Granular activated carbon (GAC)
is commonly used for removing organic constituents and
residual disinfectants in water supplies
not only improves taste and minimizes health hazards; it protects other water treatment units
exchange resins from possible damage due to such as reverse osmosis membranes and ion
oxidation or organic fouling.
is a favored water treatment technique because of its multifunctional nature and the fact that it Activated carbon adds nothing detrimental to the treated water
materials such as nutshells, wood, coal and Most activated carbons are made from raw
approximately 1,000 square meters per gram Typical surface area for activated carbon is
m /gm). However, different raw materials 2
varying in hardness, density, pore and particle produce different types of activated carbon
sizes, surface areas, extractables, ash and pH
carbons preferable over others in different These differences in properties make certain
activated carbon removes contaminants from The two principal mechanisms by which
Organics are removed by adsorption and water are adsorption and catalytic reduction
residual disinfectants are removed by catalytic reduction.
of activated carbon are Factors that affect the performanceMolecular weight
activated carbon adsorbs more effectively As the molecular weight increases, the
because the molecules are lea soluble in water
be large enough to allow the molecules to However, the pore structure of the carbon must
designed for the removal of the more difficult molecular weight molecules should be
A mixture of high and low species pH
Most organics are less soluble and more readily
adsorbed at a lower pH. As the pH increases removal decreases
increase the size of the carbon bed by twenty A rule of thumb is to
percent for every pH unit above neutral (7.0) Contaminant concentration: The higher the
contaminant concentration, the greater the removal capacity of activated carbon.
contaminant molecule is more likely to diffuse into a pore and become adsorbed The
concentrations increase, however, so do As
The upper limit for contaminants is a few hundred parts per million
require more contact time with the activated Higher contaminant concentration may
Also, the removal of organics is
water, so whenever possible, place activated enhanced by the presence of hardness in the
carbon units upstream of the ion removal units.
This is usually the case anyway since activated carbon is often used upstream of ion or membranes to remove chlorine exchangeParticle size
Activated carbon is commonly available in 8 by 30 mesh (largest), 12 by 40 mesh (most
common), and 20 by 50 mesh (finest).
removal, but at the expense of higher pressure finer mesh gives the best contact and better
mesh gives two to three times better removal A rule of thumb here is that the 8 by 30
than the 12 by 40, and 10 to 20 times better
kinetic removal than the 8 by 30 mesh Flow rate
time the contaminant will have to diffuse into a Generally, the lower the flow rate, the more
pore and be adsorbed.
carbon is almost always improved by a longer Adsorption by activated
bed of 20 by 50 mesh can be run at twice the Again, in general terms, a carbon
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.
finer mesh carbons, watch for an increased Whenever considering higher flow rates with
pressure drop. Temperature
Higher water temperatures decrease the solution viscosity and can increase die diffusion rate thereby increasing adsorption.,
temperatures can also disrupt the adsorptive bond and slightly decrease adsorption Higher
removed, but generally, lower temperatures depends on the organic compound being
seem to favor adsorption Organic Removal
Organic material in public water supplies come s from decaying plant life, which becomes soluble in water over time and exists as large, more
high-molecular weight organic acids (non-polar weak acids).
weight acids of varying sizes form.
Typical Eventually, smaller molecular
weight from a few hundred to tens of organic acid molecules range in molecular
organic acid molecules depend on a large The size, number and chemical structure of
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
takes place because all molecules exert forces to Generally, adsorption
adhere to each other
organic material because the attractive forces Activated carbon adsorbs
between the carbon surface (non-polar) and the
forces keeping the contaminant dissolved in contaminant (non-polar) are stronger than the
occur unless the organic molecules are close to The adsorptive forces are weak and cannot
the carbon's surface.
activated carbon, due to its particle size and The large surface am of the
pore configuration, allows for the adsorption to take place.
accessibility to the pores improve the Factors that decrease solubility and/or increase
performance of the activated carbon filter
Carbon filter capacity can be roughly estimated at 0. 1 pound of organics per I pound of at a flow rate of I to 2 gallons per minute per carbon
cubic foot (gpm/cu.ft.) and a bed depth of 3 feet Residual disinfectants removal
residual disinfectants (chlorine and chloramine Activated carbon can remove and destroy
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
takes minutes, removal of chlorine literally (Where removal of organics by activated carbon
activated carbon is I pound of chlorine per The chlorine capacity of new
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
of chloramine in city water supplies (pH about The predominant species
to is monochloramine
activated carbon and monochloramine also The reaction with
renders a non-oxidative chloride ion.
rate of reaction is considerably slower, the flow Since die
rate should be 0.5 gpm/cu.ft.
greater than 3 feet. and the bed depthMaterial considerations
Activated carbon beds are filters and need to be backwashed periodically.
50 percent should be incorporated into the A freeboard of about
vessel design to allow backwash inplace.
Otherwise, external backwash is required.
backwash step does not "regenerate' the carbon The
or de-adsorb contaminants. 'Me backwash step
reclassifies the bed and removes any fines or
Carbon fines are generated during transport handling and loading of activated carbon
These fines need to be backwashed out before service
are available that minimize the fines and also Pre-wetted and backwashed carbons
eliminate the problems mused by carbon dust
in a facility, Loading carbon tanks with dry
carbon is a messy, hazardous job.
pre-wetted carbon eliminates the airborne dust Using
Processed grades of activated carbon are and makes for a clean plant environment
available that include medical/pharmaceutical
pelletized carbons grades, electroplating grades, and powdered grades. grades,
removal of naturally occurring organics and Activated carbon is a proven technology for theresidual disinfectants
carbon filtration system needs to take into Designing an activated
treated, the type of activated carbon used, and account the differences in the water to be
the effluent quality and operating parameters
u]GAC System Design Parameters[/u]
Chlorine ///// Chloramine ///// Organics
Flow rate 1-3 ///// 0.5 ///// 1-2 gpm/ft.l
Min bed depth 2-3 ///// 6.0 ///// 3-5 ft
Bed life indefinite ///// 2-6w ///// 1-6m
Typical Properties of Granulated Activated Carbon
Bituminous ----Sub- Bituminous ---- Lignite ----- Nut sh
Iodine number 1,000 -1.10 --- 800-900 --- 600 000
Molasses number 235 230 300 0
Abrasion number 80-90 75 60 97
Bulk density as 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
It is reported in milligrams of elemental iodine
adsorbed per gram of GAC and determines the area available on
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.