeller impeller.
•
Chemicals introduced at the agitator blade level enhance coagulation.
•
The mechanical mixer has been used in numerous water treatment plants. Because
the mixer speed can be changed by including a variable speed drive, it is amenable to
operational changes due to changing conditions. For example, lower speeds (lower
G
-
values) are applied when polyelectrolytes are used.
