Activated Carbon Filtration
By Frank
DeSilva. Published in Water Quality Products Magazine, January, 2000
Granular activated carbon (GAC) is commonly used for
removing organic constituents and residual disinfectants in water supplies. This
not only improves taste and minimizes health hazards; it protects other water
treatment units such as reverse osmosis membranes and ion exchange resins from
possible damage due to oxidation or organic fouling. Activated carbon is a
favored water treatment technique because of its multifunctional nature and the
fact that it adds nothing detrimental to the treated water.
Most activated
carbons are made from raw materials such as nutshells, wood, coal and petroleum.
Typical surface
area for activated carbon is approximately 1,000 square meters per gram (m2/gm).
However, different raw materials
produce different types of activated carbon varying in hardness, density, pore
and particle sizes, surface areas, extractables, ash and pH. These
differences in properties make certain carbons preferable over others in
different applications.
The two
principal mechanisms by which activated carbon removes contaminants from water
are adsorption and catalytic reduction. Organics are removed by adsorption
and residual disinfectants are removed by catalytic reduction.
Factors that
affect the performance of activated carbon are:
Molecular
weight:
As the molecular weight increases, the activated carbon adsorbs more
effectively because the molecules are lea soluble in water. However, the pore
structure of the carbon must be large enough to allow the molecules to migrate
within. A mixture of high and low molecular weight molecules should be designed
for the removal of the more difficult species.
pH:
Most organics are less soluble and more readily adsorbed at a lower pH.
As the pH increases, removal decreases. A rule of thumb is to increase the size
of the carbon bed by twenty percent for every pH unit above neutral (7.0).
Contaminant
concentration: The higher the contaminant concentration, the greater the removal
capacity of activated carbon. The contaminant molecule is more likely to diffuse
into a pore and become adsorbed. As concentrations increase, however, so do
effluent leakages. The upper limit for contaminants is a few hundred parts per
million. Higher contaminant concentration may require more contact time with the
activated carbon. Also, the removal of organics is enhanced by the presence of
hardness in the water, so whenever possible, place activated carbon units
upstream of the ion removal units. This is usually the case anyway since
activated carbon is often used upstream of ion exchange or membranes to remove
chlorine.
Particle
size:
Activated carbon is commonly available in 8 by 30 mesh (largest), 12 by
40 mesh (most common), and 20 by 50 mesh (finest). The finer mesh gives the best
contact and better removal, but at the expense of higher pressure drop. A rule
of thumb here is that the 8 by 30 mesh gives two to three times better removal
than the 12 by 40, and 10 to 20 times better kinetic removal than the 8 by 30
mesh.
Flow rate:
Generally, the lower the flow rate, the more time the contaminant will
have to diffuse into a pore and be adsorbed. Adsorption by activated carbon is
almost always improved by a longer contact time. Again, in general terms, a
carbon bed of 20 by 50 mesh can be run at twice the flow rate of a bed of 12 by
40 mesh, and a carbon bed of 12 by 40 mesh can be run at twice the flow rate of
a bed of 8 by 30 mesh. Whenever considering higher flow rates with finer
mesh carbons, watch for an increased pressure drop!
Temperature:
Higher water temperatures decrease the solution viscosity and can
increase die diffusion rate, thereby increasing adsorption. Higher temperatures
can also disrupt the adsorptive bond and slightly decrease adsorption. It
depends on the organic compound being removed, but generally, lower temperatures
seem to favor adsorption.
Organic Removal
Organic material in public water supplies
comes from decaying plant life, which becomes more soluble in water over time
and exists as large, high-molecular weight organic acids (non-polar weak acids).
Eventually, smaller molecular weight acids of varying sizes form. Typical
organic acid molecules range in molecular weight from a few hundred to tens of
thousands.
The size,
number and chemical structure of organic acid molecules depend on a large number
of factors, including water pH and
temperature. Accordingly, there exists an almost infinite number of organic
acids. As a result, removing organics can be difficult and is
always site-specific.
Activated
carbon's adsorptive properties are used to remove organics. Generally,
adsorption takes place because all molecules exert forces to adhere to each
other. Activated carbon adsorbs organic material because the attractive forces
between the carbon surface (non-polar) and the contaminant (non-polar) are
stronger than the forces keeping the contaminant dissolved in water (polar).
The adsorptive forces arc weak and cannot occur unless the organic molecules are
close to the carbon's surface. The large surface am of the activated carbon, due
to its particle size and pore configuration, allows for the adsorption to take
place.
