Civil Engineering Reference
In-Depth Information
Fig. 24-21b.
Polyelectrolyte batching system (Courtesy of USFilter / Wallace and Tiernan)
Liquid polymers need no aging, and simple dilution is the only requirement for
feeding. The dosage of liquid polymers may be accurately controlled by metering
pumps or rotary wheel feeders.
Because polymers can cause slippery conditions in a treatment plant, spills should
be cleaned up immediately. Other safety precautions should also be observed, as spec-
ified by the manufacturer.
Powdered Activated Carbon
When powdered activated carbon is used, it is mixed
directly with the water, fed as a slurry, and removed by coagulation and settling. The
carbon slurry is transported by pumping the mixture at a high velocity to keep the
particles from settling and collecting along the bottom of the pipe. The velocity of the
slurry should be kept between 3 and 5 feet / sec. At velocities less than 3 feet / sec (0.91
m / s), carbon will settle out in the pipeline; and at velocities greater than 10 feet / sec
(3 m / s), excessive carbon abrasion and pipe erosion will occur. At most plants, carbon
slurries are fed at a concentration around 10.7 percent or 1 pound carbon / gallon (0.12
kg / 1) water; typically, at this concentration, either centrifugal pumps or a combination
of centrifugal pumps and eductors are used to transport the carbon slurry. Diaphragm
slurry pumps or double-acting positive-displacement pumps are used for transporting
higher concentrations. Another transport method used is a pressure pot system in which
carbon is loaded into a pressure tank and forced out by pressurizing the vessel. The
carbon slurry may be fed using a rotary wheel feeder. PAC can also be fed using an
eductor system. The dry PAC is metered into an eductor using a volumetric feeder,
and water passing through the eductor causes a vacuum that draws the PAC into the
flow stream and conveys it to the point of application. Typically, about 0.4 pounds
carbon / gallon of water is used in this application.
