By C.F. “Chubb” Michaud CWS-VI
It is well understood that high surface area and access to that area is what separates one GAC from another. GAC does, indeed, have a very high surface area—about 1000 m2/gm. That’s over 100 football fields per pound of average GAC.
GAC functions by adsorption or adhesion. Organic molecules are drawn to the clean, non-polar surface of the GAC and are held by a molecular attraction. The forces that attract the organics and pull them out of solution must be stronger than the forces that are keeping them in solution. It is understood that organics with high water solubility (alcohols, glycols, acetone or formaldehyde) are not readily adsorbed because the forces keeping them in solution are very strong. On the other hand, those with limited solubility (gasoline, chlorinated hydrocarbons and pesticides) are more easily picked up.
If a typical GAC were saturated with a hydrocarbon such as benzene so that the entire surface area of the GAC was covered one molecule deep, it would contain some 2.5 cc of benzene/gm of GAC. Since 1 gm of GAC only has a pore volume of about 0.5cc, it is easy to see that the entire surface area of GAC can not be utilized. In reality, only about 2% of the total carbon surface is actually “occupied” at break-through. Many of the pores are so small that most organics simply can not fit into it.
One of the keys to maximizing GAC utilization is matching the GAC to the job at hand. Different organics are of different sizes. Color bodies, dyes and natural organics are large molecules and are best treated by GAC with a higher percentage of larger pores (such as wood or lignite based products). Pesticides and common hydrocarbons are medium molecular weight and respond well to coal based GACs. The more volatile and lower Mol. Wgt. solvents and THMs can best be handled with shell carbons. All GACs contain large and small pores. Therefore all GACs remove most everything to some degree. The difference between them is pore size distribution. See Figure 1.
GAC granules from a single batch tend to be uniform throughout. Larger pores lead to smaller pores and provide the pathways for the adsorptive process. A poor match of the GAC and the contaminant can result in large molecules “blocking” these pathways resulting in low capacity. Smaller molecules can get “swept” out of too large a pore by the fluid stream before being “captured”. This, too, results in less than satisfactory performance.
Different sources of similar base substrates (ie the origin of the coal, wood, shell, etc) as well as the type (ie bituminous, sub-bituminous, lignite, anthracite) and the manufacturing process can produce GACs of varying pore size distributions. There are, therefore, a nearly unlimited number of GAC products—some of which will work better than others despite the fact that their data sheets show similar properties. Don’t get lulled into thinking you have found the best GAC for all seasons and stick with just one type or brand. The best advice is to try more than one GAC for different applications and go with the best comfort level.
Match the GAC to the job
Matching the “fit” of the GAC to the job is only one of the considerations. Another key is the particle size of the GAC. A typical 8×30 mesh GAC has a mean particle diameter of 2.0mm—twice that of a 12×40 mesh GAC and four times that of a 20×50 mesh product. If we assume that GAC was a perfect cube, the 8×30 mesh particle would have an external surface area of 24 mm2 (2×2 mm on a side times six sides). The 12×40 product would have an external surface of only 6mm2. However, it would take 8 such particles to equal the weight of a single 8×30. Therefore, the smaller mesh would have a total surface exposed of 48 mm2 for the same weight of GAC. The number of 20×50 particles would be twice that of the 12×40 and four times that of the 8×30. Since water can pass through all sides of the GAC surface, the finer mesh GACs will have more “accessibility” than will the coarser mesh GACs. This means more pathways into the GAC particle which results in more rapid access to adsorption sites.
We would expect the flow rates (gpm/ft3) at which we can operate the finer mesh GACs to be higher than for the coarser mesh products (with the same level of performance). Under the same conditions of operation, finer mesh GAC will have a higher capacity for the removal of a specific contaminant to a given break point. This higher “reaction” rate of finer mesh GAC is known as higher “KINETICS”. See Figure 2
In addition to higher capacities and better kinetics, finer mesh GACs are easier to backwash. See Figure 4. Proper backwash is important in maintaining good distribution, removing dirt and fines and reducing bacteria growth in GAC beds. For residential and light commercial applications there are real advantages in selecting finer mesh GACs. Units can be smaller, they will backwash more easily with limited flow rates and they will usually be less expensive to build and maintain. In addition, if you mix GAC and cation resin in a softener, the coarser sized 8 and 12 mesh products require much higher backwash rates than do softeners. As a result, they drop to the bottom where they can do little to protect the resin bed. A 20 mesh GAC, however, backwashes at slightly less than that of a softener and will rise to the top where it should be. Also, this gives ready access when it is time to change the carbon.
If the spent GAC you generate is to be returned to the manufacturer for reactivation, you probably won’t find a taker for used 20×50 mesh priduct. Even the 12×40 has limitations. Reactivation reduces the size of the GAC particle. Therefore, municipalities and larger industrial users that face disposal problems with spent GAC will generally go with the coarser 8×30 or 12×30 mesh GACs because they lend themselves to multiple regenerations.
Advantages for Selecting Finer Mesh GACs
- higher capacities
- smaller units
- easier to backwash
- less costly to build
- less costly to maintain
- easier to change GAC in mixed media units
To avoid high pressure drops, superficial flow velocities should not exceed 10 gpm/ft2 of bed area. For typical residential needs of 5 gpm, a tank with a cross section of 0.5 ft 2 or 10 inch diameter should be used. A backwash of only 3 gpm would be needed for the 20×50 mesh and 5 to 6 gpm for the 12×40 (compared to 8 or more for the 8×30). Coconut shell GACs require higher backwash flow rates than do coal based (because of higher density). Increase backwash flow 15 to 20% for shell carbons.
About the author
C.F. ‘Chubb’ Michaud, CWS-VI, is the CEO and technical director of Systematix Company, of Buena Park, Calif., which he founded in 1982. An active member of the Water Quality Association, Michaud is a member of the Board and of the Board of Governors and currently chairs the Commercial/Industrial Section (since 2001). He has served on the Board of Directors of the Pacific WQA since 2001 and chairs its technical committee. He was a founding member of (and continues to serve on) the technical review committee for WC&P and has authored or presented over 100 technical publications and papers. He can be reached at Systematix Inc., 6902 Aragon Circle, Buena Park CA 90620, telephone (714) 522-5453 or via email at cmichaud@systematixUSA.com