Environmental Engineering Reference
In-Depth Information
surface area of an adsorber, the greater its power. Each activated carbon con-
tains a huge number of pores and crevices into which organic molecules
enter and are adsorbed onto the activated carbon surface.
Activated carbon has a particularly strong attraction for organic molecules
such as the aromatic solvents benzene, toluene, and nitrobenzene; the chlo-
rinated aromatics polychlorinated biphenyls (PCBs), chlorobenzenes, and
chloronaphthalene; phenol and chlorophenols; the polynuclear aromatics
acenaphthene and benzopyrenes; pesticides and herbicides; chlorinated ali-
phatics such as carbon tetrachloride and chloroalkyl ethers; and high-molec-
ular-weight hydrocarbons such as dyes, gasoline, amines, and humics.
Two forms of activated carbon are used in water treatment: powdered and
granular. Powdered activated carbon is often used for taste and odor control.
Its effectiveness depends on the source of the undesirable tastes and odors. It
is also effective in removing the organic precursors that react with chlorine
to form harmful THM compounds after disinfection.
Powdered activated carbon is a finely ground, insoluble black powder that
can be added at any point in the treatment process ahead of the filters. It is
fed to water either as a dry powder or as a wet slurry. Although adsorption is
nearly instantaneous, a contact time of 15 minutes or more is desirable before
sedimentation or filtration.
Activated carbon media must periodically be replaced with new or regen-
erated activated carbon. Replacement cycles can vary from 1 to 3 years for
taste and odor treatment to as little as 4 or 5 weeks for removal of organics.
The activated carbon regeneration process involves: (1) removing the spent
carbon as a slurry, (2) dewatering the slurry, (3) feeding the carbon into a
special furnace where regeneration occurs (i.e., the organics are driven from
the carbon surface by heat), and (4) returning it to use.
Activated alumina
(a highly porous and granular form of aluminum oxide)
is also an adsorptive medium used in water treatment. It is used primarily
to remove arsenic and excess fluoride ions. Water is percolated through a col-
umn of alumina media, and a combination of adsorption and ion exchange
performs the actual removal of arsenic and fluoride ions. Like the regenera-
tion process used to restore used activated carbon to full potency, activated
alumina also requires periodic regeneration, accomplished by passing a
caustic soda solution through the media. Excess caustic soda is neutralized
by rinsing the activated alumina with an acid. Disposal of these wash waters,
laden with toxic arsenic and fluoride ions, must be done in accordance with
applicable laws.
Note:
Powdered activated carbon is much more difficult to regenerate
than granular activated carbon. Granular activated carbon is sometimes
used in the filter bed itself, combining both filtration and adsorption in
one treatment unit. The major problem associated with granular activated
carbon systems is suspended solids in the water plugging up the bed.