Factors that decrease solubility and/or increase accessibility to the pores
improve the performance of the activated carbon filter. Carbon filter
capacity can be roughly estimated at 0. 1 pound of organics per 1 pound of
carbon at a flow rate of 1 to 2 gallons per minute per cubic foot (gpm/cu.ft.)
and a bed depth of 3 feet.
Residual
disinfectants removal
Activated carbon can remove and destroy
residual disinfectants (chlorine and chloramine) through a catalytic reduction
reaction. This is a chemical reaction that involves a transfer of electrons from
the activated carbon surface to the residual disinfectant. In other words,
activated carbon acts as a reducing agent.
Activated
carbon's removal of chlorine reduces the chlorine to a non-oxidative chloride
ion. The reaction is very fast and takes place in the first few inches of
a new activated carbon bed. (Where removal of organics by activated carbon
takes minutes, removal of chlorine literally takes seconds). The chlorine
capacity of new activated carbon is 1 pound of chlorine per pound of carbon at a
flow rate of 3 to 5 gpm/cu.ft. and a bed depth of 3 feet.
Chloramine removal by activated carbon is a much slower reaction. The
predominant species of chloramine in city water supplies (pH about 7 to 8) is
monochloramine. The reaction with activated carbon and monochloramine also
renders a non-oxidative chloride ion. Since the rate of reaction is
considerably slower, the flow rate should be 0.5 gpm/cu.ft. and the bed depth
greater than 3 feet.
Material considerations
Activated carbon beds are filters and need
to be backwashed periodically. A freeboard of about 50 percent should be
incorporated into the vessel design to allow backwash inplace. Otherwise,
external backwash is required. The backwash step does not 'regenerate' the
carbon or de-adsorb contaminants. The backwash step reclassifies the bed and
removes any fines or suspended matter.
Carbon fines
are generated during transport, handling and loading of activated carbon. These
fines need to be backwashed out before service. Pre-wetted and backwashed
carbons are available that minimize the fines and also eliminate the problems
mused by carbon dust in a facility, Loading carbon tanks with dry carbon is a
messy, hazardous job. Using pre-wetted carbon eliminates the airborne dust and
makes for a clean plant environment.
Processed
grades of activated carbon are available that include medical/pharmaceutical
grades, electroplating grades, and powdered or pelletized carbons.
Activated carbon is a proven technology for the removal of naturally occurring
organics and residual disinfectants. Designing an activated carbon filtration
system needs to take into account the differences in the water to be treated,
the type of activated carbon used, and the effluent quality and operating
parameters.
GAC System
Design Parameters
| |
Chlorine |
Chloramine |
Organics |
|
Flow Rate (gpm/ft.²) |
1-3 |
0.5 |
1-2 |
|
Minimum Bed Depth (ft.) |
2-3 |
6 |
3-5 |
| Bed
Life |
Almost
Indefinite |
2-6
Weeks |
1 - 6
Months |
Typical Properties of Granular Activated Carbon
| |
Bituminous |
Sub-bituminous |
Lignite |
Nut Shell |
| Iodine Number |
1,000-1,100 |
800-900 |
600 |
1,000 |
| Molasses Number |
235 |
230 |
300 |
0 |
| Abrasion Number |
80-90 |
75 |
60 |
97 |
| Bulk Density as packed LB/CF |
26-28 |
25-26 |
23 |
29-30 |
| Volume Activity |
26,000 |
25,000 |
13,800 |
0 |
Iodine and molasses numbers measure
pore size distribution. Iodine number is a relative measure of pores at
sizes of 10 to 2 Angstroms. It is reported in milligrams of elemental iodine
adsorbed per gram of GAC and determines the area available on the GAC to adsorb
low molecular weight organics.
Molasses number measures the degree a
GAC removes color from a stock solution. It measures pores greater than 28
Angstroms. These are the pores responsible for removing larger molecular weight
organics such as tannins.
Abrasion numbers represent the relative
degree of particle size reduction after tumbling with a harder material. No
reduction is rated 100, complete pulverization is zero.
ADDITIONAL READING:
Cooney, David O.,
Adsorption
Design for Wastewater Treatment,
Lewis
Publishers, Boca Raton, FL (1999)
McGowan, Wes,
Residential
Water Processing,
Water Quality Association, Lisle, IL (1997)
Meltzer, Theodore H.,
High Purity
Water Preparation,
Tall Oaks
Publishing, Littleton, CO (1993)
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