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ACTIVATED CARBON PROCESSES 14.5

• The AWWA standard for PAC specifies a minimum iodine number of 500. Higher io-
dine numbers indicate greater adsorbancy.
• Higher molasses numbers indicate greater adsorbancy.
• Moisture content should be minimized and should not exceed 8%.
• AWWA standard specifies a minimum apparent density of 0.25 g/cm 3.
• Ash content, which is an impurity that reduces adsorptive capacity, should be mini-
mized.
• Smaller PAC particles adsorb organic compounds more rapidly than larger particles, so
PAC is manufactured as a very fine powder. Typically, 65% to 95% of commercially
available PAC passes through a 325-mesh (44-/zm) sieve.

Selecting a PAC Application Point
PAC is primarily used to control tastes and odors. Important factors that should be con-
sidered in selecting a PAC application point include the following:

• The contact time between PAC and organics in the source water is important and de-
pends on the ability of the carbon to remain in suspension. Providing a minimum of 15
min of contact time is generally sufficient for most taste and odor compounds. Con-
siderably longer contact times may be needed for 2-methylisoborneol (MIB) geosmin
removal.
• The surface of PAC particles should not be coated with coagulants or other water
treatment chemicals before the PAC has had adequate contact time with the source
water.
• PAC should not be added concurrently with chlorine or potassium permanganate be-
cause PAC will adsorb these chemicals. The best location for PAC addition is usually
at the head of the plant, either in the source water pipeline or in a basin dedicated to
rapid or flash mixing chemicals. Applying PAC at the earliest point in the treatment
process allows the longest contact time possible before the application of other chem-
icals. If PAC must be added later in the treatment process, dosages may be higher to
account for shorter contact times and interference by other chemicals, such as coagu-
lants and chlorine.
If PAC is added to the sedimentation basin effluent or filter influent, particular
care must be taken in filter operation. Because of its small carbon particle size,
PAC can pass through a filter and cause black water complaints from consumers. An
approach used by some systems is to add carbon at two or more application points,
with part of the carbon added to the source water and smaller doses added before fil-
tration to remove remaining taste- and odor-causing compounds. Table 14.2 summa-
rizes some of the important advantages and disadvantages of PAC addition at each of
these points.
PAC is increasingly being used in conjunction with microfiltration or ultrafiltration
membrane systems. Not all membrane filtration products are compatible with PAC so the
types of PAC that can be used may be limited. Therefore, it is vital that the use of PAC
be discussed with the membrane filtration supplier when considering this application.
PAC dosage depends on the type and concentration of organic compounds present.
Common dosages range from 2 to 20 mg/L for nominal taste and odor control, but
doses can exceed 100 mg/L to handle severe taste and odor episodes or spills of organic
chemicals.