Professor POU/POE March 2006
Topic: Activated Carbon Bed Expansion and Service Flow.
Written by: David M. Bauman /Technical Editor from Water Technology Magazine
Question:
Our water treatment dealership is in the southern US where there is a growing market for carbon filters due to high chlorine levels. My equipment supplier offers a combination softener/filter and a separate cabon filter. The combination filters don't remove chlorine for long, so I'm giving up on them. My supplier also tells me that I should be using a 12-inch diameter carbon filter. This is larger than my softener. Is this necessary? Since I've already had activated carbon filters plug up on me, wouldn't a larger one be even more of a problem?
Answer:
The combination softener/carbon filter that you're referring to is not offered by most manufactures due to the small volume and short life of the activated carbon and due to the inability of a dealer to separate the spent carbon should rebedding the tank be desired. The two media mix rather thoroughly and stay that way even after backwashing. Sales literature often shows the carbon sitting nicely on top of the cation resin, but it doesn't stay that way for long. These softeners are somtimes called "city" units because the carbon starts on the top, where it would protect the resin from breaking down from
chlorine oxidation in addition to removing the taste and odor.
You might have two reason for using carbon:
For chlorine taste, and odor, where the carbon can be placed anywhere in the system; and/or
For resin protection, where the carbon must be placed ahead of the resin. You didn't mention which application you had in mind, we we'll
assume both. Don't take the resin-protection issue lightly, because depending on the chlorine level, resin can be destroyed in one to two years from high
city chlorine levels. This is especially troublesome in cites that purposely keep the feed dose high in order to maintain a chlorine residual at the most
remote parts of their distribution system.
I agree that abandoning the combination units is a good idea.
Filters and softener diameters:
It is almost always the case that the filter should be a greater diameter than the softener. This is due to the fact that the time it takes to filter particules or remove chlorine is much greater than the time it takes for softeners to exchange ions. Another way you might hear this expressed is that the "kinetics" of one medium are faster or slower than another. One thing in your favor, however, is that chlorine removal is a much faster process than other activated carbon uses, such as organic removal. You are absolutely right that you should be concerned about a larger filter when a smaller one appeared to plug up. Lack of filter bed expansion during backwashing can cause this.
Bed expansion is defined as the resulting present when you divide the expanded portion of a filter bed by its resting depth. A 20-inch-deep bed that expandds to 30 inches has a 50 percent expansion. Some filter literature will tell you to have a 30 to 40 percent bed expansion to maintain a filter, but I have always designed them to have a 40 percent to 50 percent. If you were dealing with private wells I might say that the well can't produce a high enough flow rate to backwash certain filters. But your customers have city water, where the flow rate should be more than ample.
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