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Shannon
Citizens' Committee
Carbon
filter.
 | Many years ago, using activated carbon
for water filtration was more art than science.
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| Now, science understands more and more
how carbon works and what it eliminates from water.
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| Activated carbon can eliminate some
Volatile Organic Compounds like benzene, TCE, toluene, ...
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| Activated carbon does not eliminate
certain contaminants like microbes, sodium (Na), nitrates, fluoride (F),
water hardness ...
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| Activated carbon is made of
tiny clusters of carbon atoms stacked upon one another. The carbon source is from
a variety of materials such as peanut or coconut shells, wood or coal. The raw carbon
source is slowly heated to dehydrate it. It is then heated more in the absence of air
(oxygen) which crystalizes it and produces a high carbon material (Fig. A). |
| The carbon is activated by passing
oxidising gases through the materiel at extremely high temperatures. This activation
process produces the pores (Fig. B) that result in high adsorptive properties.
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| Activated carbon is a highly porous
material, it as an equivalent surface area of 60 to 150 acres to the pound
(1 acre = 4046,77 m2).
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| Activated carbon eliminates volatile
compounds by adsorption in trapping them at the porous surface.
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| Adsorption is the attraction of molecules
at the surface of carbon (Fig. C). The forces of physical
attraction to the carbon (molecular level electrical forces) are stronger than the
forces that keep these molecules in suspension in a solution.
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| The adsorption
process depends on the following factors :
1) Physical properties of the activated carbon, such as pore
size, distribution,
equivalent area ...
2)
The chemical nature of the carbon source, amount of oxygen (O) and hydrogen (H)
associated with the carbon
3)
The chemical composition and concentration of contaminant
4)
The temperature and pH (acidity) of the water
5)
The time exposure of water to activated carbon (flow)
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| At the start, activated carbon filters
where installed in two and finally in 33 residences where the level of TCE
in wells was at or over 5 ppb or 5 µg/l. This value corresponds to the
American norm of the US EPA, United States Environmental Protection Agency.
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| After a certain time,
the adsorption capacity of the activated charcoal is diminished because there is less and
less charcoal surface to adsorb TCE. And even, after a sufficient time, the charcoal
adsorption capacity is exhausted. The TCE level will start to augment after the filter. It is time to change the activated
charcoal.
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| But when does
this happen ? We cannot know this with certainty because the adsorption capacity depends
on several factors. And so, du to time and testing cost factors, a way to remedy
this fact would be to install 2
filters in series. In this way,
if the TCE level at the output of the first filter starts going up, the second filter will
adsorb the TCE that passed through the first one. The next thing to do is to change the first filter (the
charcoal) with a position
rotation with the remaining filter.
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| Of the 33
residences where the TCE level in wells was at or over 5 ppb or 5 µg/l, there
was 1, and 3, then 6 and now 12 (13) residences in which 2 filters in series where
installed (05/2001).
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| In water, there is something else than
TCE. There are particles and matters in suspension, sand, etc. Du to
the flow and pressure, by passing by and between the activated charcoal
particles, there are some of these particles and matters in suspension who run
alongside, touch, hit, and stick to the charcoal surface. An effect
of this is a clogging of charcoal pores and this in turn eliminates a
certain surface that should adsorb some TCE. The lifetime of the filter is
so diminished.
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| A way to partially reduce this phenomenon
is to have water circulating in the opposite direction of normal flow. Water circulating
in this way will dislodge a certain quantity of those particles and matters in suspension
who where clogging pores and in this way, free additional surface for TCE adsorption.
This function of inverse circulation of water is called "retro-circulation
cycle" or "regeneration cycle" or "backwash".
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| After
a certain number of those backwash cycles, it becomes more difficult to eliminate certain
particles and maters well hooked to the charcoal. This in turn diminishes the surface
of charcoal in contact with the water and in this way reduces the adsorption of
TCE.
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| When
in the backwash cycle, the well water (or water source) is brought and pushed under
pressure trough the charcoal but in the inverse direction of normal circulation. Some
of the particles that where held on the charcoal surface will dislodge and will
be rejected with the water. This evacuated water is thus soiled and is
possibly improper for consumption. If this water is rejected, it will
contaminate with matters that where in suspension in the water.
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| The
backwash cycle does not have as a goal and does not dislodge TCE from the surfaces of the
activated charcoal. This cycle dislodges some of the particles and matters in
suspension that piled up on the charcoal. For TCE, the electrical forces that
produces the capture by adsorption are stronger than the mechanical forces produced by the
inverse flow of the water during the backwash cycle. So, the TCE stays on
the surfaces of the charcoal.
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| Water
that feeds the backwash cycle comes from the well and is in this way not treated for TCE
adsorption. But, this water comes in contact with the activated charcoal, so part of
the TCE in the water will be possibly adsorbed at the charcoal surface. But, like it was
cited earlier, one of the influential factors on adsorption by charcoal, is the time
exposure of the contaminated water with the charcoal (flow). In this way, rejected
water is possibly still contaminated with TCE and soiled with particles and
maters in suspension. If this water is rejected, it will contaminate with
matters that where in suspension in the water and with TCE or other organic
contaminants of the same class.
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| Activated
charcoal filters does the work for which it is employed and does it well, but one must be
very vigilant time wise.
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Fig. A
Fig.
B
Fig. C