What
is Granulated Activated
Carbon?
How
is it Used For
Water Filtering?
Carbon is a substance that has a long history
of being used to absorb impurities and is the most powerful absorbent known to
man. One pound of carbon contains a surface area of 125 acres and can absorb
thousands of different chemicals. For centuries, sailing vessels used it to
store drinking water for long voyages. Carbon is also commonly used as an
effective antidote for swallowed poisons.
Activated carbon is carbon which has a slight
electro-positive charge added to it, making it even more attractive to chemicals
and impurities. Loose granular activated carbon (GAC) is used extensively in
most of the commonly encountered water filters today. Most of these filters have
ratings as simple taste and odor filters or as pre-filters designed to remove
initial dirt, rocks, sediment, etc. Very few of these GAC filters are effective
at true filtration (i.e. removal of substances of health concern). Below are
some problems:
- Channeling: When water is
forced through a substrate of GAC (which has the consistency of sand or
coffee grounds) it takes the path of least resistance and makes its own
channels through the GAC in the filter. What this means is that the water
being filtered is not contacting the GAC for enough time to allow complete
absorption of impurities to take place. Many impurities seep through along
with the "filtered" water.
- Bacterial growth: When
the water passes through the GAC, some bacteria (primarily heterotrophic)
are trapped in the substrate, too. The problem is that these trapped
bacteria multiply prolifically in the warm, moist oxygen-containing
environment among the grains of GAC. Because the GAC is loose, there is no
barrier to keep any bacteria growing inside the filter from coming out with
the water pushed through each time the filter is used. There have been
extensive tests showing certain filters have thousands more bacteria coming
out of them than initially went into them! Some companies add silver
nitrate, a known poison, to the GAC to aid in keeping the bacterial growth
down, but this method has limited effectiveness. There are also serious
questions about adverse health effects from the silver nitrate itself!
- Effectiveness decreases
rapidly: Because the amount of GAC is usually minimal in most GAC filters,
they quickly become saturated and overwhelmed with normal household usage.
The filters must be changed quite often, which also adds to the expense of
filter maintenance.
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