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Adsorption | Activated Carbons Figure 2: Isotherm for PCB molecule tive in this application compared to a quality coal-based carbon. The surface loading of adsorbate on activated carbon varies with the concentration and conditions in the fluid stream. In order to evaluate the economic potential of an application, the activated carbon isotherms can be developed for the particular compound at a given set of conditions. Many isotherms are already available for various compounds including PCBs. They can be obtained from carbon manufacturers, purifications companies and EPA literature. They can also be developed in the lab using simple procedures. Figure 2 illustrates an isotherm for a PCB molecule with one chlorine atom on TIGG 5D 1240 coal-based activated carbon. As with any testing, these isotherms are performed under controlled laboratory conditions. 20 • February 2010 • www.filtnews.com Actual performance in the field can be affected by any number of factors associated with the treatment system. When dealing with PCB contaminated groundwater, the solubility of the PCB isomers molecules in the water can typically range 20-60ppb with solubilities generally below 1 ppm. Above these levels the PCB’s will be found as free product. As illustrated by the isotherm, PCBs are readily adsorbed by activated carbon, with the example of the PCB isomer with only one chlorine atom (the lowest affinity for all PCB isomers) showing excellent loading on the carbon, even at 1 ppb levels. The result is that effluent levels below 1ppb are achievable. Treatment of this water is dependant not only on keeping the carbon “clean” for proper kinetic transference of the molecules, but also the contact time allowed for the adsorption to take place. Field experiences has shown that often under turbid conditions the PCB levels in the effluent after the carbon adsorbers can be as high as 3-5ppb. The reason for the higher than expected levels in the effluent is that the PCBs will attach themselves to colloidal material in the water or any carbon fines and pass through the bed without being adsorbed. In order to decrease these residual levels upstream and downstream filtration is required. Typi-
Figure 3: Typical PCB removal system. (Varies according to specific applications.) cally a 5-10 micron bagfilter is installed prior to the carbon bed and a 0.5-micron bag filter is installed after the carbon bed, prior to discharge. These processes remove most suspended solids that may be entering the carbon and essentially “plugging” the bed of the carbon thus limiting adsorption, and capturing any solids that may be making their way through to the effluent. In addition to the pre- and postfiltration of the carbon bed, the carbon bed needs to be properly sized. Both the bed surface area and the carbon bed depth affect the efficiency of removal. About seven to eight minutes empty-bed contact time (EBCT, or time to pass fluid through a give actual volume of carbon present as a theoretically open volume) is optimal for proper adsorption. Typically, a minimum of three feet carbon bed depth is required. The surface area is typically designed to promote a superficial velocity of four to six gallons per minute per square foot. Slower velocities can be used but very low velocities should be avoided as this may promote the occurrence of channeling, or the liquid seeking a path of least resistance through the carbon bed, resulting in poor distribution (Figure 3). Overall, activated carbon adsorption is an effective way of reducing PCB contamination in groundwater. Successful results can be achieved with a properly designed system that addresses both prefiltration and post-filtration, along with proper carbon selection and bed design parameters including bed surface area, depth and contact time. FN For more information contact: TIGG Corporation 1 Willow Avenue, Oakdale, PA 15071 Tel: 1-800-925-0011 x101 or 1-724-703-3020 x101 Fax: 1-724-703-3026 Websites: www.TIGG.com or www.TIGGtanks.com www.filtnews.com • February 2010 • 21
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