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14.6 CHAPTER FOURTEEN

TABLE 14.2 Advantages and Disadvantages of Different PAC Application Points

Point of addition Advantages Disadvantages
Intake Long contact time, good Some substances may adsorb that
mixing otherwise would be removed by
coagulation, thus increasing the activated
carbon usage rate
Slurry contactor Excellent mixing for the A new basin and mixer may have to be
preceding rapid design contact time; no installed; some competition may occur
mix interference by coagulants; from molecules that otherwise would be
additional contact time removed by coagulants
possible during flocculation
and sedimentation
Rapid mix Good mixing during rapid mix Possible reduction in rate of adsorption
and flocculation; reasonable because of interference by coagulants;
contact time contact time may be too short for
equilibrium to be reached for some
contaminants; some competition may
occur from molecules that otherwise would
be removed by coagulation
Filter inlet Efficient use of PAC Possible loss of PAC through the filters
and into the distribution system

Source: Adapted from I. N. Najm et al., 1991.

Basis for Applying Powdered Carbon
Although some treatment plants feed PAC continuously at a low dosage, a far greater
number of plants keep a supply available for use during emergencies, such as organic pol-
lution, that may last only a day or so. Other plants may apply PAC for only a few weeks
out of the year during periods of poor source water quality, such as during algae blooms.
A major dilemma facing the designer is how to provide a PAC feed and storage system
to meet the full range of feed rates. PAC system design must also deal with how best to
provide for a chemical that is potentially dusty, difficult to handle, and difficult to store
for long periods, but is needed on a moment's notice.
The designer must first consider the specific needs of the owner, from which a suit-
able PAC feed strategy can be developed. The following questions should be asked be-
fore the design is begun:
• How often, over the course of a typical year, is carbon needed?
• What is a reasonable range of PAC feed rates to meet historical source water quality
characteristics?
• What is the worst-case scenario for PAC feed?
• How quickly can a shipment of PAC be delivered to the plant site?
A PAC system must be ready when needed, quick to respond, and flexible over a wide
range of needs.

Powdered Carbon Application Equipment
Powdered activated carbon can be fed as a powder using dry feed equipment or as a slurry
using metering pumps or rotodip feeders. A dry feed system should be considered if PAC