Page 148 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

3.16 CHAPTER THREE

the pore size, and the source method used to manufacture the carbon. The characteristics of
organic impurities also play a role in removal. The impurities’ molecular weight, polarity,
pH, temperature, and concentration are important factors in the rate of adsorption. This
filter is much more effective in stripping compounds of low molecular weight because those
of high molecular weight tend to be poorly adsorbed.
One problem with activated carbon filters is the growth of bacteria due to trapped organ-
ics, mostly because of the removal of chlorine. This can be controlled by frequent cleaning
of the filter with steam, hot water (80 °C or higher), or dilute caustic soda backwash.
The activated carbon media gradually lose their adsorptive capacity and have to be peri-
odically replaced or reactivated. Small amounts of media cannot be economically reactivated.
Coal-based media can be activated, but all other types are usually replaced. Reactivation is
accomplished by heating the media to a temperature of 1600 to 1800 °F. Expect a 10 to
20 percent loss of media during reactivation. After treatment, the carbon will lose some of
its smaller adsorption pores, reducing its capacity to retain trace level contaminants. If this is
an important factor, the media must be replaced with virgin material rather than reactivated.
The indication for replacement is an excessive pressure drop through the unit found through
periodic testing of the water quality.
A generally accepted conservative method used to select the size of the filter is to use a
3
figure of 5 gpm/ft of media. Use manufacturers’ specifications to select the housing size,
amount, and type of filter media for specific applications and flow rates.
Activated carbon filters used for pretreatment should allow for a face velocity of about
2
2 to 4 gpm/ft of cross-sectional area. A minimum depth of 24 in is recommended for most
feedwater streams, but the actual depth should be selected to achieve the recommended
contact time with the flowing water. These two figures should be adjusted depending on the
quantity and type of contaminants. Tests have shown that 1 g of activated carbon is capable
of removing 1 g of residual chlorine. Because of anticipated breakthrough of organic material,
the media should be replaced approximately every 6 months.

CARTRIDGE DEPTH FILTERS

Cartridge depth filters consist of comparatively thick, replaceable, preformed filter media
that are placed inside a housing. The particle size retained by a depth filter is not precisely
defined because of the random nature of the fiber matrix. The advantage of the depth filter is
that it has superior particle load because the filter is capable of holding particles throughout
its entire matrix.

Filter Media
Depth filter media vary and consist of wound fiber (such as cotton, polypropylene, or rayon)
or resin-bonded laminates (such as cellulose, acrylic rayon, or fiberglass). Filter cartridges
are of the wound or pleated type. The wound fiber type is used most often.
Wound filters have the same effect as a stack of woven cloth, with the filter wound
around a round mandrel instead of flat. The characteristics that affect filter operation are
the type of fiber, fiber diameter, and cartridge-winding techniques such as yarn tension,
winding pattern, and spacing.
Fibers are available in two types, staple and monofilament. Staple fibers such as cotton
and wool are spun into a thread or yarn, which is then wound onto the mandrel. Staple fibers
have a fuzzy surface, or nap, that provides a high surface area for adsorption in addition to
a long, random path for particle interception.
Monofilament fibers, such as those of polypropylene and other plastics, are not spun but
are manufactured in any desired diameter. These fibers must be texturized in some fashion

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