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          Regroupement des Citoyens

                      de Shannon !

Shannon Citizens' Committee

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RedLINE.gif (1766 octets)

Carbon filter.

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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 ...

bulletActivated 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).

Carbone00.bmp (12862 octets)Fig. A

Carbone01.bmp (12862 octets)Fig. B

Carbon002.bmp (30942 octets)Fig. C

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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.