Activated Carbon
The main objectives of drinking
water treatment are to produce high quality
water that is safe for human consumption, has aesthetic appeal, conforms to
state and federal standards, and is economical in production. One of the tools
that helps to achieve these goals is activated carbon.
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Close-up
of Sample Activated Carbon Particles |
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Activated carbon is useful in
drinking water treatment because it acts as an adsorbent, and can effectively remove
particles and organics from water. These organics are of great concern in water
treatment because they react with many disinfectants, especially chlorine, and
cause the formation of disinfection-by-products, DBP's.
These DBP's are often carcinogenic
and therefore highly undesirable. This problem has been addressed most recently
by the new standards that were created by the 14th Title of the Public Health
Service Act. This Act, most commonly known as the Safe Drinking Water Act set
maximum contaminant levels (MCL's) in treated water for such DBP's as volatile
organic chemicals and trihalomethanes.
Activated
carbon is one of the best tools which can be used to reduce risks to human
health and provide an aesthetically pleasing product at reasonable cost.
Adsorption
is the process by which Activated Carbon removes substances from water.
Defined, adsorption is "the collection of a substance onto the surface
of adsorbent solids." It is a removal process where certain particles are
bound to an adsorbent particle surface by either chemical or physical
attraction. Adsorption is often confused with Absorption, where the
substance being collected or removed actually penetrates into the other
solid.
Close-ups
of Carbon Surface and Pores
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The
reason that activated carbon is such an effective adsorbent material is due
to its large number of cavernous pores. These provide a large surface area
relative to the size of the actual carbon particle and its visible exterior
surface. An approximate ratio is 1 gram = 100 m2 of surface area.
Scanning
Electron Microscope Images of Pores
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Activated
Carbon uses the physical adsorption process whereby attractive van der Waals forces pull the solute out of solution and onto its surface. Once the
solute is bound to the carbon is it considered "removed" from the
water. The animation below illustrates this process where the organics are
drawn toward the activated carbon by these forces.
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Activated carbon adsorption
proceeds through 3 basic steps
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Adsorption
efficiency decreases over time and eventually activated carbon will need to
be replaced or reactivated. Isotherms are empirical relations which are used
to predict how much solute can be adsorbed by activated carbon. The three
most well-known isotherms are the Freundlich, Langmuir and Linear. In
environmental engineering and specifically
drinking water treatment application the most commonly used isotherm is the
Freundlich. Shown to the right is the Freundlich isotherm equation in general
form.
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The two graphs below illustrate a general Freundlich isotherm equation and a
sample breakthrough curve. Each individual type of GAC has its own isotherm
curve and breakpoint characteristics. These help to predict the adsorptive
capacity of particular activated carbons and give a design estimate for
adsorptive life. Reactivation becomes necessary once the breakpoint has been
reached.
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Properties of Activated Carbon
The
process of activated carbon generation begins with the selection of a raw carbon
source. These sources are selected based on design specifications since
different raw sources will produce activated carbon with different
properties. Some of the more common raw sources include wood, sawdust,
lignite, peat, coal, coconut shells, and petroleum residues.
Characteristics
of importance in choosing carbon types include pore structure, particle size,
total surface area and void space between particles. After selection of a
source, preparations for use are made. These preparations most often include
dehydration, carbonization, and activation. Dehydration and carbonization
involve slow heating of the source in anaerobic conditions. Chemicals such as
zinc chloride or phosphoric acid can be used to enhance these processes. The
stage of activation requires exposure to additional chemicals or other
oxidizing agents such as a mixture of gases. Depending upon the specifics of
the process and the source carbon, the newly activated carbon can be
classified according to density, hardness, and other characteristics.
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Carbonized Coconut
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Activated Coconut
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As
mentioned previously, another important characteristic of activated carbon is
the isotherm or breakpoint characteristic for each particular type. This is
most often determined by modeling, testing, cost analysis, and pilot studies.
The pilot studies ensure that the chosen carbon type effectively removes the
desired substances for the particular raw water source and allows the plant
to reach desired levels of quality before treatment continues. Once a
breakpoint of a particular carbon has been determined the plant operators
know approximately how long the carbon will effectively function. As this
time approaches the carbon must be changed to ensure adequate removal.
The
"spent" carbon, as it is called, is removed and sent for
re-activation treatment. This is done primarily with granular activated
carbon because PAC particles are too small to be effectively re-activated.
This process allows for recovery of approximately 70% of the original carbon.
This number also allows for any physically lost in the shipment process. The
re-activated carbon is then mixed with a portion of new carbon for higher
effectiveness and is then returned to its place in the plant process.
Reactivation Process Specifics
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#
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Stage
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Temperature
(degrees C)
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Action
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1
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Drying
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<
100
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GAC
dewatered to 50% of original weight
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2
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Desorption
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100
- 649
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volatile
materials driven off
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3
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Pyrolysis
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100
- 649
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heavy
organics burnt leaving residue
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4
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Gasification
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>>649
and >>1038
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vapors
and residues from previous stages driven out of pores
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Applications in the water industry
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The
water industry uses activated carbon in several forms, typically powdered and
granular, to deal with a variety of undesirable aspects in raw water. To the
right is an image of pilot scale carbon columns at a water treatment plant.
Seasonal
application of powdered activated carbon (PAC) at the raw water intake or
rapid mix unit is used by some plants to correct short term raw water quality
problems such as algal blooms. PAC is basically used to correct taste and
odor problems which are primarily an aesthetic quality of the water. Other
uses such as residual ozone destruction and chemical contamination prevention
exist, but are not as well documented. Contact time is needed to allow
adsorption to occur. The PAC is removed from the water by the processes of
coagulation, flocculation, and sedimentation. Once the PAC has been separated
from the water it is disposed of along with sedimentation sludge. Some
non-traditional systems such as sludge-blanket clarifiers also use PAC
(Hoehn, 1996).
Comparison of Powdered and Granular Activated Carbon
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PAC
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PAC
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GAC
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GAC
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Granular
activated carbon is typically found in beds or filter columns as a Granular
Activated Carbon (GAC) Cap and will treat water continuously when raw water
quality problems exist year round. The GAC Cap is typically found above the
filter media as a distinct layer. In some applications the sand layer can be
replaced by GAC. When GAC is used for long term applications it can be more
economical since the carbon can be reactivated following decreased adsorption
efficiency.
If a GAC cap follows ozonation in the treatment process, a biological layer can
be cultivated in the granular activated carbon cap. Since ozone disinfection
leaves no residual disinfectant this biological layer is able to grow freely
and metabolize some of the organics in the water, enhancing the overall
removal by the GAC column.
Type
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Size(typical)
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Usage
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Economics
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Granular
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12x30,
12x40, 8x30 mesh
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6
month to 2 years
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more
$
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Powdered
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less
than 150 mesh
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short
term, as needed
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less
$
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