When to change the activated carbon?
The granular activated carbon used in drinking water treatment processes has two functions: to retain organic contaminants and to remove free residual chlorine from the predisinfection stage.
Although both functions are important, the first one is often lost sight of because, if the activated carbon fails in it, the negative effects are not noticed in the short term.
However, if the activated carbon does not dechlorinate the water well, it remains with the smell and taste produced by the chlorine, which is very perceptible and unpleasant. Therefore, the criterion by which the performance of activated carbon is usually evaluated is the absence of free chlorine in the water treated by it.
Unfortunately, the ability of activated carbon to remove free chlorine is much greater than its ability to adsorb organic matter. This is because the removal of chlorine occurs by a chemical reaction in which the carbon is very efficient. Therefore, when you wait until you detect free chlorine in the treated water, as a criterion for changing the activated carbon, it is likely that much earlier the carbon has stopped retaining organic contaminants.
These organic contaminants are present in every natural water body: wells, rivers, lakes, meltwater… Many of them are synthetic (they did not exist before the industrial era) and they harm man since he is not adapted to them, since they were not present in the environment during the long evolutionary period of the human species. Drinking water standards set maximum allowable levels for them on the order of parts per billion. Among these contaminants are: benzene, toluene, hexachlorobenzene, total trihalomethanes, all pesticides, etc.
Assume that well water is chlorinated up to 1 mg / l of residual chlorine, and that this water has a COD (Chemical Oxygen Demand) of 20 mg / l. After chlorination, the water is treated in a tank with granular activated carbon, with an empty bed contact time of 5 min, for eight hours per day, six days per week. This carbon bed will effectively retain organic contaminants for 12 months. However, it will completely eliminate free chlorine for more than two or three years. Therefore, it is nor correct to change the carbon bed until the water treated by it contains a noticeable residual of free chlorine.
How often is it recommended to change the activated carbon?
As a practical criterion, it is recommended to change the activated carbon every year. And this, from an economic point of view, does not have a great impact on the cost of treated water. For the example mentioned, if the price of the activated carbon is $ 35.00 MXN / kg, and it is changed every year, its cost per liter of treated water is $ 0.0006 MXN (that is, 0.6 cents per cubic meter of water).
What are the advantages of changing activated charcoal every year?
Another advantage of changing the activated carbon per year is that the change is made in a planned manner, and thus avoids that one day unexpectedly the treated water contains chlorine that will give the final product or force to stop the process at an inopportune moment. The change of the carbon, also allows to inspect internally the equipment that contains it and to carry out the labors of maintenance originated by the observations made in this inspection (to eliminate leaks in valves, to renew the epoxy coating of the internal surface of the tank, to backwash correctly to avoid the petrification of the carbon bed, etc.).
What is written in this article is the reason why all the international companies that purify drinking water, change the activated carbon once a year.