Activated carbon (AC) is one of the most widely used media for water filtration. It's effective at removing chlorine taste and odor, and it's recognized as being safe.

However, most activated carbon filters have an indicated lifespan of about 4-6 months, after which they need to be replaced. But is there any risk to using an AC filter past its lifespan other than the fact that it won't effectively filter contaminants anymore?

I've read in passing that an AC filter past its lifespan can actually release contaminants in water, because some contaminants in the influent water may be preferentially adsorbed by the AC, replacing existing contaminants previously trapped in the AC and causing them to be released in the effluent water. Is this something that has been explored in research?

In particular, has it been shown that using a saturated AC filter can cause the levels of certain contaminants — e.g. a trihalomethane such as bromoform, or perhaps fluoride or lead — to rise to a level that is well above the EPA maximum contaminant level?

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    $\begingroup$ The only danger I see is that the activated carbon isn't doing its job right, so you've basically answered your own question in the last paragraph. As for research: you will find plenty of articles on preferrential adsorption at the expense of desorption. $\endgroup$ – William R. Ebenezer Jul 2 '19 at 15:46
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    $\begingroup$ You may also get fine carbon particulates working their way through the filter medium with time. $\endgroup$ – Jon Custer Jul 2 '19 at 16:20

Every adsorbent has a saturation capacity depending on its active surface area. The idea to replace carbon filter is not only due to the saturation capacity but imagine fungi, bacteria may be adsorbed as well. It is worth replacing.

  • $\begingroup$ Thanks. I understand that it's well worth replacing, but what I'm wondering is how real the risk is of contaminating the water with previously trapped compounds such as trihalomethanes, or perhaps even heavy metals like lead which may have a small affinity to AC. $\endgroup$ – N4v Jul 5 '19 at 15:03
  • $\begingroup$ This is a difficult question, because you have to see what is the equilibrium constant of adsorption of trihalomethanes on AC and in water. Assume an activated carbon sample which is saturated with trihalomethanes. When you pass ultrapure water through a column of AC, depending on the equilibrium constant, you might be able to detect trihalomethane in that ultrapure water. $\endgroup$ – M. Farooq Jul 5 '19 at 16:52

Is this something that has been explored in research?

Definitely yes, and the keywords you're looking for are: adsorption, desorption, and fluid transport in porous media. Activated carbon is a nanoporous material with high specific surface area, and the kinetics of adsorption, desorption and transport in liquid phase in such media are well understood from a fundamental point of view.

You can look at papers, reviews and book chapters like this one to look more in depth into the questions. At the macroscopic scale, the behaviour of the complete filter can be predicted by equations of fluid dynamics, local thermodynamic equilibrium, and first-order kinetics.

  • $\begingroup$ Upvote. I'm not sure why someone downvoted your answer... I understand the mechanics of adsorption and desorption, but is it realistic that a saturated AC filter could cause the effluent water to have unsafe levels of lead, for example? $\endgroup$ – N4v Jul 5 '19 at 15:28

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