Drinking water treatment professionals have long held fast
to the belief that granular activated carbon (GAC) based on bituminous coal
provides the best performance for their demanding application. That’s
why, when an article in 1999 cited evidence that a lignite-based GAC
outperformed a bituminous-based carbon, industry experts were surprised and
more than a bit skeptical.
The results of the study published in the article detailed
how the Fresno Sole Source Aquifer in California selected lignite-based carbon
after reviewing a manufacturer’s comparison test. It offered evidence
that lignite-based GAC treated 35 percent more water than the bituminous GAC
before reaching saturation. Furthermore, the test showed that lignite GAC had a
30 percent longer life than bituminous GAC.
“The study clearly was misleading,” says Andy
McClure, marketing manager for Calgon Carbon Corp. “The data were 100
percent correct, but what the report didn’t say was that the study
compared lignite-based carbon with bituminous-based carbon that was produced
offshore through a direct activation process. That’s a totally different
product than the reagglomerated bituminous coal-based carbon most water
treatment professionals use.”
Direct Activated Versus Reagglomerated
“People who have used both direct activated and
reagglomerated carbons believe there’s a difference,” says Bob
Little, water quality supervisor for the City of Fresno. Most of that
difference can be attributed to how a GAC is made.
Reagglomerated carbons are manufactured through the
following process.
• A
high-grade raw material is pulverized to a powder.
• A
binder is added.
• The
product is reagglomerated into briquettes.
• The
briquettes are crushed.
• The
briquettes are sized.
• The
carbon is baked.
• The
carbon is thermally activated.
Offshore carbons often are produced through a cost-cutting
manufacturing process. Direct activation begins with an inexpensive raw
material and proceeds directly to crushing, sizing, baking and activation. To
save production costs, the pulverizing, binding and reagglomerating steps are
eliminated. While direct activation results in a lower price-per-pound carbon,
it compromises long-term product performance in most applications.
“The extra steps in making high performance
carbon—the reagglomeration process—means a lot to us,” says
John Yoshumara, manager at Stockton EW in California. “How well a carbon
performs is directly related to its internal pore structure.”
The internal pore structure of a carbon granule can be
compared to the infrastructure of roads in the United States. There are
superhighways (macropores), highways (mesopores), regular roads and dirt roads
(micropores). The larger pore structures (super highways and highways) provide
faster access to where the organic removal occurs. The tighter pore structure
(regular roads and dirt roads) is where the majority of the organic molecules
are removed through adsorption. By eliminating the steps of grinding, binding
and reagglomerating, offshore carbons exhibit fewer superhighways and highways
that allow organics to travel to the dirt roads, where adsorption takes place.
In many demanding applications, the lack of additional carbon pore
infrastructure equates to reduced performance and shorter bed life.
“The activation process obviously controls how the
carbon performs. Even if the source material was identical, a direct activated
GAC is simply not going to perform the same as a reagglomerated product,”
McClure states.
Differences between high-performance and offshore products
affect different applications to varying degrees. Offshore products initially
can be less expensive on a dollar-per-pound basis. However, by removing fewer
organic contaminants, they generally require more frequent changes. The
adsorption capacities of many offshore carbons are significantly lower.
Typically, they are less resistant to abrasion, which results in higher
transfer losses (backwash) and fines. The offshore products have approximately
6 percent fines, compared to 0.2
percent for high-performance carbon. In addition, offshore carbons can have
higher ash content, resulting in more leachables and lower adsorption
capacities. They have approximately 14 percent ash, compared to 5–7
percent for high-performance carbon. Based on fines (lost in backwash) and ash,
the offshore products offer the following equation.
6%
+ 7% = 13% unusable product or 13 percent higher cost based on pounds
Whether choosing offshore or high performance activated
products, buyers should be aware that any activated carbon not manufactured in
ISO-certified facilities offers no guarantee of ingredients or other materials
that may have been mixed in.
Source Material
Bituminous coal, anthracite, peat, wood and coconut each
affect a carbon’s inherent pore structure, influencing its properties and
performance. Nevertheless, the consistency and quality of the source material
also is extremely relevant.
“Water treatment professionals need to go beyond
simply specifying coal-based carbon for their job,” advises Dennis
Bitter, industrial account manager for Calgon Carbon. “They need to know
the source of the base for any activated carbon under consideration.”
Carbon suppliers always should disclose the source of the
starting base of their products along with details on their manufacturing
process so that buyers can make intelligent comparisons.
“Our top concern always has been the quality of the
product,” says Yoshumara, whose Stockton California treatment facility
remains loyal to high performance carbon. “We want to verify the quality
of the source material.”
Another California treatment facility with experience in
both types of carbon maintains that often it is a challenge to discover details
about offshore carbon. “Both the offshore and high performance carbons I
used were based on coal,” says Bob Hayward, general manager of Lincoln Avenue Water Co. in Altadena.
Performance You Can Count On
The problem that most often is associated with direct
activated carbon performance is uniformity. “At Fresno, we’ve seen
a lot of offshore carbon situations where one load will last 15 months at a
station and the next load will last 22 months—and nothing’s changed
as far as the water quality or concentration of the contaminant,” Little
says. “At some of our multiple-vessel sites, we’ve seen one or two
of our vessels reach port four with detectables, while another vessel is still
non-detect at port two. Supposedly it’s all the same carbon, but the
offshore performance is widely variable.”
Performance inconsistency is most likely caused by lack of
control and adherence to standards during the carbon activation process.
Reagglomeration plants take advantage of technology such as
digital readouts to ensure temperature and other variables remain constant.
Offshore carbons often are produced using more manual labor. “It really
is the difference between fine-tuning the control of the process—maintaining
established quality standards—versus mass production where you’re
just cranking out carbon,” McClure says.
A Clean Comparison
At the request of the City of Fresno, Calgon Carbon
duplicated the lignite versus bituminous test using a reagglomerated
bituminous-based product instead of the offshore media. “We ran column
studies using samples of the Fresno water in 1999,” McClure explains.
This time, the results were much different. “The
reagglomerated bituminous carbon was outperforming the lignite material by a factor
of three when the column test concluded—and it was still running at the
time,” a researcher noted. Although the original study was undertaken as
a way to show the advantage of lignite over coal, it actually succeeded in
proving the true difference between offshore, direct activated GAC and high
performance reagglomerated carbon.
Learning from the Past
The test results come as no surprise to many California
water treatment professionals. The experiences of Altadena’s Lincoln
Avenue Water Co. attest to the performance of reagglomerated carbon. “We
had used both. The high performance carbon lasted longer than the offshore
carbon—in fact, we experienced twice the carbon life from the
reagglomerated product over the direct activated GAC,” Hayward claims.
“We suspected from the start that the offshore carbon wouldn’t
deliver the same kind of performance as the product we had been using, but I
guess we had to experience it for ourselves.”
One year after the carbon dilemma began, Fresno is taking
great care to use high performance, reagglomerated carbon. Based on the test
results, Fresno again is purchasing bituminous coal-based GAC to product its
high quality water.
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