By Robert T. Deithorn and Anthony F. Mazzoni
The particle size of activated carbon affects how any POU treatment system will perform. The smaller the particle size of the activated carbon, the faster the rate of removal, whether by adsorption or chemical reaction, because the contaminant has less distance to go to reach the pores in the center of the activated carbon particle. The faster the rate of adsorption, the better the POU system will perform. To illustrate the particle size effect on performance, let's look at how an 8x30 mesh activated carbon resized to a 12x40, 20x40 and 20x50 mesh performs in a de-chlorination study.
An influent chlorine concentration of approximately 5 ppm and a surface-loading rate in 1-inch columns of 24 gpm/ft2 was used (Figure 2) in the study. This gave an empty bed contact time of 10 seconds. Four columns were run in a parallel mode using the same feed water and the same amount of activated carbon packed in 1-inch columns. In Figure 2 the data shows that the column with the smallest particle size, the 20x50 mesh, was able to maintain the treatment objective of 1 ppm through the entire 60-hour experiment, whereas the 8x30 mesh product broke through the treatment objective after 5 hours. Although the smaller particle size activated carbon performed better at the high flow rate, the potential for pressure drop problems must be considered.
Another major operating variable that impacts adsorption is the concentration of the adsorbate relative to its solubility. Adsorption capacity increases at higher concentrations of the adsorbate. Still, even at the lower contaminant concentrations found in drinking water, a significant activated carbon capacity remains to provide effective removal of these compounds.
Other operating factors that can influence activated carbon performance are temperature and pH. While adsorption capacity can increase with de- creasing stream temperatures, the temperature effect is minimal. How- ever, practical operation of POU treatment systems is recommended at ambient or colder temperatures. The pH of the water is important from the standpoint of its effect on solubility of the particular contaminant. Some organic compounds can exist in a more disassociated, polar form due to a pH shift, and would be less amenable to adsorption because of their increased solubility.
In summation, the two most important operating conditions for water treatment systems are flow rate of the water stream and concentration of the adsorbate relative to its solubility in water. As a general rule, lower flow rates allow a greater contact time with a unit volume of activated carbon, thereby improving the ability of the available activated carbon pore surfaces to attract and adsorb molecules. High flow rates can result in inefficient use of a carbon's capacity by not allowing the time for an adsorbate to migrate through the activated carbon pore to adsorption surfaces; however, it may be possible to compensate for high flow rates with a smaller particle size.
Conclusion: There are several considerations involved in the selection of an activated carbon for POU treatment.
A typical laundry list of items for evaluation includes the type of contaminant to be removed (chlorine, general taste and odor, color bodies, specific organics, or all of these). The treatment objective, handling requirements and purity of the effluent water.
Treatment devices with short contact times require the smallest particle size activated carbon practical for efficient use in contaminant removal. Also, activated carbons with a higher density and a high Iodine number allow more pounds of activated carbon to be placed in a given volume unit and provide more total surface area (volume activity) in any given container. If the system is to be back-washable, the use of a high abrasion number activated carbon is recommended so that the activated carbon will not break up during back-washing. Finally: it is important to know how your activated carbon is produced, the raw materials used and whether or not the raw material has been chemically treated which might affect the quality of the water coming out of the treatment unit.